HSC Physics Question Thread

[kiwiberry]

Hi!

I'm having trouble pinpointing exactly why the answer to this is Q, is it because of the right hand grip rule? Could you explain in detail how the force acts towards Q?

Thank you very much!

Hey! In the coil, current flows from top to bottom at the front. Using the right hand grip rule, this induces a magnetic field towards the right - so there's a north pole at the right end of the coil. The copper ring experiences an increase of flux towards the right, and by Lenz's Law, a current will be induced such that it gives rise to a magnetic field which opposes this change of flux. This means that the ring will create a magnetic field towards the left, and there will be another north pole left of the ring. The two north poles repel each other, so the ring will be pushed towards Q. Does that make sense?

[justwannawish]

Hey!

In reality, year 12 content doesn't really build on year 11 knowledge. So, when you reach year 12, you could effectively forget the content you learned in Year 11, and still be absolutely ok.

The more important thing that you should get out of year 11 is the skills, and your study techniques. Practicals, writing the answers to extended response questions, using physics terminology; those are the sorts of skills that are invaluable when you get to Year 12. Similarly, if you get into good study habits now, and learn the ways in which you learn the best, you'll get a massive jump on Year 12.

However, overall, I wouldn't worry too much if you speed through some Year 11 content. You won't at all be at a disadvantage

Ah that's good to know and eased my worries. Thank you!

[limtou]

Please explain the multiple choice below :')
Answers: 3. A and 13. C
(for 3, I managed to get the answer with the use of Earth's radius 6380km but it wasn't given in this question? Is there perhaps a way to get the answer without using Earth's radius?)
Also apologies for the split q. 13

[winstondarmawan]

Hey, just a question about power loss. So the formula is P(loss)=(I^2)R, and thus power loss is directly proportional to the square of the current, and directly proportional to resistance. Thus, it is preferred to have high voltages as when voltage is stepped up current is stepped down. However, what happens if we substitute V^2/R^2 into I^2? P(loss) becomes V^2/R, and now power loss is directly proportional to the square of the voltage and inversely proportional to the resistance. Then why is voltage stepped up>
Can someone explain the flaw in my thought because I confused myself when I thought about this..

[kiwiberry]

Hey, just a question about power loss. So the formula is P(loss)=(I^2)R, and thus power loss is directly proportional to the square of the current, and directly proportional to resistance. Thus, it is preferred to have high voltages as when voltage is stepped up current is stepped down. However, what happens if we substitute V^2/R^2 into I^2? P(loss) becomes V^2/R, and now power loss is directly proportional to the square of the voltage and inversely proportional to the resistance. Then why is voltage stepped up>
Can someone explain the flaw in my thought because I confused myself when I thought about this..

Hey! This is a pretty confusing idea - let me link you to this great explanation Jamon wrote a while back:

Step into my office

Legit, the moment in my first year ELEC lecture when I finally figured out why this is, best moment ever. Not difficult! Just a little intricate.

Right, so \(P=VI\) gives the power dissipated/lost in a circuit element (EG - the wires of a transmission network), as a product of the current through the element and the voltage across the element. The formula is identical to \(P=I^2R\) and \(P=\frac{V^2}{R}\). So why does only one work?

Notice what I emphasised in the text above - Across the element. Meaning, the voltage at one end minus the voltage at the other, the potential difference. This goes right back to Electrical Energy in the Home, circuit analysis in Year 11 involved analysing voltage drops across resistors.

The problem is that we are almost always given the voltage going into the transmission wires, never the voltage ACROSS those wires. We might put 100,000V into the wires, but that's not the voltage across them. The voltage across them is the difference between 100,000V and the voltage at the other end, which is calculable as \(V=IR\), Ohm's Law!

