Post by: jamonwindeyer on October 31, 2016, 11:30:23 am
Just click the spoilers below to see our suggested answers!
Multiple Choice
Q1: D - Transformers don't convert AC to DC; they are called rectifiers. Decrease the voltage to prevent damaging the phone.
Q2: C - Westinghouse was AC, and lower currents reduce energy loss
Q3: C - Magnetic fields only exert a force on MOVING particles
Q4: A - Alloys were discovered first, then ceramic compounds
Q5: D - \(E=\frac{V}{d}=\frac{15}{0.005}=3000Vm^{-1}\)
Q6: D - Speed of light is constant in all frames of reference (special relativity)
Q7: B - Lenz's Law opposes the motion of the magnet; pushes it away when it is coming in, pulls it back when it is going away
Q8: D - D is exposed to the greatest rate of change of magnetic flux, and so experiences the largest EMF (and thus current).
Q9: B - Back EMF will oppose Supply EMF until an equilibrium is reached, thus limiting the speed of the motor
Q10: C - The observer is in the same frame of reference as the light and doors, so length contraction does not affect observations.
Q11: B - \(\lambda=\frac{hc}{E}=\frac{6.626\times10^{34}\times3\times10^8}{3.5\times1.602\times10^{-19}}=3.5\times10^{-7}\)
Q12: B - By the right hand grip rule or its variants
Q13: C - More intensity equals more photons, but they have the same energy
Q14: B - By Kepler's 3rd Law
Q15: A - The light causes formation of electrons and holes which both carry current through the external circuit.
Q16: A - This is a loudspeaker being used in reverse, and so acts as a generator. Use of the right hand grip rule and Lenz's Law gives direction
Q17: C - The lift slowing down generates a slight upwards acceleration on the ball. This increases flight time.
Q18: C - \(v=\sqrt{ar}=\sqrt{3.6\times9.8}=5.9\)
Q19: A - In the Muon's reference frame, classical physics works. Earth views an increase in lifetime due to time dilation.
Q20: B - The changes in polarity of the magnetic field and the current ensure the force torque is always clockwise, while the commutator ensures it does not reverse with every half turn (very similar to a universal motor, if anyone learned that)
Q2: C - Westinghouse was AC, and lower currents reduce energy loss
Q3: C - Magnetic fields only exert a force on MOVING particles
Q4: A - Alloys were discovered first, then ceramic compounds
Q5: D - \(E=\frac{V}{d}=\frac{15}{0.005}=3000Vm^{-1}\)
Q6: D - Speed of light is constant in all frames of reference (special relativity)
Q7: B - Lenz's Law opposes the motion of the magnet; pushes it away when it is coming in, pulls it back when it is going away
Q8: D - D is exposed to the greatest rate of change of magnetic flux, and so experiences the largest EMF (and thus current).
Q9: B - Back EMF will oppose Supply EMF until an equilibrium is reached, thus limiting the speed of the motor
Q10: C - The observer is in the same frame of reference as the light and doors, so length contraction does not affect observations.
Q11: B - \(\lambda=\frac{hc}{E}=\frac{6.626\times10^{34}\times3\times10^8}{3.5\times1.602\times10^{-19}}=3.5\times10^{-7}\)
Q12: B - By the right hand grip rule or its variants
Q13: C - More intensity equals more photons, but they have the same energy
Q14: B - By Kepler's 3rd Law
Q15: A - The light causes formation of electrons and holes which both carry current through the external circuit.
Q16: A - This is a loudspeaker being used in reverse, and so acts as a generator. Use of the right hand grip rule and Lenz's Law gives direction
Q17: C - The lift slowing down generates a slight upwards acceleration on the ball. This increases flight time.
Q18: C - \(v=\sqrt{ar}=\sqrt{3.6\times9.8}=5.9\)
Q19: A - In the Muon's reference frame, classical physics works. Earth views an increase in lifetime due to time dilation.
Q20: B - The changes in polarity of the magnetic field and the current ensure the force torque is always clockwise, while the commutator ensures it does not reverse with every half turn (very similar to a universal motor, if anyone learned that)
Short Answer (Brief Solutions)
Question 21A
- Additional ozone particles
- Friction with satellite causes deaccleration -> Orbital Decay
Question 21B
\(F_g=\frac{GMm}{d^2}=\frac{6.67\times10^{-11}\times50\times6\times10^24}{8000000^2}=312.66N\) (2dp)
Question 22A
Moving the bar magnet near Coil B to expose it to a changing magnetic flux. Brief explanation of this and Faraday's Law for 2 marks.
Question 22B
Initially, velocity increases (negatively) due to gravity. Strong eddy currents then apply an upwards acceleration which forces the magnet back upwards. Good explanation of Eddy Currents, why they are formed, and why they oppose the magnets motion, would be needed for the full 4 marks.
Question 23A
Horizontal deflection plates cause the cathode ray to sweep across the screen. Vertical deflection plates are connected to an input source such that vertical axis becomes representative of the amplitude of the input signal. Both come together to create the typical CRO graph.
Question 23B
\(Bqv=Eq \implies B=\frac{E}{v}=\frac{10}{5.2\times10^4}=0.2mT\)
The field is applied UP the page.
Question 24
The following material is relevant:
- Resistance is related to the ease at which electrons can flow through the lattice
- Normally, electrons require energy to move through the lattice
- Superconductors below critical temperature have low enough lattice vibration to form phonons around moving electrons (regions of positive change)
- This allows formation of cooper pairs that flow through the lattice experiencing zero effective resistance
Question 25A
Team A shows an inverse-square relationship over a wide range of data points. Team B took data over a narrow range, and thus shows only a linear relationship.
Question 25B
Team B's data was not taken over a wide range of distances, and thus misappropriates the nature of the relationship between force and distance. Answers that say this in varying ways (with good explanation) would be worth 3 marks.
Question 26
Typical societal impact question. Discuss the ability to move energy production out of regional centres (because we can now distribute), discuss improved energy efficiency and thus lower costs of power, discuss the improvements to quality of life due to appliances that can now operate on different input voltages (household transformers), and other useful information. Clear and well rounded answers that show clear links to SOCIETY and don't just explain what a transformer does will score well.
Question 27A
Equipment to Mention: "Spark gaps", ultraviolet light sources, large voltage sources (induction coils could blend the voltage/gap equipment). Outline the observations that ultraviolet light increased the sparking in the typical experiment. 1 mark for equipment, 1 mark for experiment and observations made with ultraviolet light, one mark for link to photoelectric effect.
