Understanding Induced Current and EMF in Electromagnetic Induction

[jsmith613]

I agree with FOIWATER's last post, there doesn't necessarily need to be an induced current. It's just that the EMF is always such that the induced current would decrease the change in magnetic flux.

ok, so youre saying I can just imagine there is a current when there can't be and see if the situation works then...

 

[FOIWATER]

There won't be a current, there is no path for it to flow

The difference between current and EMF seem to be confusing the issue for you.

EMF can exist without a path for the EMF to force electrons around a circuit.

 

[jsmith613]

There won't be a current, unless the wheels falls off and you run into the car.

ye ye but the aim of the induced emf is to create a current that IF IT EXISTED would oppose the change in mag flux??

 

[FOIWATER]

let me write something that you should remember rather than "opposes change in flux"

when an emf is induced, there is no opposition to the change of flux.. What is happening, is that the field collapses on the conductor, in the opposite direction as the field exists. right?

(for example, a field expands around an inductor when current passes through it, on the negative half wave of the AC cycle, it collapses)

When it collapses, faradays induction law states a voltage is induced. but the EMF direction DEPENDS on DIRECTION of both the field and the direction of conductor motion.

In the example of an inductor, the field collapsing induces a voltage that opposes the voltage that causes the initial current to flow OK?

So you have an opposition to the initial current, which limits the flux, which is why it looks like it opposes the flux. but it doesn't ... it's a long string of events leading up to that.

You should really look at my previous post about the inductor, and consider it step by step, it's very interesting and explains so many things that people seem to just accept.

I personally have a much harder time understanding electric fields, as opposed to magnetic ones, which is why I bought some textbooks about it... but anyway..

Regarding your example about the car in Earth's field, the direction of the induced EMF depends on the direction the car is driving on Earth and if it is traveling in forward or reverse. unless you can somehow reverse Earth's field :) if a current did flow, it would flow in the same direction as the voltage that was induced forced it around the circuit.

 

[FOIWATER]

I HATE analogies, but this one isn't going to lead you astray...

now this isn't directly related to what you asked but you should like it considering what you asked..

Imagine an inductor coil in the most basic circuit.

AC voltage applied to the inductor causes current to flow through the inductor.

The current produces a magnetic field around the inductor. If the source was DC, the field would not change with time, it would be static. Which is why there is no inductive reactance in a DC circuit, it explains why inductive reactance is frequency dependent. Think of the field expanding and collapsing as the AC goes from peak to peak! like a set of lungs the field expands, and collapses, expands and collapses.

When it collapses... which it does not do in a DC circuit until power is removed... but in an ac circuit it does every negative half cycle, faraday's induction law states a voltage MUST be induced, as sophiecentaur said, it is not of equal magntitude as the source voltage, it must be smaller since some energy is lost in the transfer of energy types, and some flux is not linked even with a good core, etc.

The voltage that is induced, causes a current to flow. The current that flows... flows in the OPPOSITE direction as the INITIAL CIRCUIT current, because the current that is now flowing is a result of the field COLLAPSING, not increasing like before. This explains why we use Q to represent power in an inductor, and we represent it as VARS not in WATTS. We do not dissipate energy in an inductor, we store it in a field and every so often, empty it back into the circuit. When we empty the inductor, it appears to oppose the source.

 

[jsmith613]

Regarding your example about the car in Earth's field, the direction of the induced EMF depends on the direction the car is driving on Earth and if it is traveling in forward or reverse. unless you can somehow reverse Earth's field :) if a current did flow, it would flow in the same direction as the voltage that was induced forced it around the circuit.

thanks for the previous thing on inductors...you're right..its the kinda stuff we take for granted and don't learn from first principles...so thanks for explaning it to me so well :)

The reason I am fixing on this example (of the car) is that every A-level textbook I have seems to mention it and this is one form of the example given.
The EMF is described to have magnitude, Blv (length of conductor * velocity).

So if I was moving forward, had my conductor pointing upwards and the horizontal compoenent of the Earth's mag field pointing from side to side then there would be an induced EMF to the right??
now going back to your inductor posts would I be correct in thinking that your saying I would only produce a magnetic field to oppose a change in an already existsing magnetic field (e.g: the collapsing/reforming AC current in an adjacent wire). In the car example, there is no chanigng mag field anyway (I am just sweeping an area) so there is no need to produce a current anyway to oppose the change in magnetic field??

If this is correct, then let's look at another physical phenomena: magnets falling in a copper tube. If we imagine the copper tube as stacks of copper circlets (which I presume is valid, right??) then as the magnet falls the associated magnetic field with a copper circlet is chaning (despite there being no current). As the associated magnetic field is chaning a current flows in each copper circlet to create a magnetic field to OPPOSE this change of changing magnetic field.

