Understanding Induced Current and EMF in Electromagnetic Induction

[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.

 

[jsmith613]

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.

I thought it would be if the tube was long enough because once in the tube there is no change in flux linkage within the tube? The longer the magnet, I would have though, the less time it speds inside the tube so surely the less straight the middle section is??

 

[truesearch]

here is a drawing of the falling magnet as it passes through a coil. as the blue end passesthrough you will get the blue part of your trace.
as the red end passes through you will get the red part of the trace.
The size and width of each part of the trace are to do with the fact that the falling magnet is accelerating.
As the middle section of the magnet passes through there is a flux but it is not changing !
Try to picture the field lines for a bar magnet, in the centre they are parallel

 

[sophiecentaur]

The currents will be different at different parts of the tube over its length, remember.

 

[sophiecentaur]

The currents will be different at different parts of the tube over its length, remember.

 

[sophiecentaur]

The currents will be different at different parts of the tube over its length, remember.

 

[sophiecentaur]

The currents will be different at different parts of the tube over its length, remember.

 

[jsmith613]

The currents will be different at different parts of the tube over its length, remember.

I think people are missing the question I had:

if the tube was A LOT longer and the magnet was v. short would the emf be 0 for all the time the magnet was in the LONG tube?

 

[sophiecentaur]

I think people are missing the question I had:

if the tube was A LOT longer and the magnet was v. short would the emf be 0 for all the time the magnet was in the LONG tube?

What EMF where?

 

[jsmith613]

What EMF where?

the EMF induced in the tube?
if I drop a magnet into the tube for the entire time the magnet is in the tube, won't the net emf be zero and hence our graph would have a curved ends with a long flat bit in between
a little bit (but not exactly like:)

/\_________________
........\/

Ignore the ... they are there just to position the V

(this is an emf-time graph)

 

[cabraham]

Is an induced current (conventional current) in the same direction or in the opposite direction to the induced emf.
I ask this in relation to electromagnetic induction. We can predict the direction of the current using the right hand rule but how do I know the direction of emf?

Another question, related to this, is how could an EMF be induced (and hence a current be induced) in a strip of wire, such as an aerial, if it is not in a complete circuit? Surely the whole point of EM induction is to oppose the change in magnetic flux. If a current cannot be induced, how is this done? For example when a car moves forward, an EMF is induced across the ends of the aerial BUT there is no complete circuit

Thanks a lot guys!

Good question. Is the induced current in the same direction as the induced emf? My answer would be that Ohm's law is always in effect. The emf will be oriented in relation to the current and impedance per Ohm's law. For a resistance, positive current enters the positive terminal of the resistance. The positive current enters the positive emf terminal and exits the negative emf terminal. In a resistor, charge carriers are losing energy due to collisions with the material lattice structure.

In an aerial, current and voltage are induced even though the circuit is not a closed loop. This perplexed investigators in the 19th century until Maxwell added a displacement current term to Ampere's Law and published it in 1873. Though an open circuit, the E & H fields incident on the antenna exert forces on the electrons in the wire. The electrons move through the wire. Before they get to the other end the polarity reverses and the electrons reverse direction. For high enough frequencies, the circuit is not a true open, but an infinitessimal series of resistrors, capacitors, conductors, and inductors.

As long as the wavelength of the E/H fields is short compared with the antenna length, a substantial current can exist. The impedance of the antenna actually looks like a pure resistance. With capacitors, displacement currents exist, and the current leads the voltage by about 90 degrees. In an antenna, displacement current exists with about 0 degrees of phase shift between I & V.

The key to understanding induction is that it only works in the ac domain, i.e. time-changing flux (or motion plus static flux which is equivalent). In the ac domain, there is no such thing as a perfect open or short. A superconducting loop has inductive reactance and an open loop has capacitance. Thus current and voltage are both induced together. You can't have one w/o the other. Make sense? I hope I helped.

Claude