Question 27B
You could discuss a few things here; Hertz experiment would work, as would Einstein's formulation. Einstein was likely the more common answer. Discuss the photon model of light and how Einstein used it to explain the photoelectric effect for full marks.
Question 28
Some things to cover (not exhaustive)
- Law of Conservation of Momentum: Momentum in closed system must be conserved
- Momentum of rocket is always equal and opposite to momentum of fuel (which is assumed a constant)
- As mass of rocket decreases, but momentum is constant, velocity must increase (acceleration)
- Decrease in mass also causes increase in acceleration by Newton's 2nd Law (seen in diagram)
Question 29
Far out, what the hell is this. Super broad; pick three technologies and explain the discoveries that lead to them. Things from I2I are what would likely work best in this question, since discovery/application is the idea of the topic to begin with. CRO's, Photo Cells, Solar Cells (of course the electric generator needs to be one of them, with Faraday's Law being the discovery that spurred it).
Question 30A
Calculate change in GPE of mass:
This is the energy that would be converted to electrical energy to the light bulb (ignoring losses), so this is the maximum energy.
Question 30B
When the switch is closed, the DC generator will in actuality have Back EMF which opposes the fall of the mass. It falls more slowly as a result. When the switch is open, those oppositional 'eddy currents'/'back emf' cannot flow, and so the mass falls under the normal effects of gravity on Mars. Some explanation to this effect (with a tad more detail perhaps) would earn the 3 marks.
- Additional ozone particles
- Friction with satellite causes deaccleration -> Orbital Decay
Question 21B
\(F_g=\frac{GMm}{d^2}=\frac{6.67\times10^{-11}\times50\times6\times10^24}{8000000^2}=312.66N\) (2dp)
Question 22A
Moving the bar magnet near Coil B to expose it to a changing magnetic flux. Brief explanation of this and Faraday's Law for 2 marks.
Question 22B
Initially, velocity increases (negatively) due to gravity. Strong eddy currents then apply an upwards acceleration which forces the magnet back upwards. Good explanation of Eddy Currents, why they are formed, and why they oppose the magnets motion, would be needed for the full 4 marks.
Question 23A
Horizontal deflection plates cause the cathode ray to sweep across the screen. Vertical deflection plates are connected to an input source such that vertical axis becomes representative of the amplitude of the input signal. Both come together to create the typical CRO graph.
Question 23B
\(Bqv=Eq \implies B=\frac{E}{v}=\frac{10}{5.2\times10^4}=0.2mT\)
The field is applied UP the page.
Question 24
The following material is relevant:
- Resistance is related to the ease at which electrons can flow through the lattice
- Normally, electrons require energy to move through the lattice
- Superconductors below critical temperature have low enough lattice vibration to form phonons around moving electrons (regions of positive change)
- This allows formation of cooper pairs that flow through the lattice experiencing zero effective resistance
Question 25A
Team A shows an inverse-square relationship over a wide range of data points. Team B took data over a narrow range, and thus shows only a linear relationship.
Question 25B
Team B's data was not taken over a wide range of distances, and thus misappropriates the nature of the relationship between force and distance. Answers that say this in varying ways (with good explanation) would be worth 3 marks.
Question 26
Typical societal impact question. Discuss the ability to move energy production out of regional centres (because we can now distribute), discuss improved energy efficiency and thus lower costs of power, discuss the improvements to quality of life due to appliances that can now operate on different input voltages (household transformers), and other useful information. Clear and well rounded answers that show clear links to SOCIETY and don't just explain what a transformer does will score well.
Question 27A
Equipment to Mention: "Spark gaps", ultraviolet light sources, large voltage sources (induction coils could blend the voltage/gap equipment). Outline the observations that ultraviolet light increased the sparking in the typical experiment. 1 mark for equipment, 1 mark for experiment and observations made with ultraviolet light, one mark for link to photoelectric effect.
Question 27B
You could discuss a few things here; Hertz experiment would work, as would Einstein's formulation. Einstein was likely the more common answer. Discuss the photon model of light and how Einstein used it to explain the photoelectric effect for full marks.
Question 28
Some things to cover (not exhaustive)
- Law of Conservation of Momentum: Momentum in closed system must be conserved
- Momentum of rocket is always equal and opposite to momentum of fuel (which is assumed a constant)
- As mass of rocket decreases, but momentum is constant, velocity must increase (acceleration)
- Decrease in mass also causes increase in acceleration by Newton's 2nd Law (seen in diagram)
Question 29
Far out, what the hell is this. Super broad; pick three technologies and explain the discoveries that lead to them. Things from I2I are what would likely work best in this question, since discovery/application is the idea of the topic to begin with. CRO's, Photo Cells, Solar Cells (of course the electric generator needs to be one of them, with Faraday's Law being the discovery that spurred it).
Question 30A
Calculate change in GPE of mass:
This is the energy that would be converted to electrical energy to the light bulb (ignoring losses), so this is the maximum energy.
Question 30B
When the switch is closed, the DC generator will in actuality have Back EMF which opposes the fall of the mass. It falls more slowly as a result. When the switch is open, those oppositional 'eddy currents'/'back emf' cannot flow, and so the mass falls under the normal effects of gravity on Mars. Some explanation to this effect (with a tad more detail perhaps) would earn the 3 marks.
Medical Physics
A (i) - Hard X-rays have a higher frequency than soft X-rays. Explain that hard X-rays are most useful for medical imaging.
A (ii) - Cheap, easily accessible, less radiation exposure than other methods while still providing excellent detail of skeletal structures. Expanding on these ideas would yield 3 marks.
B (i): Piezoelectric materials consist of dipoles which align themselves to electric fields. Applying an AC current causes the material to vibrate due to the alignment of these dipoles. Exposing it to sound waves results in an AC in the material. This allows for production and receiving of ultrasound.
B (ii) At the first boundary, \(Z_1=Z_2\), so there will be no reflected ultrasound. At the second boundary:
^2}{8.364\times10^6)^2}\approx37%)
So 37% of the pulse is reflected. This will be registered by the ultrasound receiver and indicate the location of the bone.
C: Doppler ultrasound is regular ultrasound, but the frequency of reflected pulses is measured to check for Doppler Effect. This allows movement in the body (EG - detecting blood flow in the heart) to be detected. CAT instead combines X-ray images from multiple angles into a 3D image. This allows better visualisation of size/position (EG - size of fluid buildups in the lungs). An answer to this effect, with the mechanics of each expanded and more detail on the use, would get 4 marks.
