# Why is enduced emf zero between poles? Shouldnt it be constant?

Watch
Announcements
#1
I dont understand cii (in attached files)

E = Change in flux / ∆t.
When we move the coil between the poles (where there is uniform B), the area is changing hence there is change in flux. But it is moving at a constant speed, so shouldnt emf be constant between the poles? How is it 0? Please help!

this is a quite similar question. Except it doesnt ask for a graph, but if you do the working for enduced emf at different distances moved by the wire (and calculating time to move the particular distance by s=d/t as we are given the constant speed) you can easily find it out to be constant and not zero!
Last edited by username5091846; 9 months ago
0
9 months ago
#2
if something stays at zero that's a constant value too

the question is about the difference between hall probes and coils

hall probes give a reading proportional the strength of the magnetic field they're in

coils give a reading proportional to the rate of change in flux linked by the coil

in the question c ii the plane of the flat coil is oriented parallel to the faces of the pole pieces - so the axis of the coil is parallel to the field.
B field is constant in the area through which the coil is moving
so the quantity of flux linked by the coil as the coil is moved through the uniform field is constant
so the rate of change of flux linked by the coil is zero - the amount of flux being linked as the leading edge of the coil advances is equal to the amount of flux being unlinked at the trailing edge...
so the EMF reading from the coil is zero

the small coil is different from the setup shown in the top image, in Q5 they've drawn in the test leads to the ammeter in which gives a little extra hint that it's a large one turn coil where the stiff copper wire is one side of the coil that's being moved through the B field while the rest of the coil remains fixed relative to the field - in this situation the amount of flux being linked changes over time.
0
#3
(Original post by Joinedup)
in the question c ii the plane of the flat coil is oriented parallel to the faces of the pole pieces - so the axis of the coil is parallel to the field.
B field is constant in the area through which the coil is moving
so the quantity of flux linked by the coil as the coil is moved through the uniform field is constant
so the rate of change of flux linked by the coil is zero
check this out. A wire is being moved in a uniform field, it says change in area the wire/coil moves through also results in change in flux. B is uniform here too. Yet emf is enduced. Why doesnt this apply to cii? Surely there is change in area between the poles throughout the distance the coil travels in cii, even if B is same, and knowing it moves at constant speed it can be said the emf induced should have been non zero and constant. However, this is incorrect. Really confused.
Last edited by username5091846; 9 months ago
0
9 months ago
#4
(Original post by crudux_c)
check this out. A wire is being moved in a uniform field, it says change in area the wire/coil moves through also results in change in flux. B is uniform here too. Yet emf is enduced. Why doesnt this apply to cii? Surely there is change in area between the poles throughout the distance the coil travels in cii, even if B is same, and knowing it moves at constant speed it can be said the emf induced should have been non zero and constant. However, this is incorrect. Really confused.
in cii you have a small coil of fixed size and shape, all parts of the small coil are moving through a large and uniform magnetic field at the same speed - the amount of flux going through the coil doesn't change with time...

in the other case you have a flexible 1 turn coil where one side of the coil remains stationary and another side of the coil (the rigid copper wire) is moving through the uniform field - so the amount of flux going through the coil is changing with time.

you only get an emf when the amount of flux going through the coil changes
1
#5
(Original post by Joinedup)
in the other case you have a flexible 1 turn coil where one side of the coil remains stationary and another side of the coil (the rigid copper wire) is moving through the uniform field -
Isnt the whole of the 1 turn coil/wire going through the uniform field too just like the coil? Like, all parts at once like the coil? I thought it was.
0
9 months ago
#6
(Original post by crudux_c)
Isnt the whole of the 1 turn coil/wire going through the uniform field too just like the coil? Like, all parts at once like the coil? I thought it was.
Doesn't look like it in the diagram IMO.

maybe you could think of loops and coils like those childrens fishing nets with a fixed size hoop
https://www.active.com/Assets/active...nd+Day/net.jpg

and the B field as being a flowing river that's going at a uniform velocity

the volume of water going through the net per second can change if you dip the hoop in and out of the water or if you rotate the angle the hoop makes to the direction of flow - but keeping it in the river and moving it side to side or up and down doesn't change the volume/second going through the net.

in the diagram with the stiff copper wire the movement of the wire is like dipping the net in and out of the water surface - it's changing the flux linkage / volume per second of water going through the net over time - so it causes induction.

in the diagram with the small coil that stays inside the uniform field at a constant angle to the field - that's like having the net submerged in the water at a constant angle to the flow and just moving it side to side - the flux linkage / volume per second doesn't change over time - so no induction happens.

