HSC Physics Question Thread

[MysteryMarker]

Hey guys

Can someone please explain to me how the answer to this question is A? I thought that by using np/ns = Is/Ip,  Is = (np x Ip)/ns, thus by decreasing the number of turns in the secondary coil, the current in the secondary coil would increase and so too would the deflection underwent by the pointer in the galvanometer.

[jamonwindeyer]

Hey guys

Can someone please explain to me how the answer to this question is A? I thought that by using np/ns = Is/Ip,  Is = (np x Ip)/ns, thus by decreasing the number of turns in the secondary coil, the current in the secondary coil would increase and so too would the deflection underwent by the pointer in the galvanometer.

I saw this in the 2015 Paper; it's a trick, and I reckon a HEAP of people would have put B as an answer.

Your reasoning is correct, but with a minor oversight. Let me first explain why A is correct.

We have a voltage source attached to the primary coil. The voltage, current and resistance of this coil are related by Ohms Law:



But the resistance in the primary coil is dependent on the number of turns in the coil. So, we could say \(V=kIn\), where \(k\) is just some constant (perhaps, the resistivity of the wire). So if we decrease the amount of turns in the primary coil, without decreasing the voltage, that MUST mean that the current in the primary coil increases as a result.

By the law of conservation of energy, the power into a transformer is equal to the power out of a transformer (ignoring losses). We've maintained the same voltage at the input, but increased current, so the input power has increased. At the other end, our current increases to compensate for this. For totality, the effects at the output (in the secondary coil) are:

- The voltage increases due to the changed turn relationship between the coils, \(V_s=\frac{V_pN_s}{N_p}\)
- The current increases due to the increased amount of power going INTO the transformer

So the question remains, why is B incorrect? Well this is where we get tricky, because well, I'm not sure. It is, in general correct, at least to my understanding. My only explanation for why A is chosen would be that the deflection is likely greater. I also notice that the voltage source is DC (not sure if that is intentional), which could mean that the deflection is only brief, not constant, while the switch is flicked. However, that shouldn't change the reasoning in my opinion, all should still work as you'd expect.

I'm yet to find a good reason why A is chosen over B. I know why A is correct, but not why B isn't, ahaha

[RuiAce]

It was the question I got scabbed a mark away from (along with probably a lot of the state). I'm still convinced it's B.

Which would mean somehow I'd have to contradict off Jamon's argument.



Generally with these questions regarding transformers, I've found that the only thing we can assume constancy is power. For some reason resistance cannot be taken as constant. So since Jamon substituted resistance for n*resistivity, since n is obviously constant it must be that the resistivity changes somehow.

In other words, k is a variable, not a constant.

[pels]

Hey guys.

Got some questions for physics.

For this specific dotpoint,

process information to identify some of the metals, metal alloys and compounds that have been
identified as exhibiting the property of superconductivity and their critical temperatures

What exactly are we required to know here? In my notes I have Aluminium and Tin and their corresponding Critical Temperatures, but is that all we need to know for that point?

Additionally, what are the differences between AC Synchronous Motors and AC Induction motors, in terms of the principles used and components/structure?

Also, what is the difference between magnetic flux and magnetic flux density. Is it simply that flux density is B X A?

Also one last question. What year 11 material in important to know for physics, even though most of it is directly linked to year 12 content.

Thanks.

[RuiAce]

Also, what is the difference between magnetic flux and magnetic flux density. Is it simply that flux density is B X A?

Also one last question. What year 11 material in important to know for physics, even though most of it is directly linked to year 12 content.

Thanks.

________________________

Nothing much. Although I found that remembering how to do prelim calculations helped me here and there (e.g. just simple things such as q = I * t, V = W/q)


Also with those superconductors I think just memorised their names and critical temperatures like you suggested. Pick one of each though, so for the allow you could do yttrium barium cuprate. Letting someone else take over from here though

[pels]

Thanks heaps.

Still need that explanation of motors and generators.

Other random question I thought of:
Is the only difference in motors and generators the fact that they convert energy differently (e.g. electrical to mechanical and mechanical to electrical), as well as the split ring commutator vs the slip rings?

Cheers

[RuiAce]

I don't remember what a 'synchronous' motor is.

The split and slip rings - not quite. But yeah the energy conversion is just reversed between a motor and a generator.

The split and slip rings distinguish between DC and AC

[jamonwindeyer]

Thanks heaps.

Still need that explanation of motors and generators.

Other random question I thought of:
Is the only difference in motors and generators the fact that they convert energy differently (e.g. electrical to mechanical and mechanical to electrical), as well as the split ring commutator vs the slip rings?

Cheers

I got you! So an AC Synchronous Motor is just your typical motor structure; slip rings, rotor, stator, etc. Exactly as you learn for DC Motors

An AC Induction Motor is a different animal entirely. The rotation is a consequence of Lenz's Law, induced by a rotating magnetic field. There is a section on it in the second post in this thread! Let me know if I can explain it in a little more depth for you though

[pels]

Yeah i might need an explanation on it still.

Cheers

[jakesilove]

Yeah i might need an explanation on it still.

Cheers

In my HSC year, I used this video to understand Induction motors. Don't worry if you don't perfectly get what's going on; you just need to be able to describe it!

[pels]

Thanks for all the help guys.

 

[dylan862]

Best way to study for astrophysics??

[jamonwindeyer]

Best way to study for astrophysics??

Speaking in general for Physics rather than for astrophysics particularly - Past papers! Practice makes perfect for a subject like Physics, do as many practice questions as you can you might want to read this article I wrote that taks about this a tad

You might also want to try things like dot point brainstorms; brainstorming everything you remember about a specific dot point. Palm cards for terminology work too. Lots of cool stuff to try! But past papers/practice questions should take precedence

[Neutron]

Yo!

Can someone please explain to me how a transistor operates as a switch or amplifier? I've never properly understood it :/ and also what are transistor radios and how do they work?

Cheers!

[jamonwindeyer]

Yo!

Can someone please explain to me how a transistor operates as a switch or amplifier? I've never properly understood it :/ and also what are transistor radios and how do they work?

Cheers!

Hey Neutron! To properly understand the workings of a transistor is beyond the HSC course. I'm studying transistors now; definitely nothing nice you need to understand PN junctions, but not how they are used in a transistor, so you are all set! I can help you understand PN junctions if that is something you need help with

Transistor Radio = Radio that uses transistors in its circuitry (again, no more detail required)

It seems strange that the operation of a transistor isn't covered, but tbh, it's a little too complex to properly understand at Year 12 level