D: This question requires a breakdown of how PET works; a positron emitting pharmaceutical such as Fluorine 18 is attached to glucose (usually) to form FDG molecules, which are injected into the patient. The location of the molecules is indicated by the release of positrons, which annihilate with electrons to form gamma ray pairs which are detected by gamma cameras. Etc etc. This would get 3/4 marks, but you would probably then need to provide an example of how it is used in diagnosis, and why it is useful in certain scenarios, to get all the way there. Scans of brain activity would be an option.
E: Full explanation of MRI. It should cover:
- The Fundamentals: Spin, precession and parallel-anti parallel alignments, and the Larmor Frequency (2? marks)
- The effect of exposing precessing nuclei to the Larmor Frequency and the idea of absorbed energy (1 mark)
- Explanation of relaxation of these nuclei back into parallel alignments, including reference to T1/T2 (2? marks)
- Emitted radio wave detection and combination into an image (1? mark)
Mark values are indicative, there is more than one way to skin a cat ;)
A (ii) - Cheap, easily accessible, less radiation exposure than other methods while still providing excellent detail of skeletal structures. Expanding on these ideas would yield 3 marks.
B (i): Piezoelectric materials consist of dipoles which align themselves to electric fields. Applying an AC current causes the material to vibrate due to the alignment of these dipoles. Exposing it to sound waves results in an AC in the material. This allows for production and receiving of ultrasound.
B (ii) At the first boundary, \(Z_1=Z_2\), so there will be no reflected ultrasound. At the second boundary:
So 37% of the pulse is reflected. This will be registered by the ultrasound receiver and indicate the location of the bone.
C: Doppler ultrasound is regular ultrasound, but the frequency of reflected pulses is measured to check for Doppler Effect. This allows movement in the body (EG - detecting blood flow in the heart) to be detected. CAT instead combines X-ray images from multiple angles into a 3D image. This allows better visualisation of size/position (EG - size of fluid buildups in the lungs). An answer to this effect, with the mechanics of each expanded and more detail on the use, would get 4 marks.
D: This question requires a breakdown of how PET works; a positron emitting pharmaceutical such as Fluorine 18 is attached to glucose (usually) to form FDG molecules, which are injected into the patient. The location of the molecules is indicated by the release of positrons, which annihilate with electrons to form gamma ray pairs which are detected by gamma cameras. Etc etc. This would get 3/4 marks, but you would probably then need to provide an example of how it is used in diagnosis, and why it is useful in certain scenarios, to get all the way there. Scans of brain activity would be an option.
E: Full explanation of MRI. It should cover:
- The Fundamentals: Spin, precession and parallel-anti parallel alignments, and the Larmor Frequency (2? marks)
- The effect of exposing precessing nuclei to the Larmor Frequency and the idea of absorbed energy (1 mark)
- Explanation of relaxation of these nuclei back into parallel alignments, including reference to T1/T2 (2? marks)
- Emitted radio wave detection and combination into an image (1? mark)
Mark values are indicative, there is more than one way to skin a cat ;)
Quanta to Quarks (courtesy of ParallelPlatypus)
These solutions are courtesy of the user, ParallelPlatypus, absolute legend!
Ai) Neutrons and protons. Protons are positive, neutrons neutral.
ii)
• Electrostatic force is a stronger repulsive force than gravity is attractive at these distances, hence another force is required.
• The strong force, mediated by quark anti-quark gluons, binds protons and neutrons together, and is essentially a residual colour force
• Hence, the strong force balances the electrostatic force and allows the nucleus to be stable.
Bi)
• Relative intensity of spectral lines
• Hyperfine spectral lines
• Zeeman effect
ii)n=4
c) Pauli = Pauli’s exclusion principle which used the four quantum numbers to provide an explanation of the unique states of electrons in their orbits (e.g. 2,8,18 etc.) Pauli explained the quantum arrangement of electrons in the hydrogen atom, making Bohr’s model more credible and justified.
Heisenberg = Matrix mechanics which supported Schrodinger’s probabilistic wave function- lead to the conceptualisation of the quantum probability cloud adaptation of the Bohr model. The Heisenberg Uncertainty principle implies an inaccuracy in measurement which lends further support to an uncertain interpretation of orbital positions and momentums.
d) This one is a bit vague to tackle- I would include an explanation of what synchrotrons, cyclotrons and linear accelerators are, what they achieve and why this is important. I.e. Accelerators make charged particles reach near light speeds and then causes extremely high energy collisions. In the process, exotic sub-atomic particles are revealed from decomposition processes. These can be observed using electromagnetic and magnetic detection devices which record extensive amounts of data from within the extremely short duration of the collision. This data when analysed, can reveal the particles of the standard model. I included a table of the leptons, quarks and force bosons just to make sure. I also mentioned the discovery of the Higgs Boson which lent great support to the standard model.
e) Pretty standard question- you should mention his work in neutron bombardment experiments in which he discovered that slower thermal neutrons were more effective in producing fission. He worked in the Manhattan Project and created the first working nuclear reactor. This involved pioneering moderators like graphite and control rods made of cadmium or boron. This is reflected in modern nuclear reactors which have similar components. You can then talk about the impacts of nuclear reactors on society. E.g. clean energy and the production of isotopes (where I dumped my prepared isotopes, I did industrial and medical, and related this to their societal impact). You could also relate the discussion of the rate of nuclear reaction and controlled/uncontrolled reactions to nuclear weaponry + plutonium 239 production from nuclear reactors. Then you can discuss the pros and cons of nuclear weapons which are well documented. This was my take on the question :)
Ai) Neutrons and protons. Protons are positive, neutrons neutral.
ii)
• Electrostatic force is a stronger repulsive force than gravity is attractive at these distances, hence another force is required.
• The strong force, mediated by quark anti-quark gluons, binds protons and neutrons together, and is essentially a residual colour force
• Hence, the strong force balances the electrostatic force and allows the nucleus to be stable.
Bi)
• Relative intensity of spectral lines
• Hyperfine spectral lines
• Zeeman effect
ii)n=4
c) Pauli = Pauli’s exclusion principle which used the four quantum numbers to provide an explanation of the unique states of electrons in their orbits (e.g. 2,8,18 etc.) Pauli explained the quantum arrangement of electrons in the hydrogen atom, making Bohr’s model more credible and justified.