---
needless to say this analogy isn't entirely perfect
0
#7
(Original post by Joinedup)
Doesn't look like it in the diagram IMO.

maybe you could think of loops and coils like those childrens fishing nets with a fixed size hoop
https://www.active.com/Assets/active...nd+Day/net.jpg

and the B field as being a flowing river that's going at a uniform velocity

the volume of water going through the net per second can change if you dip the hoop in and out of the water or if you rotate the angle the hoop makes to the direction of flow - but keeping it in the river and moving it side to side or up and down doesn't change the volume/second going through the net.

in the diagram with the stiff copper wire the movement of the wire is like dipping the net in and out of the water surface - it's changing the flux linkage / volume per second of water going through the net over time - so it causes induction.

in the diagram with the small coil that stays inside the uniform field at a constant angle to the field - that's like having the net submerged in the water at a constant angle to the flow and just moving it side to side - the flux linkage / volume per second doesn't change over time - so no induction happens.

---
needless to say this analogy isn't entirely perfect
I get the idea, thanks!

I just noticed the stiff copper wire is being raised vertically between the poles and the other situation the coil is being horizontally moved between the poles. Does that in any way explain why emf is enduced in one case and is 0 (of course other than the emf peaks on entering and leaving) in the other?
Last edited by username5091846; 9 months ago
0
#8
Just to confirm, the emf induced in the stiff copper wire stays constant and non zero throughout the time its being raised between the poles, right?
0
9 months ago
#9
F = qvxB and all are constant so force is constant yes
0
9 months ago
#10
(Original post by crudux_c)
I dont understand cii (in attached files)

E = Change in flux / ∆t.
When we move the coil between the poles (where there is uniform B), the area is changing hence there is change in flux. But it is moving at a constant speed, so shouldnt emf be constant between the poles? How is it 0? Please help!

......

I am not sure why do you say that the induced emf in this is zero.

Both the mark scheme and examiner report did not state that the induced emf is zero.

0
#11
(Original post by Eimmanuel)
I am not sure why do you say that the induced emf in this is zero.

Both the mark scheme and examiner report did not state that the induced emf is zero.

No, I did not say enduced emf here should be zero. I believe it should be constant (because the wire is being moved at constant speed and hence rate of change of flux is constant) and NON ZERO (8×10^-3 V) all the way throughout in between the poles.

In the other question (I'm attaching it below) where a coil is moved between the poles of a uniform magnet at uniform speed did I say the emf should be constant and non zero just as in the case above because it seemed the exact same situation. However, the mark scheme to it says the induced emf is zero between the poles with a spike in EMF just on entering and leaving the uniform magnetic field. Why isnt the EMF non zero and constant here too? Can you please help clear things up for me? I appreciate Joinedup's help earlier a lot, but would love to have a bit more clarity to the problem from you.

This is the question I'm talking about (cii)
Last edited by username5091846; 9 months ago
0
9 months ago
#12
(Original post by crudux_c)
No, I did not say enduced emf here should be zero. I believe it should be constant (because the wire is being moved at constant speed and hence rate of change of flux is constant) and NON ZERO (8×10^-3 V) all the way throughout in between the poles.

In the other question (I'm attaching it below) where a coil is moved between the poles of a uniform magnet at uniform speed did I say the emf should be constant and non zero just as in the case above because it seemed the exact same situation. However, the mark scheme to it says the induced emf is zero between the poles with a spike in EMF just on entering and leaving the uniform magnetic field. Why isnt the EMF non zero and constant here too? Can you please help clear things up for me? I appreciate Joinedup's help earlier a lot, but would love to have a bit more clarity to the problem from you.

This is the question I'm talking about (cii)

The two questions describe 2 different scenarios.
The question c(ii) describes the situation that is shown on the right diagram below. However, the coil in the question c(ii) is small, so the diagram on the right side should be modified with a much smaller coil.

There is only a change of magnetic flux linkage with the coil as the coil enters or leaves the uniform magnetic field. As a result, there is induced emf when the coil enters or leaves the uniform magnetic field.
0
#13
(Original post by Eimmanuel)
There is only a change of magnetic flux linkage with the coil as the coil enters or leaves the uniform magnetic field. As a result, there is induced emf when the coil enters or leaves the uniform magnetic field.
Why isn't this the case with the other question where a wire is being moved between poles of uniform flux density exactly similarly? I believe an EMF is induced throughout the wires movement and not just on entering and leaving there? Why isn't the change in magnetic flux across the wire constant there too as in the case with the small, flat coil?

