VCE Physics Question Thread!

[Zealous]

Thanks for the help. I'm still wrapping my head around it all so i have a few questions still. I understand everything up to the part in bold. I understand the direction of the changing flux, i just don't think i understand the opposing directions properly. Why is it a downwards force. i thought the magnetic field direction was the important part, it needs to oppose the changing flux?

I think of it as the system resisting the change and wanting to be back in its original state.

So initially, there are magnetic field lines going from N to S of the coil, or from the top to the bottom of the coil. When the coil is moved to the right, we are removing it from that initial magnetic field, and the coil wants that magnetic field back. In order to create a magnetic field going from the top to the bottom, there needs to be a clockwise current.

Conversely, for Q6, initially there is no magnetic field through the coil and we are forcing it into a position with a magnetic flux. The coil doesn't like that and wants to be back with no magnetic field, so will create a magnetic field from S to N (or bottom to top) to cancel out the increas in magnetic field. This will cause a counterclockwise current. Think of it as the system wanting to get back to where it was initially.

This guy does an amazing job of explaining it: https://www.youtube.com/watch?v=qWu82nJS42I

[odeaa]

This guy does an amazing job of explaining it: https://www.youtube.com/watch?v=qWu82nJS42I

holy shit i love that guy, ive watched legit every one of his videos

[Floatzel98]

Thanks guys. Those explanations really helped. That video really helped as well. That guy is awesome haha.

One very last thing. I think I'm still having trouble with using the right hand rules with coils/solenoids etc. Do you put your thumb in the direction of current or in the direction of the magnetic field, and then your fingers would wrap around the coil and be the current? I've been doing most questions the first way since that is what my teacher told me to do, but i seem to get mixed answers.

Also some other questions. Using the same question i posted above as a reference. How long does a induced EMF and current as well as the magnetic field produced last for? Would the current just stay in the loop until it got discharged somewhere?

I think of it as the system resisting the change and wanting to be back in its original state.

This actually helps a lot. I'll definitely think of it like this from now on

[odeaa]

Thanks guys. Those explanations really helped. That video really helped as well. That guy is awesome haha.

One very last thing. I think I'm still having trouble with using the right hand rules with coils/solenoids etc. Do you put your thumb in the direction of current or in the direction of the magnetic field, and then your fingers would wrap around the coil and be the current? I've been doing most questions the first way since that is what my teacher told me to do, but i seem to get mixed answers.

Also some other questions. Using the same question i posted above as a reference. How long does a induced EMF and current as well as the magnetic field produced last for? Would the current just stay in the loop until it got discharged somewhere?
This actually helps a lot. I'll definitely think of it like this from now on

for normal wires, thumb in direction of current (along the wire) and fingers in the direction of field
while you can apply this to a coil or solenoid, its easier just to go fingers=current and thumb=field

as for what happens to the current in the loop, I have no idea lol. In terms of VCE, I've done all the past papers back to 1997 and havent seen anything remotely related to that, and its not in the study design afaik

[Floatzel98]

Probably a simple question but I'm getting a bit confused on this:

A student builds a simple alternator consisting  of a coil containing 500 turns, each of area 10cm^2, mounted on an axis that can rotate between the poles of a permanent magnet of strength 80mT. At a frequency of 50Hz, it is found that the peak voltage produced is 12.6V

a) What are the peak to peak and RMS voltages

b)If the frequency is doubled to 100hz, how will the peak and RMS voltages change?

B is what i'm having trouble with. I'm kind of having a mind blank trying to put my maths and physics skills together here, but how does the increased frequency produce a great EMF? I understand it when talking about a greater change of flux since it is now moving faster, but i don't really understand it graphically. Well basically the whole electric power generation chapter in my textbook goes over it all in like 2 pages. I understand that they differentiate the flux ( and that it equals the induced EMF, but i don't understand the angular velocity part of the equation (). The book also does some stuff to be able to use frequency but I'm just not understanding it at all.

Thanks

[Floatzel98]

One more question. With the attached; I was able to do the question with a bit of intuition, but I actually don't understand how everything is set up, especially with the change in part b).I can't seem to visualize where the transformer was originally and where he then puts it. If someone was maybe to draw a diagram of how it was set up, that would be amazing.

Thanks

[Kel9901]

Probably a simple question but I'm getting a bit confused on this:

A student builds a simple alternator consisting  of a coil containing 500 turns, each of area 10cm^2, mounted on an axis that can rotate between the poles of a permanent magnet of strength 80mT. At a frequency of 50Hz, it is found that the peak voltage produced is 12.6V

a) What are the peak to peak and RMS voltages

b)If the frequency is doubled to 100hz, how will the peak and RMS voltages change?