Heisenberg = Matrix mechanics which supported Schrodinger’s probabilistic wave function- lead to the conceptualisation of the quantum probability cloud adaptation of the Bohr model. The Heisenberg Uncertainty principle implies an inaccuracy in measurement which lends further support to an uncertain interpretation of orbital positions and momentums.
d) This one is a bit vague to tackle- I would include an explanation of what synchrotrons, cyclotrons and linear accelerators are, what they achieve and why this is important. I.e. Accelerators make charged particles reach near light speeds and then causes extremely high energy collisions. In the process, exotic sub-atomic particles are revealed from decomposition processes. These can be observed using electromagnetic and magnetic detection devices which record extensive amounts of data from within the extremely short duration of the collision. This data when analysed, can reveal the particles of the standard model. I included a table of the leptons, quarks and force bosons just to make sure. I also mentioned the discovery of the Higgs Boson which lent great support to the standard model.
e) Pretty standard question- you should mention his work in neutron bombardment experiments in which he discovered that slower thermal neutrons were more effective in producing fission. He worked in the Manhattan Project and created the first working nuclear reactor. This involved pioneering moderators like graphite and control rods made of cadmium or boron. This is reflected in modern nuclear reactors which have similar components. You can then talk about the impacts of nuclear reactors on society. E.g. clean energy and the production of isotopes (where I dumped my prepared isotopes, I did industrial and medical, and related this to their societal impact). You could also relate the discussion of the rate of nuclear reaction and controlled/uncontrolled reactions to nuclear weaponry + plutonium 239 production from nuclear reactors. Then you can discuss the pros and cons of nuclear weapons which are well documented. This was my take on the question :)
Post by: proficles on October 31, 2016, 02:15:14 pm
Post by: Zyzyzyzy on October 31, 2016, 02:17:17 pm
Post by: imtrying on October 31, 2016, 02:27:59 pm
Post by: alexander.chu1 on October 31, 2016, 02:39:01 pm
Post by: bluealder on October 31, 2016, 02:39:30 pm
Post by: Tom McMahon on October 31, 2016, 02:43:19 pm
Post by: pels on October 31, 2016, 02:44:09 pm
16 is wrong, since mechanical energy is creating electrical energy ie generator. Then using Lenz Law current is Y -X so B
Actually, the loudspeaker is an application of the motor effect.
Post by: noonedoesnt on October 31, 2016, 02:46:34 pm
Post by: bluealder on October 31, 2016, 02:47:09 pm
Actually, the loudspeaker is an application of the motor effect.
But it's not asking that, it's asking what the speaker is acting as, in this case mechanical energy is converted to electrical. Read the top bit of the question.
Post by: thomasdo1 on October 31, 2016, 02:47:51 pm
Actually, the loudspeaker is an application of the motor effect.
No, there was no power supply (amplifier)
Post by: ParallelPlatypus on October 31, 2016, 02:51:59 pm
Post by: jakesilove on October 31, 2016, 02:55:00 pm
Post by: EjonMP on October 31, 2016, 02:59:59 pm
Post by: ParallelPlatypus on October 31, 2016, 03:02:43 pm
Post by: and1_98 on October 31, 2016, 03:05:34 pm
Got the calculation; my explanation for the 'open switch' part wasn't too sound though.
Had to go back and essentially guess a majority of the multiple choice due to time constraints towards the end... they were insanely difficult under pressure.
Thought a lot of the questions (bar the Transformer/society and 6 mark recount of 3 technologies) actually required a sense of understanding of concepts regarding the course; which is what I like to see.
A decent exam - hard mc - and a lot to do in 3 hours
Post by: jamonwindeyer on October 31, 2016, 03:08:32 pm
Correct me if wrong, but I though 16 was A because mechanical movement creating electrical energy (generator) and the right side of coil would be north to attract to south and reduce motion (Lenz's Law), and by right hand grip rule would be A?
I agree, there was no power supply, and the cone is pushed providing a current, therefore it is a generator. I disagree with the current direction though- we are looking at conventional current, so the thumb should point to the left, and the fields go from the north to south, therefore towards the center, therefore current should be going down on the front side, and travel from X to Y (conventional). So I would say A is the correct answer. (Hope it is anyway). I think the initial answer C is correct for the elevator- we experience weightlessness as the elevator slows, which implies a lower downwards acceleration, therefore it should not fall as fast.
You guys (and others) are totally right, thanks for that spot! I totally missed it! I've made that change, legends! ;D
Elevator is also incorrect
Care to justify? Jake and I are pretty confident on that one :)
If the magnets were in the same direction, it would be stronger, but the magnets have their poles near each other- so the fields would be very small and only go from N to S in small contained areas- hence very minimal interaction with coil.
Great answer Platypus, thanks heaps! :)
Post by: ParallelPlatypus on October 31, 2016, 03:09:41 pm
Post by: bohimei on October 31, 2016, 03:10:24 pm
Post by: ParallelPlatypus on October 31, 2016, 03:14:26 pm
Post by: Swagadaktal on October 31, 2016, 03:15:22 pm
Just a q: q23b the field applied has to go upwards coz the electron is moving to the right - so current is going to the left (conventional current) , electric field going into page so the force is going downwards (using RH Slap)? So to make sure it doesnt go downwards the field has to go upwards?
(like im just learning here not questioning solutions or anything :P)
Post by: jamonwindeyer on October 31, 2016, 03:15:32 pm
How is 10/(5.2x10^4) = 0.1 mT ?
Yeah it should be around 1.92*10^-4 T, they probably put 5*2 in calculator thinking the dot was a multiplication.
Cheers guys, rounding error, changed to 0.2mT which is the same as the answer above if you round to that level of significance ;D
Post by: bluealder on October 31, 2016, 03:16:44 pm
I agree, there was no power supply, and the cone is pushed providing a current, therefore it is a generator. I disagree with the current direction though- we are looking at conventional current, so the thumb should point to the left, and the fields go from the north to south, therefore towards the center, therefore current should be going down on the front side, and travel from X to Y (conventional). So I would say A is the correct answer. (Hope it is anyway). I think the initial answer C is correct for the elevator- we experience weightlessness as the elevator slows, which implies a lower downwards acceleration, therefore it should not fall as fast.
Yeah my bad, messed up the grip rule
Post by: jamonwindeyer on October 31, 2016, 03:17:29 pm
Fuck I'm from VCE - took a quick look at your exam and holy shit its wayyyy harder than VCE physics holy fuck
Just a q: q23b the field applied has to go upwards coz the electron is moving to the right - so current is going to the left (conventional current) , electric field going into page so the force is going downwards (using RH Slap)? So to make sure it doesnt go downwards the field has to go upwards?
(like im just learning here not questioning solutions or anything :P)
Hey Swag ;) it was a brutal one this year!
