HSC Physics Question Thread

[RuiAce]

Don't quite understand how to go about this question.

Recently the weapons unit of the UN discovered the components for a 100m long cannon barrel in an unspecified country. The country's defence agencies have denied that this is an offensive weapon to be used against neighbouring countries. They have explained that is is to be used to launch communication satellites into orbit. Explain why this is not considered to be a plausible explanation. (3 marks)

Hint: Think about accelerating too quickly causing the satellite to burn, and excessive g-forces if people are meant to be on the satellite.

[Neutron]

Yo! Could someone please explain these two multiple choices for me? For the first one, if back emf was equal to the applied voltage wouldn't that mean the total voltage is zero so the coil wouldn't even spin? (No voltage=no current) what ahah and for the second one, the heat loss within metal is due to the current's interaction with the resistance right? So I get that with lower resistance you can get more current (and since its I^2 then that will increase power loss) but if you had high resistance, wouldn't the ultimately heat loss be greater? Like if there was no resistance but the current was really high, there'd still be no power loss so idk in this case does a higher current win out? :/ cheers

Neutron

[Happy Physics Land]

(Image removed from quote.) (Image removed from quote.)

Yo! Could someone please explain these two multiple choices for me? For the first one, if back emf was equal to the applied voltage wouldn't that mean the total voltage is zero so the coil wouldn't even spin? (No voltage=no current) what ahah and for the second one, the heat loss within metal is due to the current's interaction with the resistance right? So I get that with lower resistance you can get more current (and since its I^2 then that will increase power loss) but if you had high resistance, wouldn't the ultimately heat loss be greater? Like if there was no resistance but the current was really high, there'd still be no power loss so idk in this case does a higher current win out? :/ cheers

Neutron

Hey Neutron!

I will start with question 19. This question used to always bother me, because my reasoning when answering this question is the same as you (i.e. P=IR^2 and high resistance would result in more power loss and heat dissipation etc.). I reckon it's a badly written question and the question should really specify "the metallic base of the cooking pot".

The base of the pot must be a metal in order for eddy current to be induced and flow. So compared to all the other materials such as plastic which has high resistance, all the materials used to make the base of the pot would have low resistance since they are metals. Really dodgy question, I honestly agree with your reasoning but the answer is C as per BOSTES.

Question 16 was a bit difficult, and I almost ignored the minute detail as well until when Jamon Windeyer (Credit pointed out to me that it is an IDEAL ELECTRIC MOTOR. This means that there will be NO FRICTION and hence the motor will retain its angular momentum and no extra torque is needed for the motor to continue rotating. In real life situations when friction is present, option A would be correct because we need the supply current to create torque to overcome the effect of friction (T = nBIAcostheta). But since here we have an ideal electric motor with no friction, nothing needs to be overcome and no work is done since no force (F=BIL) is applied to help it rotate. Hence back emf = applied voltage (i.e. no net emf, no current) and hence the answer is C. Tricky!

Best Regards
Happy Physics Land

[and1_98]

I initially thought C... Then turned to A... Answer says A. Anyone care to explain?

[student123456]

Hi. Would anyone be able to help me answer this question please?

A photon is incident on a hydrogen atom in the ground state.
Explain, using de Broglie’s hypothesis, why the photon is not absorbed
by the hydrogen atom. (3 marks)

[imtrying]

Hey😄
Under the dot point about Planck and Einsteins differing views about whether science research is removed from political and social forces, I definitely see the political forces,like the use of scientific research for use by military and governments, but what were the social forces? Would it be things like WW2 German anti-Semitism impacting on Jewish scientists?

Also, a bit confused on how Hertzs experiment relates to the photoelectric effect.

[Neutron]

I initially thought C... Then turned to A... Answer says A. Anyone care to explain?

Hah yew this is something I can answer so in order for diffraction to occur, the wavelength must be similar to the slit size right? Cause if the slight was too big the wave would just pass right through (so in this case, if the wavelength was smaller than the slit, it would just pass right through with no diffraction). So in order for the Braggs to observe a diffraction pattern, the wavelength of the X Ray's would have to be on the same order of magnitude (fancy way of saying same size) as the atomic spacings within the crystal! Also, loving the creativity of option D heh, hope this helps!

Neutron

[jakesilove]

I initially thought C... Then turned to A... Answer says A. Anyone care to explain?

Brilliant answer Neutron, nothing to add except that this is just something you need to know, but not necessarily understand.

[jakesilove]

Hi. Would anyone be able to help me answer this question please?

A photon is incident on a hydrogen atom in the ground state.
Explain, using de Broglie’s hypothesis, why the photon is not absorbed
by the hydrogen atom. (3 marks)

Hey! Check out posts HERE and HERE for an answer

[jakesilove]

Hey😄
Under the dot point about Planck and Einsteins differing views about whether science research is removed from political and social forces, I definitely see the political forces,like the use of scientific research for use by military and governments, but what were the social forces? Would it be things like WW2 German anti-Semitism impacting on Jewish scientists?

Also, a bit confused on how Hertzs experiment relates to the photoelectric effect.

