HSC Physics Question Thread

[Happy Physics Land]

Hello! How do you do this question? (The Answer is A)

Hey FallonXay!

A difficult question definitely! (A lot of difficult ones today). At the first sight it might seem like a motors problem.But sneaky sneaky it's actually a problem about Faraday and Lenz's law! Let's see how that comes into the question!

Now all the coils are turning in the clockwise direction for all the options, and all the coils are initially placed at a horizontal plane, 90 degrees to the direction of B-field. At this time, it receives the maximum amount of flux - amount of magnetic field lines that penetrates through the area of the coil. (FLUX! NOT CHANGE IN FLUX!) Now as the coil rotates, the plane is no longer 90 degrees to the magnetic field lines and consequently it loses magnetic flux. As it continues to rotate it loses more and more flux and consequently this change in flux results in an emf being induced (Faraday's law) and this causes an eddy current to be induced inside the coil. The eddy current will be a direction such that it opposes the original change in flux (Lenz's law). So, as we lose magnetic flux downwards, the eddy current wants to flow in a direction so that it can produce more magnetic flux downwards to compensate for the loss. Using our Right Hand Coil Rule, our thumb point downwards, our fingers curling in a clockwise direction, hence the answer is A.

Good question.

Best Regards
Happy Physics Land

[znaser]

Bostes put the answer down as b and excel put it down as d. I'm pretty sure though that the answer is d, but i could be mistaken. Thanks

[zoe_rammie]

Bostes put the answer down as b and excel put it down as d. I'm pretty sure though that the answer is d, but i could be mistaken. Thanks

Hmm that's a bit strange...I would've said D as well, considering the fact that
Induced EMF = - rate of change of Magnetic flux over time

I wonder what other people say.

[Happy Physics Land]

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

Yo for the AC motor, does the one in the second picture exist? (The one that looks like an AC generator) Or is it just induction, universal and synchronous? Cause Google doesn't seem to think it's a motor but for a question we had to talk about how the AC motor uses the motor effect and the other three don't speaking of, what actually is the synchronous motor?

Hey Neutron!

Synchronous motor isnt in the HSC syllabus so you dont really need to know about it. Clearly by the way Im speaking here I dont really know what it is (hehe soz soz ). Jake and Jamon are a bit busy right now so they can come help later.

In terms of that question on whether or not AC slip ring motors exist, I can tell you that yes it does but not in that form. AC slip ring motors can only work with induction motors (something I learnt from engineering studies, not in the syllabus). And if in the exam they want you to provide a motor that works on AC, just talk about universal or induction motor.

I'm not too capable here, I will inform Jake/Jamon about your question.

Best Regards
Happy Physics Land

[zoe_rammie]

Hey Proficles!

We can first eliminate A which is definitely not correct because gravitational potential energy can only be increasing when you move from a lower orbit to a higher orbit. It's like you climbing from the 1st floor of a building to the 40th floor of the building, your gravitational potential energy has increased. Same principle.

C and D can be a bit more confusing and when you look at it both seem viable, but of course its multiple choice and you have to choose the best answer. Indeed it's quite hard to see which one is a better option especially under exam conditions since both options talk about work and energy.

Here is what I think. There are two reasons why I would pick C:

1. The change in gravitational potential energy between two orbits is defined by the formula E_p=-GMm/r (1/r_f - 1/r_i). Now this explicitly says the same thing as option C, that the change in gravitational potential energy is just the difference between the final orbit and the initial orbit.
2. Option C is more specific about the geostationary orbit part whereas D mentions "a very large distance away” but this doesnt really fit the question, which specifically mentions “geostationary orbit”. You can be 10,000,000,000km from the centre of Earth (hypothetically speaking) and still be in Earth's gravitational field but probably only experiencing a gravitational field strength of 0.001N/kg. So it doesnt quite suit our scenario here.

A tough question, but option C definitely seem more reasonable.

Best Regards
Happy Physics Land

Imma also just add, (And I agree with you that the answer is most likely C)
D says "a very large distance away”, which, for all you know, could mean farther away than the high orbit itself.
So because it is pretty vague as to which direction it'd be moving, I'd choose C.

[Happy Physics Land]

Hey guys

Need help with this question, cause i don't have the answers . I believe the answer is C because the light from A and B will take the same time to travel to A' and B' but the flashes will be witnessed by O' differently as the observer moves into the flash from B' whereas he moves away from the flash at A'. Is this correct? Appreciate any help given

Cheers.

http://prnt.sc/d09xas ( Not letting me upload my 968KB file :/ )

Hey Mystery Marker!

This question is a bit ambiguous, here is why.

If the two charges are in a different inertial frame of reference as the observer, then your reasoning and answer will definitely be correct, no doubt about it.

However, if the two charges are in the same inertial frame of reference as the observer, i.e. they are somehow attached to the train and travel with the train like what the diagram has illustrated, then the answer would be A. Because if the two charges are moving at a speed v with the train, then for the observer O', he isnt approaching any one of the charges as they are all in the same frame of reference and so light would travel equal distance towards O' from point A' and B'.

But yes if its the first scenario where the train is approaching an external point of reference B' then you are definitely correct.