I really hope I have understood what you have said :)

 

[FOIWATER]

Yeah the car would have a (very small) emf induced in it due to Earth field, you're right.

I wouldn't say that fields are created to oppose already existing, changing fields. All you have to remember, is that fields are created when there is relative motion between conductors, and already existing fields. In your example there is no change in field, but the result would be the same even if you were. All you need is relative motion to induce a voltage, whether the conductor (car) or magnetic field (earth field) is moving, it makes no difference. as long as there is relative motion between the two, there will be the same end result.

If a magnetic fell in a copper tube, it would induce a voltage in the copper, in the same way a copper penny would have a voltage induced into it if it was thrown into a large magnetic field. The field is not produced to OPPOSE the original field, the fact that the two exist is because one 'creates' the other, that is all

 

[truesearch]

there is an awful lot of confusion in this thread ! The facts are easily stated
When a conductor experiences a changing magnetic flux an emf is induced
The emf is in a direction that opposes the changing flux.
This means that the emf will try to make a current flow in the conductor to oppose the changing flux. The conductor is the source of the emf (NOT PD) so current will flow (if there is any) from the - end of the conductor to the + end then through the external circuit (if there is one).
Current flows from the - terminal of a battery (source of emf) to the + terminal of the battery then through the external circuit.

 

[jsmith613]

The field is not produced to OPPOSE the original field, the fact that the two exist is because one 'creates' the other, that is all

But they would be in the opposite direction though, right?
so if the magnet is north face down as it falls, the upper part of the coil would produce a north pole upwards??

Would I also be correct to assume that the entire time the magnet is inside the coil the NET emf is zero because the top of the magnet produces a current that cancels the current produced by the bottom of the magnet?

 

[DragonPetter]

there is an awful lot of confusion in this thread ! The facts are easily stated
When a conductor experiences a changing magnetic flux an emf is induced
The emf is in a direction that opposes the changing flux.
This means that the emf will try to make a current flow in the conductor to oppose the changing flux. The conductor is the source of the emf (NOT PD) so current will flow (if there is any) from the - end of the conductor to the + end then through the external circuit (if there is one).
Current flows from the - terminal of a battery (source of emf) to the + terminal of the battery then through the external circuit.

Hmm . . are you using conventional current? In conventional current, current flows out the + terminal of a battery and into the - terminal.

 

[jsmith613]

Hmm . . are you using conventional current? In conventional current, current flows out the + terminal of a battery and into the - terminal.

conventional current IS WHAT WE USE

 

[DragonPetter]

conventional current IS WHAT WE USE

Then what he said is incorrect.

 

[truesearch]

I don't use anything else in electrical circuits.
Conventional current flows from th + to the - around the EXTERNAL circuit.
It then flows from - to + through the source of emf (battery, generator, moving wire...whatever)

 

[truesearch]

With reference to an inductor connected to a battery (a DC circuit).
When the switch is opened to break the circuit the collapsing magnetic flux does induce an emf in the inductor and this emf opposes the changing flux so it is in a direction to KEEP the current flowing !

 

[DragonPetter]

I don't use anything else in electrical circuits.
Conventional current flows from th + to the - around the EXTERNAL circuit.
It then flows from - to + through the source of emf (battery, generator, moving wire...whatever)

Ah, I did not see you were describing flow in the internal circuit of whatever battery it is that you are using as the source voltage.

 

[jsmith613]

Ah, I did not see you were describing flow in the internal circuit of whatever battery it is that you are using as the source voltage.

going back a while now, surely in the eqn
V = IR
I is NOT conventional??

 

[truesearch]

i believe that it is understood that current means conventional current in physics teaching

 

[jsmith613]

i believe that it is understood that current means conventional current in physics teaching

ok, thanks, i presume this is in EVERY SITUATION then

 

[truesearch]

It has to be otherwise there would be total confusion !
Do you realize what is meant by conventional current? it is the direction in which + charges would flow.
Confusion usually comes about when you talk about electron flow because electrons are - charged.
Electrons travel in the opposite direction to conventional current.

 

[jsmith613]

I fear that due to all these new posts people will miss one of my other questions in post #39
could someone please confirm if I am correct with what I said there

that said, I would like to thank everyone for all their help :)

 

[truesearch]

re#39
Have you seen this experiment with the magnet dropped down a copper tube?...the effect is awsome.
If the bottom of the magnet is a N pole then current will flow around the copper tube ahead of the N pole to repel the magnet. So a N pole is induced
The top of the magnet is a S pole so the induced current in the copper tube above the magnet will flow to attract the magnet. So a N pole is again induced.
The induced current flows in the same direction, following the falling magnet, trying to stop it moving because this is what causes the changing magnet flux through the conductor.