I shall be very thankful if you could explain the difference between both the scenarios. It'd clear off any confusion. And help with my understanding.
0
9 months ago
#14
(Original post by crudux_c)
Why isn't this the case with the other question where a wire is being moved between poles of uniform flux density exactly similarly? I believe an EMF is induced throughout the wires movement and not just on entering and leaving there? Why isn't the change in magnetic flux across the wire constant there too as in the case with the small, flat coil?

I shall be very thankful if you could explain the difference between both the scenarios. It'd clear off any confusion. And help with my understanding.

(Original post by crudux_c)
Why isn't this the case with the other question where a wire is being moved between poles of uniform flux density exactly similarly? ...
Which other question are you referring? In which post(s)?
0
#15
(Original post by Eimmanuel)
Which other question are you referring? In which post(s)?
the reasoning I found (on an A Level Physics site) for the emf to be zero while the flat coil moves between poles is that because B is constant between poles and so change in flux linkage occurs only on entering, leaving the field and not while the coil moves between poles, hence emf is only enduced while entering or leaving the field and is 0 moving between poles.

the field is constant here too at 89 mT. So, change in flux should only occur and hence emf should be enduced just when the wire enters or leaves the field and not in between poles where change in flux should remain 0 because flux linking the wire doesnt change (this is the logic used in the above flat coil question to determine when change in flux and hence enduced emf occurs).

Can you explain the difference between both the scenarios? And how does the induced emf exactly vary when the wire is moving between the poles of 89 mT please? That would really aid my understanding. Thanks a lot for your time.
Last edited by username5091846; 9 months ago
0
9 months ago
#16
(Original post by crudux_c)
the reasoning I found (on an A Level Physics site) for the emf to be zero while the flat coil moves between poles is that because B is constant between poles and so change in flux linkage occurs only on entering, leaving the field and not while the coil moves between poles, hence emf is only enduced while entering or leaving the field and is 0 moving between poles.

the field is constant here too at 89 mT. So, change in flux should only occur and hence emf should be enduced just when the wire enters or leaves the field and not in between poles where change in flux should remain 0 because flux linking the wire doesnt change (this is the logic used in the above flat coil question to determine when change in flux and hence enduced emf occurs).
Before I explain the difference, ensure you really know how to write or spell induced emf NOT enduced emf.

The reasoning is a flaw. There is only coil entering the uniform magnetic field and no coil moving through the uniform magnetic field or leaving the uniform magnetic field.
In fact, joinedup had explained the situation well at post #6.

(Original post by crudux_c)
....Can you explain the difference between both the scenarios? And how does the induced emf exactly vary when the wire is moving between the poles of 89 mT please? That would really aid my understanding. Thanks a lot for your time.
The situation depicts in Fig. 5.1 “looks” like what is shown on the right-side diagram as shown below (shown in Fig. (a) & (b)).

The area of “coil” in Fig 5.1 is much larger than the region of the uniform magnetic field. So when the copper wire moves through the magnetic field, the uniform magnetic field is “enclosed” inside the coil (shown in Fig. (b)). This is why I say the “coil” is only entering the uniform magnetic field when the copper wire moves through the magnetic field.
0
#17
(Original post by Eimmanuel)
Before I explain the difference, ensure you really know how to write or spell induced emf NOT enduced emf.

The reasoning is a flaw. There is only coil entering the uniform magnetic field and no coil moving through the uniform magnetic field or leaving the uniform magnetic field.
Firstly, thanks for correcting me on how to spell "induced".

Secondly, I wish to confirm what do we really mean by saying that the small flat coil only enters the uniform magnetic field but not moves through it or leaves it like the way the copper wire does in fig 5.1?

Also, do you think the fact the coil is moved horizontally and the copper wire on the other hand is moved vertically between poles plays any part in the reasoning why emf is induced in one case and not in the other?

how do you explain induced emf in this worked example? The area of the wire seems just about the size of the field region here and not greater than it. All parts of it are entirely moving through the magnetic field region at once like in the case of the small flat coil, so why is emf induced here then?