B is what i'm having trouble with. I'm kind of having a mind blank trying to put my maths and physics skills together here, but how does the increased frequency produce a great EMF? I understand it when talking about a greater change of flux since it is now moving faster, but i don't really understand it graphically. Well basically the whole electric power generation chapter in my textbook goes over it all in like 2 pages. I understand that they differentiate the flux ( and that it equals the induced EMF, but i don't understand the angular velocity part of the equation (). The book also does some stuff to be able to use frequency but I'm just not understanding it at all.

Thanks

With doubled frequency comes halved period (T=1/f), so the denominator is halved and hence the EMF (both rms and peak) is doubled.

[Adequace]

 

With doubled frequency comes halved period (T=1/f), so the denominator is halved and hence the EMF (both rms and peak) is doubled.

This might be a dumb comment, but is that because f + T has to equal 1 or something like that?

Anyway, I have a Unit 2 SAC tomorrow. I've not really been doing much tonight apart from revising my notes and cheat sheet which makes me feel unproductive. I completed the checkpoints questions, textbook questions and questions I could find online. Is there anything else I could've done?

I want to develop a strong work ethic towards test and know what to do when I have nothing to do especially when units 3/4 come along, if that makes sense.

[odeaa]

This might be a dumb comment, but is that because f + T has to equal 1 or something like that?

Anyway, I have a Unit 2 SAC tomorrow. I've not really been doing much tonight apart from revising my notes and cheat sheet which makes me feel unproductive. I completed the checkpoints questions, textbook questions and questions I could find online. Is there anything else I could've done?

I want to develop a strong work ethic towards test and know what to do when I have nothing to do especially when units 3/4 come along, if that makes sense.

Imo, theres really not that much point doing any more for a 1/2 if you have done all that already. thats what I did for all my sacs in year 11 and it still set me up with a good work ethic in year 12. I wouldnt waste time on something like physics if you dont have any practise questions to do (really theres not you can do), and just do an english essay or something like that, in the long run its time better spent

if you really want to study but have literally no questions to do, write your own exam, making it as hard as possible but still covering as much of the course as you can, and then write detailed solutions (probs wont have time but if you really want something to do)

also, i dunno if you have it, but buy the neap questions book. Really good quality questions, better than checkpoints imo (didnt even know they had checkpoints for 1/2)

[Floatzel98]

Just checking, can we add N, number of turns, into the equation . My book *cough*heinemann*cough doesn't explicitly state this anywhere but some questions are coming up with it and the only way I can get the answer is if I add N into it.

[paper-back]

Just checking, can we add N, number of turns, into the equation . My book *cough*heinemann*cough doesn't explicitly state this anywhere but some questions are coming up with it and the only way I can get the answer is if I add N into it.

Yeah, you add N(number of turns) to the equation

[Floatzel98]

I just saw a questions where we had to find an estimate for the magnetic force on a 'vertical lightning conductor' that was perpendicular to the Earths magnetic field. They gave us the current and they estimated a length for it. But they estimated Earths magnetic field to be .  My textbook (this question was in checkpoints) usually uses and online it says it ranges from to . Would we actually ever get a question on the exam like this anyway that i should't worry about it?

Thanks

[Kel9901]

I just saw a questions where we had to find an estimate for the magnetic force on a 'vertical lightning conductor' that was perpendicular to the Earths magnetic field. They gave us the current and they estimated a length for it. But they estimated Earths magnetic field to be .  My textbook (this question was in checkpoints) usually uses and online it says it ranges from to . Would we actually ever get a question on the exam like this anyway that i should't worry about it?

Thanks

I can state with certainty that there won't be a question that requires you to use estimates that aren't given.

[timton]

QUESTION

Context: Generator for a small town.
Why would you make the turn ratio for a step up and step down generator the same?

Is it to lower power loss?

[silverpixeli]

QUESTION

Context: Generator for a small town.
Why would you make the turn ratio for a step up and step down generator the same?

Is it to lower power loss?

Not exactly sure what you're asking, I think you mean a step up/down transformer which is connected to a generator. Also, by 'the same' do you mean e.g. 1:100 for the step up and 100:1 for the step down?

If that's the case, then there's not really any advantage to this, it just means that when you step up the power coming out of the generator to transmit it, and then step it down later, the stepping up and the stepping down exactly cancel each other out and the ratio of voltage to current after transmission is the same as the ratio coming out of the generator, before transmission.