 

[jsmith613]

re#39
Have you seen this experiment with the magnet dropped down a copper tube?...the effect is awsome.
If the bottom of the magnet is a N pole then current will flow around the copper tube ahead of the N pole to repel the magnet. So a N pole is induced
The top of the magnet is a S pole so the induced current in the copper tube above the magnet will flow to attract the magnet. So a N pole is again induced.
The induced current flows in the same direction, following the falling magnet, trying to stop it moving because this is what causes the changing magnet flux through the conductor.

oh, ok, so at what stage in the cycle will the EMF induced be zero:

http://www.a-levelphysicstutor.com/images/fields/EMI-dropped-mag02.jpg

 

[truesearch]

#52
Where did you get that trace from?
It is what you would get if one end of the magnet was dropping through a coil of wire.
Imagine a N pole approaching the coil and emf will be induced to oppose (repel) the falling magnet pole. when the pole passes through the coil an emf in the opposite direction will be induced to oppose (attract) the falling magnet pole.
The emf increases and takes less time because in falling the magnet pole is accelerating.
This is not exactly the same as the magnet falling in the copper pipe because both poles are in the pipe together (unless the magnet is very long)
(ps...I am signing off tonight)

 

[jsmith613]

re#39
Have you seen this experiment with the magnet dropped down a copper tube?...the effect is awsome.
If the bottom of the magnet is a N pole then current will flow around the copper tube ahead of the N pole to repel the magnet. So a N pole is induced
The top of the magnet is a S pole so the induced current in the copper tube above the magnet will flow to attract the magnet. So a N pole is again induced.
The induced current flows in the same direction, following the falling magnet, trying to stop it moving because this is what causes the changing magnet flux through the conductor.

enjoy you sleep!

so what would the graph look like for the magnet in the tube?
also, take a look at this animation: http://regentsprep.org/Regents/physics/phys08/clenslaw/default.htm

it seems to contradict you. The impression I get is that this animation ignores the "pole" of the magnet and takes into account the amount of flux linked.
The lower coil has an increasing magnetic flux DOWN so I want to oppose this so I point by thumb UPWARD and I get a current counter-clockwise

The upper coil has a decreasing magnetic flux DOWN and I want to oppose this so I point my thumb DOWN and get a current clockwise.

This therefore implies that if the magnet is WITHIN the tube, the EMF is zero for all that time. right?

here is another site that backs up the last one: http://www.thenakedscientists.com/H...-science/exp/mysterious-forces-eddy-currents/

 

[sophiecentaur]

It really concerns me that, in the middle of a very useful discussion like this one, I see that there is still some confusion about when Current is "Conventional Current". This is the fault of the teaching of Electricity in School which claims to be 'helping' kids by telling them about Electrons flowing. The rational is that it somehow makes an abstract thing like electricity more tangible by introducing a concrete idea. It clearly doesn't help - it just adds confusion and tempts kids to say "they got it wrong didn't they?". The laugh is that there are many teachers who, themselves, only think in terms of electron flow because the concept of the true nature of electricity is quite beyond them (at least half of them being either Biologists of Chemists, in UK schools). We are all going to hell in a handcart.

And the MEAN induced EMF will be zero for the falling magnet. It starts off one way and ends up the other as the magnet falls through each elemental ring of the tube - following Lenz's law all the way, as it has to do by opposing both the increase and the decrease in flux.

 

[jsmith613]

And the MEAN induced EMF will be zero for the falling magnet. It starts off one way and ends up the other as the magnet falls through each elemental ring of the tube - following Lenz's law all the way, as it has to do by opposing both the increase and the decrease in flux.

so if I was to trace a graph of EMF (Voltage) against time (seconds) what would it look like?

 

[BruceW]

I don't think it's that bad to tell kids about electrons flowing. They learn about atoms anyway. If kids were not taught about atoms and electrons, then I would agree with you. Anyway, back to post #39:

But they would be in the opposite direction though, right?
so if the magnet is north face down as it falls, the upper part of the coil would produce a north pole upwards??

Remember that the induced current is such that it reduces the change in magnetic flux. In the part of the coil above the magnet, the magnetic field is decreasing since the magnet is falling away from it. So the induced current will create a magnetic field In the same direction as the field which is created by the magnet.

 

[sophiecentaur]

A horizontal line with an S shaped wiggle in it.

 

[jsmith613]

A horizontal line with an S shaped wiggle in it.

so like this:
http://www.a-levelphysicstutor.com/images/fields/EMI-dropped-mag02.jpg

or would there be a straight line in the middle with curved ends?

 

[sophiecentaur]

It would depend on how long the magnet was. If long enough, the field wouldn't be changing whilst the middle section was passing. Your graph would be right for a short one.