I'm really sorry for being annoying and cross questioning a lot. Just hoping to get my head around this.
Last edited by username5091846; 9 months ago
0
9 months ago
#18
(Original post by crudux_c)
Firstly, thanks for correcting me on how to spell "induced".

Secondly, I wish to confirm what do we really mean by saying that the small flat coil only enters the uniform magnetic field but not moves through it or leaves it like the way the copper wire does in fig 5.1? ...
Let me name the question of Hall probe question as 9702_w12_q43_q5 and the question of the copper wire moving through the magnetic field as 9702_w10_q43_q5.

The coil that I referred to in post #16 to describe the situation in 9702_w10_q43_q5 is NOT really the small coil that was mentioned in 9702_w12_q43_q5. In post #16, I use the word “look” with “”.

The Fig (a) and Fig (b) in post #16 shows the position of the coil at two different times. Fig (a) is at an earlier time and Fig (b) in the later time to illustrate a coil entering the uniform magnetic field which “looks” like the situation in 9702_w10_q43_q5.

IMO, it is meaningless to talk about the coil moving through uniform magnetic field in 9702_w10_q43_q5.

I am not sure what are you trying to confirm the “meaning” of coil entering, leaving and moving through the uniform magnetic field.

Post #18 Fig 1a illustrates a coil just enters a uniform magnetic field from left to right if we consider the sequences of the diagram from top to bottom.

However, the same figure (Post #18 Fig 1a) can also illustrate a coil just leaves the uniform magnetic field from right to left if we consider the sequences of the diagram from bottom to top.

Post #18 Fig 1b illustrates a coil moves through a uniform magnetic field.
0
9 months ago
#19
(Original post by crudux_c)
....Also, do you think the fact the coil is moved horizontally and the copper wire on the other hand is moved vertically between poles plays any part in the reasoning why emf is induced in one case and not in the other? ....
Not really sure what are you asking.

Are you referring the coil that moves horizontally to 9702_w12_q43_q5 while the copper wire that is moved vertically between poles to 9702_w10_q43_q5? If yes, then you are missing the main key understanding of Faraday’s law of induction. In addition, there are induced emf in both questions and I don’t understand what you mean by there is induced emf in one case and no induced emf in the other case.

In fact, I can have a situation where the coil moves horizontally or vertically, and there is no induced emf. The key is how the coil moves with respect to the constant magnetic field in both situations that brings about a change in magnetic flux linkage,

PS: Let me name the question of Hall probe question as 9702_w12_q43_q5 and the question of the copper wire moving through the magnetic field as 9702_w10_q43_q5.
0
#20
(Original post by Eimmanuel)
Let me name the question of Hall probe question as 9702_w12_q43_q5 and the question of the copper wire moving through the magnetic field as 9702_w10_q43_q5.

The coil that I referred to in post #16 to describe the situation in 9702_w10_q43_q5 is NOT really the small coil that was mentioned in 9702_w12_q43_q5. In post #16, I use the word “look” with “”.

The Fig (a) and Fig (b) in post #16 shows the position of the coil at two different times. Fig (a) is at an earlier time and Fig (b) in the later time to illustrate a coil entering the uniform magnetic field which “looks” like the situation in 9702_w10_q43_q5.

IMO, it is meaningless to talk about the coil moving through uniform magnetic field in 9702_w10_q43_q5.

I am not sure what are you trying to confirm the “meaning” of coil entering, leaving and moving through the uniform magnetic field.

Post #18 Fig 1a illustrates a coil just enters a uniform magnetic field from left to right if we consider the sequences of the diagram from top to bottom.

However, the same figure (Post #18 Fig 1a) can also illustrate a coil just leaves the uniform magnetic field from right to left if we consider the sequences of the diagram from bottom to top.

Post #18 Fig 1b illustrates a coil moves through a uniform magnetic field.
But in 9702_w12_q43_q5 it clearly says the coil moves along the line xy. And xy is a line between the poles of the magnet. Which means the coil is moving in the magnetic field. So why do you say it only enters the magnetic field and not move in it? (In post #16 you say so).
Last edited by username5091846; 9 months ago
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

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

### Poll

Join the discussion

#### Current uni students - are you thinking of dropping out of university?

Yes, I'm seriously considering dropping out (99)
13.2%
I'm not sure (32)
4.27%
No, I'm going to stick it out for now (234)
31.2%
I have already dropped out (18)
2.4%
I'm not a current university student (367)
48.93%