Force on electron due to electric field goes out of the page, since the field goes into the page. Using Right Hand Slap, the magnetic field needs to be upwards to generate an oppositional force into the page ;D
Post by: Evan123 on October 31, 2016, 03:19:14 pm
Hope that made sense :)
Edit: I don't understand why it is C (I think it is A)
Post by: gabe.ralph on October 31, 2016, 03:20:55 pm
Post by: Lockyb98 on October 31, 2016, 03:21:57 pm
Post by: bohimei on October 31, 2016, 03:26:49 pm
Post by: ParallelPlatypus on October 31, 2016, 03:27:27 pm
Post by: bohimei on October 31, 2016, 03:28:56 pm
Would Q17 be A? As the elevator is slowing down as it reaches the TOP floor therefore time in flight would be shorter?
Time of flight is irrelevant here. The elevator is still moving up BUT slowing down, the ball is NOT slowing down along with the elevator. Because the ball isn't slowing down it'd actually move up relative to the elevator which is slowing down. Hence the dotted line above the solid line.
Post by: synix on October 31, 2016, 03:36:46 pm
Post by: jamonwindeyer on October 31, 2016, 03:38:01 pm
Time of flight is irrelevant here. The elevator is still moving up BUT slowing down, the ball is NOT slowing down along with the elevator. Because the ball isn't slowing down it'd actually move up relative to the elevator which is slowing down. Hence the dotted line above the solid line.
You are totally right, but time of flight is one relevant way to explain the phenomenon. It IS in the air for longer after all ;D
Imagine you are in an elevator, when you first take off from the ground, you experience a stronger downwards 'gravity'- just like a rocket ship taking off, this would cause the path shown in A. Just as you decelerate when you reach the top, imagine you are in the elevator, you get the fluttering feeling in the stomach, this is due to the feeling of lowered 'gravity', or a lower perceived weight. This lower weight means the flight will be longer. Pretty confident on this one. As for 15, I found this one hard... I think it should be A, I agree with the solutions, after lots of last minute research last night- light strikes the n or p type semiconductor around the boundary, creates the electron, hole pair (by exciting the electron out of the valence band), then the pn junction local electrical field pulls the electron towards the n type and the hole towards the p type. The light isn't changing the voltage, it is simply facilitating the electric field of the depletion zone achieving this.
Couldn't have said it better myself! ;D
Post by: gabe.ralph on October 31, 2016, 03:38:45 pm
Imagine you are in an elevator, when you first take off from the ground, you experience a stronger downwards 'gravity'- just like a rocket ship taking off, this would cause the path shown in A. Just as you decelerate when you reach the top, imagine you are in the elevator, you get the fluttering feeling in the stomach, this is due to the feeling of lowered 'gravity', or a lower perceived weight. This lower weight means the flight will be longer. Pretty confident on this one. As for 15, I found this one hard... I think it should be A, I agree with the solutions, after lots of last minute research last night- light strikes the n or p type semiconductor around the boundary, creates the electron, hole pair (by exciting the electron out of the valence band), then the pn junction local electrical field pulls the electron towards the n type and the hole towards the p type. The light isn't changing the voltage, it is simply facilitating the electric field of the depletion zone achieving this.But in doing so it creates a potential difference, so why not c). It just seems in a solar cell the potential difference is more important than the pairs. If both happen why is only A the answer.
Post by: stateofmind on October 31, 2016, 03:39:06 pm
Time of flight is irrelevant here. The elevator is still moving up BUT slowing down, the ball is NOT slowing down along with the elevator. Because the ball isn't slowing down it'd actually move up relative to the elevator which is slowing down. Hence the dotted line above the solid line.
a=(v-u)/t. so as acceleration is increasing, time of flight decreases. range=velocity x time. so less time equals less range
thats what I thought lol made an account just to post this :P
Post by: bohimei on October 31, 2016, 03:41:07 pm
Post by: Cindy2k16 on October 31, 2016, 03:43:20 pm
1. the multiple choice was hard. since im dumb i didnt know if voltage is stepped up or down for q1 but i guessed it right i think. Also the circular motion question freaked me out and I didn't know how to do it and did some weird calculations and still managed to get 5.9?? Also yep. fell for the speaker question and chose the wrong answer.
2. sigh. i shouldn't have looked at the answers. I told myself not to but then i did and I definitely regret it. The strangest thing was that I did the same working out for 30.a) as shown. but i didn't get that number..... I got a different value :/ so I must've done it wrong somehow....I probably would've been better off not knowing the answer (since I didn't even know if I did the question right but now I know i did it right but managed to get the wrong answer still) I didn't look at all the answers though, just some and then told myself to stop lol
3. I initially thought it was an alright exam. but the answers kind of gave me a reality check haha. I didn't see many mistakes out of the answers I glimpsed, but my mind tends to focus on what I did wrong rather than what I did right though :/
4. So for this exam I'm gonna go by my life motto of "expect the worst" ;D ;D
Edit: Last MC question...I'm an idiot I was totally blind and only saw that the power supply was connected to the coil, and didn't see the coils on either side. Got that wrong. So 18/20 for MC... Not that bad though I really would've liked the safety of only losing marks in the long response (which i refuse to look at any more lol i only looked at the last question)
Post by: jamonwindeyer on October 31, 2016, 03:44:58 pm
Just wondering if u guys plan on answering any of the options topics? Thnx :)
I've started Medical Physics and I'll probably finish it later tonight; I'll have a crack at Age of Silicon too (I've done enough at uni to feel semi-confident, and you guys can check me). We might have some for Astrophysics at some point too.
If anyone would like to share their solutions for Geophysics or Quanta, PM me! I'll pop your answers in the post above and credit your username (gotta love that AN fame) ;D
Post by: jamonwindeyer on October 31, 2016, 03:46:01 pm
But in doing so it creates a potential difference, so why not c). It just seems in a solar cell the potential difference is more important than the pairs. If both happen why is only A the answer.
The formation of the electron-hole pair by a photon doesn't affect the potential difference; that's a property of the junction. The electric field is relatively constant. The light frees an electron hole pair which can THEN be affected by the field ;D
Post by: ParallelPlatypus on October 31, 2016, 03:46:39 pm
Post by: jamonwindeyer on October 31, 2016, 03:48:04 pm
hm my thoughts:
1. the multiple choice was hard. since im dumb i didnt know if voltage is stepped up or down for q1 but i guessed it right i think. Also the circular motion question freaked me out and I didn't know how to do it and did some weird calculations and still managed to get 5.9?? Also yep. fell for the speaker question and chose the wrong answer.