Best Regards
Happy Physics Land

[Happy Physics Land]

Hi are hysteresis losses from the constant reversal of magnetisation of the transformer core? TIA

Hey Cindy!

Hysteresis loss is simply the loss of magnetic flux during the transmission process from primary coil to secondary coil. It's not really because of the constant reversal of magnetic polarity, but because the inability of iron core to PERFECTLY transmit the magnetic flux. In other words, you can reverse your magnetic polarity however you like, as long as you have an ideal perfect transmitter of magnetic flux, hysteresis losses cannot occur. Iron core is the best thing we have now and even that isnt perfectly ideal, so hysteresis losses would always occur. And because the equation vp/vs = np/ns relies on the perfect transmission of magnetic flux, energy would be lost because the equation is no longer viable since some magnetic flux is lost.

Best Regards
Happy Physics Land

[Happy Physics Land]

Bostes put the answer down as b and excel put it down as d. I'm pretty sure though that the answer is d, but i could be mistaken. Thanks

Hey Znaser!

Definitely D. If you've done 2u mathematics you should know that faraday's law is essentially a differentiation of flux. So all you had to do there was just to draw the derivative graph of the original graph and the outcome would look like a cosine graph like the one in B. But because of the implication from lenz's law, there is a negative sign in front of faraday's law and hence the cosine graph should be flipped along the x-axis, producing the answer D. So yeah Im with you there.

Best Regards
Happy Physics Land

[RuiAce]

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

Yo for the AC motor, does the one in the second picture exist? (The one that looks like an AC generator) Or is it just induction, universal and synchronous? Cause Google doesn't seem to think it's a motor but for a question we had to talk about how the AC motor uses the motor effect and the other three don't speaking of, what actually is the synchronous motor?

Hey Neutron!

Synchronous motor isnt in the HSC syllabus so you dont really need to know about it. Clearly by the way Im speaking here I dont really know what it is (hehe soz soz ). Jake and Jamon are a bit busy right now so they can come help later.

In terms of that question on whether or not AC slip ring motors exist, I can tell you that yes it does but not in that form. AC slip ring motors can only work with induction motors (something I learnt from engineering studies, not in the syllabus). And if in the exam they want you to provide a motor that works on AC, just talk about universal or induction motor.

I'm not too capable here, I will inform Jake/Jamon about your question.

Best Regards
Happy Physics Land

Yeah can confirm synchronous motor isn't in the course.

But I am pretty certain, that in your ordinary DC motor if you just
- Swap the split ring commutator with a slip ring commutator
- Swap the DC power source with an AC power source
You still have an AC motor. Just that this is your ordinary motor, not the induction motor which relies on a rotating magnetic field.

The AC currents are monitored such that the frequency of direction reversal will always aid in the running of the motor

In practice, this is probably rarer than the AC induction motor as it's quite obvious that the induction motor has its advantages. But it's definitely a real thing and does exist.

[Happy Physics Land]

hullo hullo hullo again.

Got another question aheh.

Could someone please explain the math of this? (2014 q 20)

The answer is D.

tyvm

Hey Zoe!

No worries! This question is just a bit difficult to type out by hand, I've written and drawn my solutions, hope you understand!
Don't hesitate to ask if anything's obscure!



Best Regards
Happy Physics Land

[zoe_rammie]

Hey Zoe!

No worries! This question is just a bit difficult to type out by hand, I've written and drawn my solutions, hope you understand!
Don't hesitate to ask if anything's obscure!

(Image removed from quote.)

Best Regards
Happy Physics Land

oOooOHHH gotcha gotcha. Thank you muchly!

[Happy Physics Land]

oOooOHHH gotcha gotcha. Thank you muchly!

No worries, as long as you got something out of it!

[maxpedersen]

Hey Max!

Definitely a tricky question to think about, and it is definitely NOT TRUE that eddy currents cant flow in DC motors. The reason why you are perhaps a bit confused is because you didnt recognise that eddy current is not produced by the supply emf, but the induced emf. You are quite right in that DC doesnt really produce a change in magnetic flux, but it has nothing to do with the production of eddy currents, because eddy currents is produced as a result of the rotor spinning in a magnetic field.

So when the rotor rotates in an external magnetic field, it cuts magnetic field lines and hence experiences a change in magnetic flux. According to Faraday's law, this causes an emf to be induced and according to lenz's law this emf will be in a direction such that it opposes the source of change in flux. This is what we otherwise know as back emf or induced emf. This emf creates a current, and this is what we call an eddy current. This eddy current will flow in the rotor to oppose the source of the change in flux which is the supply emf.

Evidently eddy current is always produced in a motor, regardless of the type of supply voltage that the motor is connected to.

Best Regards
Happy Physics Land

Oh right, my bad haha. Thanks for such a comprehensive answer, appreciate it!

[Cindy2k16]

When drawing the bands of conductors, should we draw the conduction band sitting right on top of the valence band or the two bands overlapping? Or is either fine? Also do we say there is no band gap?

Thanks

[levendibigd]

Hey! Could I please have a hand with Q20c   answer a) 20ms^-1 and b)14ms^-1