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Physics challenge

He he, here's a quick challenge for any of you physics bods out there.

A metal disc of radius 10cm is rotating at 3*10^3 revs/min perpendicular to a magnetic field of strength 0.01 T. What is the induced voltage between the centre of the disc and it's edge?

First correct answer wins a big special hug!

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Reply 1

Unregistered

unsure where to go from here, without a formula book?? get phi to be 0.000314, but unsure how to get an induced emf. Maybe faraday's or Lenz's Law, but where to find number of turns in a coil??!! obviously the frequency (50Hz) is involved somewhere

does anyone care

whats the answer then? i got my physics this afternoon. anyone else on aqa

Reply 4

Unregistered

You're close, just not quite there yet....

Reply 5

Unregistered

I'll give you another hour or so to get it

is it 0.0002V?

I stumbled across the answer 0.0006283, not sure if it's correct at all, (it is awful early)

It might even be zero?

mmmh

Rob

Reply 8

elpaw

its 0v isn't it, because the magnetic field isnt changing relative to the edge or the centre (it's always perpendicular) so d(phi)/dt =0.

Reply 9

Unregistered

The answer is 0.016 V

The equation is V=n(change of flux density / change of time )

Where n is the number of turns in a coil.
The rotations of the disc can be treated as turns in the coil.
-Divide 3000 turns by 60 to get 50 turns/sec
-flux density = BA (B is field strength, A=area)
-As you have worked out 50 turns in one scond, time can be treated as 1.
-so the calculation is V=nBA
-plug in the numbers and go!

So, unfortunately noone gets a big hug, and I'm stuck here hugging myself.

Reply 10

elpaw

flux density isn't changing, so change of flux density=0, so induced v = 0

Reply 11

Unregistered

Flux density is changing, as charged paricle (i.e. the electrons in the metal) are moving through an area of magnetic field. In essence, they are moving through a wire (being rotated in the disc), so if you like you could think of it a s a 10cm lebgth of wire being spun around a point. I think you'll find this does give an induced e.m.f.

Reply 12

Unregistered

To clarify my last post: when I say flux density is changing, that is not exactly true. The electron s experience a force due to moving through the field, which has the same effect as the flux density changing.

Reply 13

Eru Iluvatar

None, because the metal isn't magnetic?
Or it stops spinning?
Or is it in a different atmosphere, where it floats out of the magnetic field?

As you can tell, i dont do physics, but there are so many variables you have not disclosed, that any answer could be justified to be right.

Reply 14

Unregistered

Sorry Iluvatar, you're dead wrong. Like you said, you don't do physics. I recomend that you keep it that way.

Reply 15

Unregistered

I don't know if my last post explained it for you elpaw, but rest assured that it really does work. In the Edexcel PHY6 exam the first question was about a spinning metal disc in a magnetic field, are you on Edexcel?

Reply 16

Unregistered

Originally posted by Unregistered
The answer is 0.016 V

The equation is V=n(change of flux density / change of time )

Where n is the number of turns in a coil.
The rotations of the disc can be treated as turns in the coil.
-Divide 3000 turns by 60 to get 50 turns/sec
-flux density = BA (B is field strength, A=area)
-As you have worked out 50 turns in one scond, time can be treated as 1.
-so the calculation is V=nBA
-plug in the numbers and go!

So, unfortunately noone gets a big hug, and I'm stuck here hugging myself.

its 0v coz a CHANGE in FLUX causes an induced voltage. if the disk is spinning, then there is no CHANGE in flux as it doesnt actually move out of its original position. thus as the same a number of field lines run through it, there is no Voltage generated

correct me if im rong

Reply 17

Unregistered

Sorry, you are wrong, although in a way you are partly right. There is indeed no change in flux denity or flux linkage, but the magnetic field acts on the moving charges (electrons) in the metal, with the end result that more electrons accumulate near the inner or outer edge of the disc (depending on direction of the field). However, the equation for rate of change of magnetic flux linkage can be used, as the disc behaves as a coil, or a length of wire being swung in a magnetic field, whatever way is easier to visualise.
If you did the Edexcel Unit 6 synoptic today you would have noticed that the first question was about a spinning magnetic disc in a magnetic field, which caused a potential difference between the inner and outer edges.
Hope this clears it up. :smile:

Reply 18

Rob

Originally posted by Unregistered

If you did the Edexcel Unit 6 synoptic today you would have noticed that the first question was about a spinning magnetic disc in a magnetic field, which caused a potential difference between the inner and outer edges.
Hope this clears it up. :smile:

It was a metal disc, least i hope it was a metal disc, yes it was a metal disc, cos they said it was could be a Steel disc, It was a metal disc yeah ?

Rob

Reply 19

Unregistered

Th arnser is 2. Hyuk.