If we use the voltage across the wires, those other formulas work - \(P=VI\) and \(P=\frac{V^2}{R}\). If we just use the input voltage, they break. THIS is why we usually use \(P=I^2R\), because we don't need to look at the other end of the wires. We know how much current goes in, we know how much resistance there is - And that is all we need

This is really hard until you have a click moment, then it is really easy. If it is still a little confusing let me know, in which case I'll do a numerical example using Year 11 techniques to show you the difference

[austv99]

Hey, can I get help with this multiple choice question?
I dont understand the difference between A and C.

[Aaron12038488]

So I received an assignment for physics for which I RECEIVED A WHOLE TOPIC which was 'The Cosmic Engine'. I need assurance for a question. Complete the table below by summarising the historical models of the Universe.
Its in a table format,
and the first column states Helio- or Geocentric model?
It says 6 models, I've got Aristotle's, Aristarchus, Copernicus, Brahe's, Kepler, Galleli, Newton. Thats 7, which i found on someone else's notes. But when i searched it up, i got different answers.
 

[seventeenboi]

HIHI i need help with this question ... why does the force/time graph of dropping a metallic sheet through a constant magnetic field look like this ??
i've attached both question and answer below )
thanks heaps

[jamonwindeyer]

So I received an assignment for physics for which I RECEIVED A WHOLE TOPIC which was 'The Cosmic Engine'. I need assurance for a question. Complete the table below by summarising the historical models of the Universe.
Its in a table format,
and the first column states Helio- or Geocentric model?
It says 6 models, I've got Aristotle's, Aristarchus, Copernicus, Brahe's, Kepler, Galleli, Newton. Thats 7, which i found on someone else's notes. But when i searched it up, i got different answers.
 

Hey! Sorry, just a little unsure exactly what your question is? Are you confused about whether each model is heliocentric or geocentric?

[bsdfjnlkasn]

Hey there,

I know these are easy but I'm not getting any of the answers (B, A, C respectively). Would love some help - thank you!

[kiwiberry]

Hey there,

I know these are easy but I'm not getting any of the answers (B, A, C respectively). Would love some help - thank you!

Hey!

1) Remember that M is the mass of the central body ie. the earth, not the telescope

2) 3) Put those into a calculator and it should work out

[bsdfjnlkasn]

Hey!

1) Remember that M is the mass of the central body ie. the earth, not the telescope

2) 3) Put those into a calculator and it should work out

Awesome, thanks so much! I realised I was adding the Earth's radius to every distance - why is this wrong?

Any ideas on the following?

[winstondarmawan]

Awesome, thanks so much! I realised I was adding the Earth's radius to every distance - why is this wrong?

Any ideas on the following?

You do not add the radius of Earth because the question already gives you the ORBITAL RADIUS, and not the distance from the surface of the Earth.

Also for the question, I'm pretty sure its B. Remember that EMF is directly proportional to the rate of change of flux.
Think about the situation where the magnet is closest to the coil. If you do 3U, you can think about it in terms of Simple Harmonic Motion and that is the  position closest to the magnet is the "extreme point". At the extreme point, there is no velocity, and in this case, no change in magnetic flux, and thus no EMF.
So, if it is closest to the coil, no EMF should be present. Someone please correct me if i am wrong

[jamonwindeyer]

Hey, can I get help with this multiple choice question?
I dont understand the difference between A and C.

Hey austv, welcome to the forums!!

I agree with you tbh! Without knowing which way the battery was connected you have no way to distinguish between A and C!

[jamonwindeyer]

HIHI i need help with this question ... why does the force/time graph of dropping a metallic sheet through a constant magnetic field look like this ??
i've attached both question and answer below )
thanks heaps

Hey! Cool question, so:

- The sheet is falling under gravity, so there is a constant gravitational force on the sheet
- As the sheet enters the field, eddy currents form which act to oppose the motion. So, an upwards force is experienced, which subtracts from the gravitational force (since it is acting in the opposite direction). So, the overall force is reduced!

This is what causes the graph you see - The straight line due to gravity and the dips due to the opposing force caused by induced eddy currents!