HSC Physics Question Thread

[winstondarmawan]

That's a really dodgy question. Back EMF in motors is just the name given to the induced emf due to the rotation of the coil in the field. In a generator, that induced emf is the output voltage we want in the first place. The use of the terminology isn't appropriate here in my opinion

Assuming it just means induced emf, the answer is B. Induced emf is proportional to rate of change of flux, and the second graph matches that - It is a maximum when flux is changing the most

Shouldn't it be a negative cos graph because it is the negative of the derivative? Or am I missing something here...

EDIT: Also this question from Quanta to Quarks:
Explain why the spectroscope was important in the development of the Bohr model of the atom. (4 marks)

[Mymy409]

Hey,
I don't understand how in AC induction motors, the rotor follows the stator.
Also, do AC motors have brushes and commutators or not, coz I'm seeing different things in different books/sites.

[pikachu975]

Shouldn't it be a negative cos graph because it is the negative of the derivative? Or am I missing something here...

EDIT: Also this question from Quanta to Quarks:
Explain why the spectroscope was important in the development of the Bohr model of the atom. (4 marks)

A spectroscope is a device that allowed the observation of the hydrogen emission spectrum. When the hydrogen atoms were excited through heating or passing a current through low density gas, it released visible light of specific wavelengths that were unexplained by the Rutherford model.

This led Bohr to develop his postulates:
1) Electrons exist in allowable energy states which explained the spectral lines rather than the continuous spectra.
2) The transition of electrons is accompanied by the emission or absorption of EMR, which explained why there were visible light being emitted when the hydrogen gas was excited.
3) Angular momentum of electron orbits are quantised in multiples of h/2pi.

Therefore the spectroscope was quintessential in the development of the Bohr model as the model was formed based on the inability of the Rutherford model in explaining the hydrogen emission spectrum; which was observed by the spectroscope.

[pikachu975]

Hey,
I don't understand how in AC induction motors, the rotor follows the stator.
Also, do AC motors have brushes and commutators or not, coz I'm seeing different things in different books/sites.

In AC induction motors, the rotor wants to balance the change in flux. The only way to do this is to try and catch up to it so they rotate at the same speed. This would mean they have no relative motion and hence no change in flux. This is because the stator causes the apparent rotating magnetic field so the change in flux must be minimised by the rotor rotating. However it cannot reach the exact speed so the difference in speed is the slip speed.

AC motors DO have brushes. They have SLIP ring commutators, 2 of them, which each connect to one terminal. The brushes maintain electrical contact between the external circuit and the coil. The slip ring commutators reverse the polarity of the output current every half turn.

[kiwiberry]

AC motors DO have brushes. They have SLIP ring commutators, 2 of them, which each connect to one terminal. The brushes maintain electrical contact between the external circuit and the coil. The slip ring commutators reverse the polarity of the output current every half turn.

The slip rings don't reverse the polarity of the input current because AC already constantly changes direction, however the split-ring commutator in DC motors reverses the direction of current every half-turn to ensure unidirectional torque

[winstondarmawan]

A spectroscope is a device that allowed the observation of the hydrogen emission spectrum. When the hydrogen atoms were excited through heating or passing a current through low density gas, it released visible light of specific wavelengths that were unexplained by the Rutherford model.

This led Bohr to develop his postulates:
1) Electrons exist in allowable energy states which explained the spectral lines rather than the continuous spectra.
2) The transition of electrons is accompanied by the emission or absorption of EMR, which explained why there were visible light being emitted when the hydrogen gas was excited.
3) Angular momentum of electron orbits are quantised in multiples of h/2pi.

Therefore the spectroscope was quintessential in the development of the Bohr model as the model was formed based on the inability of the Rutherford model in explaining the hydrogen emission spectrum; which was observed by the spectroscope.

Thank you! How about this question here:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20668088_1293149414143912_1322689573_n.jpg?oh=b4f5e827f3f630af98148796735dad4a&oe=598AE96A Part b.
TIA

[kiwiberry]

Thank you! How about this question here:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20668088_1293149414143912_1322689573_n.jpg?oh=b4f5e827f3f630af98148796735dad4a&oe=598AE96A Part b.
TIA

You would draw something like this:

But replace the values on the left with the values given in the table and only draw up to n=4. The energy levels for n>4 will be above n=4

[mercurry]

Hey,
Could someone explain these to me?

7. C
8. B
9. D
Cheers

[pikachu975]

The slip rings don't reverse the polarity of the input current because AC already constantly changes direction, however the split-ring commutator in DC motors reverses the direction of current every half-turn to ensure unidirectional torque

My bad I think I'm confused with AC generators. For AC generators the slip rings change the polarity of the output current or something?