In terms of social impacts, you're absolutely right! There really isn't much else to talk about.

Hertz's experiments relates to the photoelectric effect as it was the first observation of it. He observed that EM radiation could cause 'sparking' in wires, which we now realise is due to the photoelectric effect. He didn't do any research into this, or discover the cause (he left that the Planck/Einstein), so the Hertz experiment is significant purely as a starting point

[imtrying]

In terms of social impacts, you're absolutely right! There really isn't much else to talk about.

Hertz's experiments relates to the photoelectric effect as it was the first observation of it. He observed that EM radiation could cause 'sparking' in wires, which we now realise is due to the photoelectric effect. He didn't do any research into this, or discover the cause (he left that the Planck/Einstein), so the Hertz experiment is significant purely as a starting point

Thanks

[Neutron]

Hey Neutron!

I will start with question 19. This question used to always bother me, because my reasoning when answering this question is the same as you (i.e. P=IR^2 and high resistance would result in more power loss and heat dissipation etc.). I reckon it's a badly written question and the question should really specify "the metallic base of the cooking pot".

The base of the pot must be a metal in order for eddy current to be induced and flow. So compared to all the other materials such as plastic which has high resistance, all the materials used to make the base of the pot would have low resistance since they are metals. Really dodgy question, I honestly agree with your reasoning but the answer is C as per BOSTES.

Question 16 was a bit difficult, and I almost ignored the minute detail as well until when Jamon Windeyer (Credit pointed out to me that it is an IDEAL ELECTRIC MOTOR. This means that there will be NO FRICTION and hence the motor will retain its angular momentum and no extra torque is needed for the motor to continue rotating. In real life situations when friction is present, option A would be correct because we need the supply current to create torque to overcome the effect of friction (T = nBIAcostheta). But since here we have an ideal electric motor with no friction, nothing needs to be overcome and no work is done since no force (F=BIL) is applied to help it rotate. Hence back emf = applied voltage (i.e. no net emf, no current) and hence the answer is C. Tricky!

Best Regards
Happy Physics Land

Aye happy physics land it's been a while hahaha
Yeah yeah that pot thing makes sense now heh
But with the ideal motor one (does ideal just mean we ignore everything in the real world lmao) I get that if the net voltage is zero and the current is therefore zero that coil will keep spinning anyway, but while the back emf is being induced, won't it produce a force that opposes the rotation of the coil (and thus slowing it down?) like I thought the point of back emf never equaling the supply emf being that because when they're equal, the relative motion between the coil and the magnet will be zero (force produced by back=force produced by supply) and therefore, no rate of change of flux=no emf induced.. Idk sorry i think I'm just being dumb haha

Neutron

[jamonwindeyer]

Aye happy physics land it's been a while hahaha
Yeah yeah that pot thing makes sense now heh
But with the ideal motor one (does ideal just mean we ignore everything in the real world lmao) I get that if the net voltage is zero and the current is therefore zero that coil will keep spinning anyway, but while the back emf is being induced, won't it produce a force that opposes the rotation of the coil (and thus slowing it down?) like I thought the point of back emf never equaling the supply emf being that because when they're equal, the relative motion between the coil and the magnet will be zero (force produced by back=force produced by supply) and therefore, no rate of change of flux=no emf induced.. Idk sorry i think I'm just being dumb haha

Neutron

No no you've actually got it and don't realise!

I thought the point of back emf never equaling the supply emf being that because when they're equal, the relative motion between the coil and the magnet will be zero (force produced by back=force produced by supply)

You are right! Torque produced by supply equals torque produced by back-emf, so the net torque is zero. What happens to an object where we have a zero net force? It just keeps doing whatever it was doing. Thus, the motor keeps spinning! It's the inertia of the coil that keeps it going

[Neutron]

No no you've actually got it and don't realise!

I thought the point of back emf never equaling the supply emf being that because when they're equal, the relative motion between the coil and the magnet will be zero (force produced by back=force produced by supply)

You are right! Torque produced by supply equals torque produced by back-emf, so the net torque is zero. What happens to an object where we have a zero net force? It just keeps doing whatever it was doing. Thus, the motor keeps spinning! It's the inertia of the coil that keeps it going

Ah idek what it is I don't get I think I just need time to wrap my head around this lmao I'll keep thinking about it and hopefully I eventually work out what I don't get. But just with another question, what actually is the neutrino? Like it's not found it the nucleus is it? So during beta decay, won't some of the "kinetic energy the electron is supposed to have but doesn't" be converted to create the mass of the neutrino and then the neutrino's kinetic energy? Cheers

Neutron

[imtrying]

Would I be correct in saying this?
Doping a semiconductor with a group 3 impurity increases its conductivity because there is one less electron in the valence band so electrons are not so tightly held in covalent bonds so they are more free to move to the conduction band, meaning there are more positive holes created in the valence band to carry positive charge in the opposite direction.

Doping with a group 5 impurity just means there are more electrons available to move in to the conduction band.

Haha I don't know if this is right, this part of the topic always confuses me.