Speaker question got me too; we can despair together ;) sounds like you did great on the whole in a hard section!
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2. sigh. i shouldn't have looked at the answers. I told myself not to but then i did and I definitely regret it. The strangest thing was that I did the same working out for 30.a) as shown. but i didn't get that number..... I got a different value :/ so I must've done it wrong somehow....I probably would've been better off not knowing the answer (since I didn't even know if I did the question right but now I know i did it right but managed to get the wrong answer still) I didn't look at all the answers though, just some and then told myself to stop lol
You'll get 2/3 just for the working, don't let it stress you! ;D
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3. I initially thought it was an alright exam. but the answers kind of gave me a reality check haha. I didn't see many mistakes out of the answers I glimpsed, but my mind tends to focus on what I did wrong rather than what I did right though :/
4. So for this exam I'm gonna go by my life motto of "expect the worst" ;D ;D
You will be fine; this was a tough exam and it sounds like you did really well on the whole. Don't stress, it's done now, just party! ;D
Post by: jamonwindeyer on October 31, 2016, 03:48:41 pm
I agree, that is why I find this one hard (regarding Q15), but as for the most correct, is the light independently responsible for the creation of the voltage? No. It is the electric field that produces the voltage. What I mean to say, is some random bit of light could strike a p type semiconductor, and an electron - anti electron pair would form and then recombine- there would be no voltage. Hence it is not the light causing the voltage, but the electric field across the p n junction that already exists though. That is my take on it though... tough one.
You need to be a tutor next year Platypus; you explain stuff really well, thanks heaps for your help!! ;D
Post by: bohimei on October 31, 2016, 03:50:04 pm
that pun got me good
Post by: ParallelPlatypus on October 31, 2016, 03:50:58 pm
Post by: Cindy2k16 on October 31, 2016, 03:54:57 pm
Post by: Alalamc on October 31, 2016, 04:11:18 pm
Post by: Cindy2k16 on October 31, 2016, 04:13:39 pm
Speaker question got me too; we can despair together ;) sounds like you did great on the whole in a hard section!
You'll get 2/3 just for the working, don't let it stress you! ;D
You will be fine; this was a tough exam and it sounds like you did really well on the whole. Don't stress, it's done now, just party! ;D
For the working out of 30a, is the second work for correct working out? Since I can't remember I dont know if my mistake was somehow accidentally writing in the numbers wrong D:
Post by: ParallelPlatypus on October 31, 2016, 04:15:58 pm
I realised I mixed up weak and strong force in my paper haha, I fixed it here though. I imagine a moderator can place these solutions in the official ones if they are happy with them.
These solutions have been moved to the first post in this thread!
Post by: FallonXay on October 31, 2016, 04:17:32 pm
Quanta to Quarks Solutions (My Solutions?!):
I realised I mixed up weak and strong force in my paper haha, I fixed it hear though. I imagine a moderator can place these solutions in the official ones if they are happy with them.
Ai) Neutrons and protons. Protons are positive, neutrons negative.
(Neutrons are neutral btw :)) Thanks for posting your solutions!!!
Post by: ParallelPlatypus on October 31, 2016, 04:19:19 pm
Post by: FallonXay on October 31, 2016, 04:20:25 pm
c) Pauli = Pauli’s exclusion principle which used the three quantum numbers to provide an explanation of the unique states of electrons in their orbits (e.g. 2,8,18 etc.) Pauli explained the quantum arrangement of electrons in the hydrogen atom, making Bohr’s model more credible and justified.
Heisenberg = Matrix mechanics which supported Schrodinger’s probabilistic wave function- lead to the conceptualisation of the quantum probability cloud adaptation of the Bohr model. The Heisenberg Uncertainty principle implies an inaccuracy in measurement which lends further support to an uncertain interpretation of orbital positions and momentums.
Also, I may be wrong, but weren't there 4 quantum numbers? (Principal, Orbital, Magnetic, Magnetic Spins)
Post by: FallonXay on October 31, 2016, 04:21:15 pm
Hahah woops- fixed above
Awesome ;D
Post by: FallonXay on October 31, 2016, 04:24:28 pm
e) Pretty standard question- you should mention his work in neutron bombardment experiments in which he discovered that slower thermal neutrons were more effective in producing fission. He worked in the Manhattan Project and created the first working nuclear reactor. This involved pioneering moderators like graphite and control rods made of cadmium or boron. This is reflected in modern nuclear reactors which have similar components. You can then talk about the impacts of nuclear reactors on society. E.g. clean energy and the production of isotopes (where I dumped my prepared isotopes, I did industrial and medical, and related this to their societal impact). You could also relate the discussion of the rate of nuclear reaction and controlled/uncontrolled reactions to nuclear weaponry + plutonium 239 production from nuclear reactors. Then you can discuss the pros and cons of nuclear weapons which are well documented. This was my take on the question :)
Awhhhh, nice ~ I completely forgot about radioisotopes :)
Nice thinking!
Post by: ParallelPlatypus on October 31, 2016, 04:26:29 pm
Post by: jamonwindeyer on October 31, 2016, 04:41:47 pm
So judging by the little information offered by the raw marks database.....is a raw mark of say, 85% for this exam going to award a band 6? Lower? Higher?
This was a really tough paper in my opinion, I'm expecting a fairly low Band 6 cut-off this year! Probably low 80's for a Band 6? We'll have to wait and see! :)
17/20 FOR MULTIPLE CHOICE AND LEGIT ONLY STUDIED 2 1/2 DAYS.. Award plz 😂
Dayum, legend! Well done! ;D
For the working out of 30a, is the second work for correct working out? Since I can't remember I dont know if my mistake was somehow accidentally writing in the numbers wrong D:
You'd get like 1 mark for correct approach, 1 mark for correct formulae, 1 mark for correct numerical answer? Something like that? If you literally just subbed a wrong number you'd definitely get the two marks, if you miswrote the formula as a silly mistake you'd probably still get two marks too! :)
Post by: jamonwindeyer on October 31, 2016, 04:51:39 pm
Quanta to Quarks Solutions (My Solutions?!):
I realised I mixed up weak and strong force in my paper haha, I fixed it here though. I imagine a moderator can place these solutions in the official ones if they are happy with them.
I've put them in with the main post; you are awesome!! ;D
Post by: ladyofathena on October 31, 2016, 04:54:04 pm
Didn't realise the loudspeaker wasn't connected to anything lol
Post by: jamonwindeyer on October 31, 2016, 05:06:31 pm
Ughhh so mad at myself. Only looked at MC but already lost like 5 marks and I really did not like my answers for the rest of the paper. I was on track to get a band 6 but I think the highest I'll get is in the low 80's.
