This discussion is closed.
Unregistered
Badges:
#1
Report 16 years ago
#1
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!
0
Unregistered
Badges:
#2
Report 16 years ago
#2
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
0
plasmotron
Badges: 0
#3
Report 16 years ago
#3
does anyone care
0
plasmotron
Badges: 0
#4
Report 16 years ago
#4
whats the answer then? i got my physics this afternoon. anyone else on aqa
0
Unregistered
Badges:
#5
Report 16 years ago
#5
You're close, just not quite there yet....
0
Unregistered
Badges:
#6
Report 16 years ago
#6
I'll give you another hour or so to get it
0
Unregistered
Badges:
#7
Report 16 years ago
#7
is it 0.0002V?
0
Rob
Badges: 0
Rep:
?
#8
Report 16 years ago
#8
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
0
elpaw
Badges: 15
Rep:
?
#9
Report 16 years ago
#9
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.
0
Unregistered
Badges:
#10
Report 16 years ago
#10
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.
0
elpaw
Badges: 15
Rep:
?
#11
Report 16 years ago
#11
flux density isn't changing, so change of flux density=0, so induced v = 0
0
Unregistered
Badges:
#12
Report 16 years ago
#12
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.
0
Unregistered
Badges:
#13
Report 16 years ago
#13
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.
0
Eru Iluvatar
Badges: 17
Rep:
?
#14
Report 16 years ago
#14
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.
0
Unregistered
Badges:
#15
Report 16 years ago
#15
Sorry Iluvatar, you're dead wrong. Like you said, you don't do physics. I recomend that you keep it that way.
0
Unregistered
Badges:
#16
Report 16 years ago
#16
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?
0
wugz
Badges:
#17
Report 16 years ago
#17
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
0
Unregistered
Badges:
#18
Report 16 years ago
#18
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.
0
Rob
Badges: 0
Rep:
?
#19
Report 16 years ago
#19
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.
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
0
Inbred Hick
Badges:
#20
Report 16 years ago
#20
Th arnser is 2. Hyuk.
0
X
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

What factors affect your mental health the most right now? (select all that apply)

Lack of purpose or routine (163)
15.51%
Uncertainty around my education (172)
16.37%
Uncertainty around my future career prospects (102)
9.71%
Isolating with family (71)
6.76%
Lack of support system (eg. Teachers, counsellors) (39)
3.71%
Lack of exercise/ability to be outside (91)
8.66%
Loneliness (105)
9.99%
Financial worries (47)
4.47%
Concern about myself or my loved ones getting ill (96)
9.13%
Exposure to negative news/social media (74)
7.04%
Lack of real life entertainment (eg. cinema, gigs, restaurants) (91)
8.66%

Watched Threads

View All