[jamonwindeyer]

Hey,
Could someone explain these to me?

7. C
8. B
9. D
Cheers

Sure!

7C - We consider the conservation of energy. So, the electrical energy we put in must come out as either kinetic energy, or losses in the form of heat. So the sum of those things is a constant for that reason the others are incorrect - A violates the conservation of energy, B is wrong because supply voltage is constant, and D is wrong because current will decrease, not increase

8B - A single change in magnetic flux will cause a single direction of current flow. We are introducing extra field into the page, we need field out of the page to counter it. The current needs to flow anti-clockwise for this by the right hand grip rule

9D - If you read the other three answers, they are advantages! So it has to be D by default

[jamonwindeyer]

My bad I think I'm confused with AC generators. For AC generators the slip rings change the polarity of the output current or something?

Even simpler - Slip rings literally just maintain contact. They change absolutely nothing about the output

[johnk21]

Can someone please help me with this medical physics question:

Compare and contrast the methods of X-rays and endoscopy. (4 marks)

THanks in advance

[winstondarmawan]

Hey! Can someone please look over my response for this question, and make sure I have all the relevant stuff:
Assess the effectiveness of the Bohr-Rutherford model of the atom in accounting for experimental observations. (6)
Rutherford's original model of the atom (Mainly empty space with a minute, dense, positive nucleus and small negatively charged electrons) was sufficient in accounting for the results of the gold foil experiment, as the minute nucleus was able to explain the complete deflection of 1/20000 alpha particles and the empty space was able to explain the uninterrupted path of the majority of alpha particles. However, it had several flaws which were not explainable, such as the fact that an accelerating charged particle would give off EM radiation and thus lose energy, spiralling into the nucleus, yet this was not the case. It also offered no explanation for the line spectrum of hydrogen. Thus, Bohr introduced his three postulates to supplement Rutherford's model and could explain one of the above, the line spectrum of hydrogen. This model was known as the Bohr-Rutherford model. However, the model still had some flaws:
Could not explain why electrons had a stable orbit
Could not explain various intensities and broadness of spectral lines
Could not explain hyperfine structures
These were all experimental observations that the Bohr-Rutherford model could not explain.
Judgement: As such it is effective in accounting for some experimental observations, such as gold foil experiments, however failed to account for some other experimental observations as stated above.

[blasonduo]

Can someone please help me with this medical physics question:

Compare and contrast the methods of X-rays and endoscopy. (4 marks)

THanks in advance

Hello! This is a pretty chill question!

Just find 2 contrasts and 2 comparisons.

For example:
1) x-rays are non-invasive, endoscopy is.
2) They both use EMR to find internal problems in the body.
3) Endoscopy requires the patient to "flush out" their bodily wastes while x-rays do not
4) Both a very cheap method.

There are heaps! but 4 should give you the marks

[kiwiberry]

Hey! Can someone please look over my response for this question, and make sure I have all the relevant stuff:
Assess the effectiveness of the Bohr-Rutherford model of the atom in accounting for experimental observations. (6)
Rutherford's original model of the atom (Mainly empty space with a minute, dense, positive nucleus and small negatively charged electrons) was sufficient in accounting for the results of the gold foil experiment, as the minute nucleus was able to explain the complete deflection of 1/20000 alpha particles and the empty space was able to explain the uninterrupted path of the majority of alpha particles. However, it had several flaws which were not explainable, such as the fact that an accelerating charged particle would give off EM radiation and thus lose energy, spiralling into the nucleus, yet this was not the case. It also offered no explanation for the line spectrum of hydrogen. Thus, Bohr introduced his three postulates to supplement Rutherford's model and could explain one of the above, the line spectrum of hydrogen. This model was known as the Bohr-Rutherford model. However, the model still had some flaws:
Could not explain why electrons had a stable orbit
Could not explain various intensities and broadness of spectral lines
Could not explain hyperfine structures
These were all experimental observations that the Bohr-Rutherford model could not explain.
Judgement: As such it is effective in accounting for some experimental observations, such as gold foil experiments, however failed to account for some other experimental observations as stated above.

I would also state Bohr's three postulates (electrons exist in discrete stationary states, electrons only absorb/emit EMR when transitioning between these states, angular momentum is quantised) and explicitly say that this meant that only certain wavelengths of EMR could be emitted, thus explaining the hydrogen spectrum. Otherwise looks good!!