Didn't realise the loudspeaker wasn't connected to anything lol
Neither did I ;)
Don't worry, you know about Band cut-offs and stuff right? A mark in the 80's should still nab you a Band 6! ;D you just never know with this sort of thing, stay confident! ;D
Post by: ladyofathena on October 31, 2016, 05:12:37 pm
Neither did I ;)
Don't worry, you know about Band cut-offs and stuff right? A mark in the 80's should still nab you a Band 6! ;D you just never know with this sort of thing, stay confident! ;D
Haha thanks Jamon, I do know about the cut offs but I'm always thinking the worst particularly since my friends found the exam fine. Just have to do my best for chem and then I'm done! ;D
Post by: jamonwindeyer on October 31, 2016, 05:32:24 pm
Haha thanks Jamon, I do know about the cut offs but I'm always thinking the worst particularly since my friends found the exam fine. Just have to do my best for chem and then I'm done! ;D
No stress! Get keen to finish up and party! ;D
Post by: hungrybubbles on October 31, 2016, 06:00:35 pm
Post by: Happy Physics Land on October 31, 2016, 06:04:15 pm
I feel like question 20 is D, if it were DC it would be continuously clockwise, but with AC - the brushes dont change the sides every half turn . unless they do .
But because the wire forms solenoids around the two blocks near the rotor coil, AC is also fed into that and the solenoids become electromagnets. So whilst AC is converted into DC by commutator, the polarity of the electromagnets would switch every now and then and hence it would work like a motor rotating in a clockwise direction as predicted by right hand palm rule.
Post by: gstaah on October 31, 2016, 06:08:18 pm
Post by: bethjomay on October 31, 2016, 06:13:06 pm
Post by: bethjomay on October 31, 2016, 06:14:23 pm
uh so for the 6 marker on discoveries that lead to new technologies. i put Bragg's experiment creating a better understanding of lattice structures within crystals which enabled the development of superconductivity to occur -> bcs theory -> MRI but idk if this is correct or i would get any marks?
That sounds good as part of an answer! Did you talk about generators and one other technology as well?
Post by: Aliceyyy98 on October 31, 2016, 06:16:02 pm
Post by: ParallelPlatypus on October 31, 2016, 06:20:13 pm
Post by: ParallelPlatypus on October 31, 2016, 06:22:06 pm
Post by: bethjomay on October 31, 2016, 06:27:07 pm
For 30b, I would guess maybe one for identifying Back EMF/Lenz's law as a source of resistance in the coil, one for explaining what happens when the switch opens, and another for explaining how this effects the falling object. It is a pretty generous 3 marks (in my opinion).
I had a brain snap and said that the back emf would be greater when the switch was open as there would be no load on the circuit. :P
Post by: gstaah on October 31, 2016, 06:28:28 pm
That sounds good as part of an answer! Did you talk about generators and one other technology as well?
yeah i talked about faraday-> generators and eintsteins photoelectric effect -> solarcells
Post by: stateofmind on October 31, 2016, 06:31:45 pm
Post by: jamonwindeyer on October 31, 2016, 06:32:13 pm
uh so for the 6 marker on discoveries that lead to new technologies. i put Bragg's experiment creating a better understanding of lattice structures within crystals which enabled the development of superconductivity to occur -> bcs theory -> MRI but idk if this is correct or i would get any marks?
Sounds awesome to me! ;D
I found the main section ok, but multiple choice and Quanta to Quarks were hard! I got really lost in the calculation for the option. What are people thinking for a band 6 cut off?
I'm feeling low 80's for a Band 6 cut-off, because the MC was brutal and the paper on the whole was on the difficult end of the scale :)
For Q20, this is actually the exact design of a "universal motor"- which was part of the Motors and Generators topic, so you can research that online if you wish. Basically, every time the AC switches, the magnetic solenoids also swap, which means the force is in the same direction. So essentially, the AC acts as a DC current.
Universal Motors aren't specifically part of the syllabus, but those who touched on them probably would have found the question a little easier than deriving from first principles ;D
Post by: jamonwindeyer on October 31, 2016, 06:33:35 pm
I am pretty sure the answer to MC qu 17 is C, not A as suggested. An upwards deceleration is equal to downwards acceleration. As a= (v-u)/t, the increased value of acceleration means flight time is LESS, not greater. As horizontal range=velocity x time, the reduced time means horizontal range is also reduced, as illustrated in diagram C. :)
You are correct about the upwards deacceleration being a downwards acceleration, but it's not the ball that is accelerating. It is the lift, our frame of reference! If our frame of reference is accelerating downwards, that's like the ball accelerating upwards! It's a test of relativity. Ever been in a lift that went down really quick and you feel a bit lighter? It's just like that!
Post by: Sanaz on October 31, 2016, 09:27:13 pm
Post by: jamonwindeyer on October 31, 2016, 10:31:27 pm
LMAO I MENTIONED BACK EMF hahaha for 30b.... thought i was wrong...
Nah you got it! ;D
Post by: Neutron on October 31, 2016, 11:45:00 pm
Post by: Neutron on October 31, 2016, 11:50:08 pm
Post by: jamonwindeyer on November 01, 2016, 12:34:44 am
How well do you think we needed to explain the 3 technologies question? Cause my mind blanked and the only one i could think of was Maglev trains (discovery being superconductors) but I just said that they provided high speed transportation for society through superconductive magnets without explaining the specifics :O (i don't actually know the specifics, damn my brain couldn't function during that exam)
I think the big thing was the Discovery, not so much how it is applied! :)
And also, this is probably really dumb for multiple choice question 8, the option D had more coils which means that the length of the wire is longer right? And the longer the wire, the greater the resistance.. Since V=IR and V is constant, a greater R would result in a lower I which means the galvanometer would deflect less wouldn't it? Idk whether the greater rate of change of magnetic flux is enough to exceed this (by doubling the velocity) which is why I chose option C :/ Is that some poor logic i had going on in the exam hahahah
I was debating this myself!! Your reasoning is correct, I think it is definitely valid, and further because more coils equal less current in a transformer!
Tbh, I'm yet to fully 100% commit to D as the solution. I totally understand your choice of C. I just think D is the more likely choice (happy to be convinced otherwise if anyone has any thoughts!) ;D
PS - Congrats on getting to 100 posts!
Post by: Aliceyyy98 on November 01, 2016, 08:28:58 am
Post by: jamonwindeyer on November 01, 2016, 10:06:30 am
Hey for the question on three technilogies, it asked HOW the discovery led to them, so we would have to mention how the discovery is applied right?
This is true! Hmm, how many lines did you have... 14! So you'd have to be brief in that effect. It seems like they want a "This is the discovery that was made and this is how it was made. This principle is applied in the technology in this way." Repeat 3 times.
I'd have written something like this for the generator, for example:
Faraday's experiments, which consisted of exposing coils to changing magnetic fields, established the idea of electromagnetic induction (the generation of an EMF when exposed to changing magnetic flux). This lead to the development of the electric generator, which uses induction to convert the kinetic energy of a rotor to electrical energy for external use.
Or, something like that. I interpret the question as needing to specifically mention HOW the discovery was made, because that is HOW it lead to the development of the technology. Then of course, as you say, a somewhat detailed mention of its application.
This is a weird question though; super broad, unlike anything I've seen in a Physics paper for quite a while! :)
EDIT: There is more than one way to skin a cat. Not taking my exact approach doesn't necessarily mean you can't score top marks!
Post by: Aliceyyy98 on November 01, 2016, 11:43:54 am
This is true! Hmm, how many lines did you have... 14! So you'd have to be brief in that effect. It seems like they want a "This is the discovery that was made and this is how it was made. This principle is applied in the technology in this way." Repeat 3 times.
I'd have written something like this for the generator, for example:
Faraday's experiments, which consisted of exposing coils to changing magnetic fields, established the idea of electromagnetic induction (the generation of an EMF when exposed to changing magnetic flux). This lead to the development of the electric generator, which uses induction to convert the kinetic energy of a rotor to electrical energy for external use.
Or, something like that. I interpret the question as needing to specifically mention HOW the discovery was made, because that is HOW it lead to the development of the technology. Then of course, as you say, a somewhat detailed mention of its application.
This is a weird question though; super broad, unlike anything I've seen in a Physics paper for quite a while! :)
EDIT: There is more than one way to skin a cat. Not taking my exact approach doesn't necessarily mean you can't score top marks!
Thank you Jamon!! One more question, does school play a big role in atar calculation, or is it your hsc performance more important??
Post by: RuiAce on November 01, 2016, 11:55:37 am
Thank you Jamon!! One more question, does school play a big role in atar calculation, or is it your hsc performance more important??Your own cohort's performance decides how your internal marks get aligned. But only your own cohort; previous cohorts are useless in this calculation.
And moderation is dependent on your entire cohort's performance in the final exam
Post by: hammyhtk on November 01, 2016, 07:57:38 pm
Post by: bethjomay on November 01, 2016, 08:04:57 pm
Hi guys, regarding Q23b shouldnt the direction of the magnetic field be out of the page inorder for the force to be up the page so that the electron travels undeflected, or am I really confused?
I got that too! But I was unsure about it.
Post by: jamonwindeyer on November 01, 2016, 08:08:26 pm
Hi guys, regarding Q23b shouldnt the direction of the magnetic field be out of the page inorder for the force to be up the page so that the electron travels undeflected, or am I really confused?
I got that too! But I was unsure about it.
Not quite! The electric field is going INTO the page, meaning the force on the electron is OUT of the page! I think you guys might have thought the initial field was a magnetic field, not an electric field? :)
Post by: bethjomay on November 01, 2016, 08:13:37 pm
Not quite! The electric field is going INTO the page, meaning the force on the electron is OUT of the page! I think you guys might have thought the initial field was a magnetic field, not an electric field? :)
Ah, ok. I got that it was an electric field I just couldn't visualise how the field would effect it as I'd never dealt with an electric field in or out of the page, only down or up, so I just inferred the deflection like you would for a magnetic field. :/
EDIT: I am now realising the outward deflection is actually quite obvious due to the nature of an electric field and kicking myself for not noticing :P
Post by: Ahbelle on November 01, 2016, 08:14:27 pm
Post by: bethjomay on November 01, 2016, 08:18:50 pm
For 25 A, how do you know using only the graphs that the relationship shown by team A is 'inverse-square' not simply inverse?
One way we can tell is that the graph got exponentially steeper as the distance approached zero! The curve on an 'inverse' graph would be something in the form y = 1/x which would be the same in the in the x and y directions. Someone please correct me if I'm wrong but I hope that made sense :)
Post by: jamonwindeyer on November 01, 2016, 08:23:50 pm
Ah, ok. I got that it was an electric field I just couldn't visualise how the field would effect it as I'd never dealt with an electric field in or out of the page, only down or up, so I just inferred the deflection like you would for a magnetic field. :/
Ahh fair enough! Yeah, electric fields are even simpler, just follow the direction of the field (or opposite for an electron) ;D
All good! If you calculated the magnitude correctly you'll still get most of the marks ;D
For 25 A, how do you know using only the graphs that the relationship shown by team A is 'inverse-square' not simply inverse?
Well I suppose you can immediately jump to that conclusion because Newton's Law of Universal Gravitation is inverse square in nature. Qualitatively from the graph, see Beth's explanation above (thanks Beth!). Admittedly, I probably wouldn't spot it without knowing that we are talking about gravity ;D
Post by: PunjanTriv on November 16, 2016, 09:45:07 am
We did question 10 in class and somehow our teacher said the correct answer is B. Our teacher said:
As we know the train is moving towards the right, the front door is now further away and the back door is closer. As the light is turned on the light travels at the same speed and same direction, the light will eventually reach door W first. Therefore W opens before Z.
I know you are right as well.
So the answer can be either C or B.
But which one is more correct?
Thanks
Post by: jamonwindeyer on November 16, 2016, 12:27:19 pm
Hey Jamon!
We did question 10 in class and somehow our teacher said the correct answer is B. Our teacher said:
As we know the train is moving towards the right, the front door is now further away and the back door is closer. As the light is turned on the light travels at the same speed and same direction, the light will eventually reach door W first. Therefore W opens before Z.
I know you are right as well.
So the answer can be either C or B.
But which one is more correct?
Thanks
Hey Punjan! Welcome to the forums! ;D
Your teacher is mistaken. B is incorrect.
The key in noticing why is emphasising that we are talking about what the passenger observes. The observer, in their frame of reference, sees a light source at fixed distance from two doors. So to the observer on the train, they open simultaneously.
Your teachers analysis only works for someone observing from outside the train, standing on a platform (for example). They see the back of the train catch up, so THEY will see the train door at the back open first. But the observer on the train sees them open at the same time.
The Wikipedia Article actually does a good job showing this! Scroll down to the train-platform thought experiment! ;D
I hope this makes sense!