Understanding Induced Current and EMF in Electromagnetic Induction

[jsmith613]

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!

 

[DragonPetter]

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!

Hello,

To your first question: First, when you say direction of induced emf, do you mean polarity of the emf voltage?

Think of it this way, the emf reduces the amount of current flowing, or in the perspective of current, the decreasing change in current creates an increase in the amount of emf.

The equation for induction is:

EMF = -\frac{d\Phi}{dt}

The minus sign shows that EMF opposes the change in flux.

In circuits with inductors, this looks like a voltage drop as the current passes through the inductor, so the current flows into the EMF's positive terminal, and out its negative terminal.

I hope that helps and makes sense.

 

[jsmith613]

Hello,

To your first question: First, when you say direction of induced emf, do you mean polarity of the emf voltage?

Think of it this way, the emf reduces the amount of current flowing, or in the perspective of current, the decreasing change in current creates an increase in the amount of emf.

Well I presume I mean polarity of EMF voltage although I am not quite sure of the difference :S

for the second paragraph, surely the induced EMF induces a current. if I take a strip of wire no current initially flows, so if I move it in a field and an EMF is induced a current HAS to flow (it cannot be reduced)

 

[sophiecentaur]

That's right. The Right hand rule tells you the direction of the EMF, which works even with an open circuit. The current direction follows from the EMF and the value depends on the load it drives..

 

[DragonPetter]

Well I presume I mean polarity of EMF voltage although I am not quite sure of the difference :S

for the second paragraph, surely the induced EMF induces a current. if I take a strip of wire no current initially flows, so if I move it in a field and an EMF is induced a current HAS to flow (it cannot be reduced)

What happens when current flows in an open wire? Do you think you can compare this to an antenna?

 

[sophiecentaur]

My point is that the EMF is what is induced. The Current is a secondary issue and depends upon the Impedance of the load. No current will flow in a short length of unconnected wire but there will be an EMF.
The theory of how an antenna couples to an EM wave is more involved because there is a varying E field as well as an H field. In the case of simple Magnetic induction, any E field is ignored because we aren't dealing with free space conditions.

 

[jsmith613]

That's right. The Right hand rule tells you the direction of the EMF, which works even with an open circuit. The current direction follows from the EMF and the value depends on the load it drives..

well my first question really was to they flow in the same direction.
maybe I have misunderstood emf and that it doesn't flow?

 

[sophiecentaur]

well my first question really was to they flow in the same direction.
maybe I have misunderstood emf and that it doesn't flow?

It's a good thing to get the terminology right in things like this.
An EMF doesn't "flow"; it is a Potential Difference and exists 'across' to parts of a circuit. A current will flow as a result of an applied EMF.
PD is like a difference in heights. Things can fall from one height to another - like a current can flow.

 

[DragonPetter]

Another thing to consider in the equation I first wrote out is that there is no current term. Magnetic flux can be dependent on a current, but it is not always the case. EMF is directly dependent on magnetic flux.

 

[jsmith613]

It's a good thing to get the terminology right in things like this.
An EMF doesn't "flow"; it is a Potential Difference and exists 'across' to parts of a circuit. A current will flow as a result of an applied EMF.
PD is like a difference in heights. Things can fall from one height to another - like a current can flow.

well surely one end of the emf is +/- and the other end 0
so let's say + is on the left and 0 is on the right. the emf will go from + to 0 (left to right).
if this is a closed wire, in which direction wil the current flow?

 

[DragonPetter]

well surely one end of the emf is +/- and the other end 0
so let's say + is on the left and 0 is on the right. the emf will go from + to 0 (left to right).
if this is a closed wire, in which direction wil the current flow?

The same way current flows with any other + to - voltage source.

 

[jsmith613]

The same way current flows with any other + to - voltage source.

so from - to +

therefore, the direction of the induced emf can be worked out using the LEFT hand rule, right?

 

[sophiecentaur]

so from - to +

therefore, the direction of the induced emf can be worked out using the LEFT hand rule, right?

How do you come to that conclusion?
EMF=−dΦdt and
I=EMF/Resistance
so where does your other sign change come from?
If you connect a battery, which way will the current flow? What's different for an induced emf?

 

[FOIWATER]

induced voltages result in current flow that opposes the current flow that created the field that induced the voltage.

This is the reason inductors exhibit impedance. Apply a voltage across an inductor, current flows through it, which creates a magnetic field around it. Assuming AC voltage and that field varies with time depending on frequency, the magnetic field expands and collapses over time. Each time current passing through it increases, the field increases, each time the current collapses, the magnetic field collapses. the collapsing of the field has the opposite effect that the initial current-produced magnetic field did. It induces a voltage back into your inductor, this causes current to flow which bucks the source current. This is the nature of impedance in an inductor, and why it does not account for actual power loss as well, it just stores energy in a magnetic field.

This phenomena also accounts for CEMF inside an electric motor. The reason why motors draw more current bases on load, is because of the fact EMF is induced opposing source voltage. At slower speeds, less EMF is induced that bucks the source voltage, hence why a motor draws more current when loaded.

Research lenz' law

Right hand rule gives direction of current flow based on field direction and relative motion direction.

Left hand motor (lorentz' force) rule, right hand (faraday's induction) generator rule. Notice only your two middle fingers will oppose one another, which accounts for generator action in all motors, and all loading phenomena

 

[FOIWATER]

basically, it depends how the magnetic field interacts with the conductor that is having a voltage induced in it.

If the field is expanding from the source, and collapsing on the conductor, the voltage is the opposite direction.

For a static field and relative motion of the conductor, use the right hand rule.

 

[sophiecentaur]

induced voltages result in current flow that opposes the current flow that created the field that induced the voltage.

Absolutely BUT the 'back emf' is never greater than the initial induced EMF. Add the original EMF to the EMF generated by the induced current and it will always produce a current that is less it would have been but still in the 'right' direction . If you look upon it as being analogous to the force of reaction against you when you are pushing a massive object along, that reaction force is never great enough to move the object towards you.

 

[BruceW]

\oint \vec{E} \cdot d \vec{l} = - \int \frac{d \vec{B}}{dt} \cdot d \vec{A}
This is the equation for induction, just written out in more detail. On the right-hand side of the equation is an area integral, where the direction of the area vector is the normal to the surface. And on the left-hand side is a line integral over a closed path bounding the same area.

The convention in physics is that the line integral goes anti-clockwise around the area which it bounds. So from this, you can work out the direction of the electric field.

 

[jsmith613]

How do you come to that conclusion?
EMF=−dΦdt and
I=EMF/Resistance
so where does your other sign change come from?
If you connect a battery, which way will the current flow? What's different for an induced emf?

well then as there is no change in sign I would presume therefore that EMF and current are in the same direction

 

[sophiecentaur]

Right. (Thank god - or nothing would work. )

 

[FOIWATER]

yeah i totally agree, there has to be power lost during the transfer of energy, as well all the flux that the current created can not be linked, much is leaked.

I know you know this. :D

 

[jsmith613]

induced voltages result in current flow that opposes the current flow that created the field that induced the voltage.

This is the reason inductors exhibit impedance. Apply a voltage across an inductor, current flows through it, which creates a magnetic field around it

First, I simply thought that the induced voltage opposed a change in flux linkage (there does not need to be a current producing it - e.g: a permamnent magnet), right??

Also what happens if there is no circuit...no current...therefore no field. so why induce an EMF??

 

[BruceW]

First, I simply thought that the induced voltage opposed a change in flux linkage (there does not need to be a current producing it - e.g: a permamnent magnet), right??

That's right.

 

[FOIWATER]

No, the reason for flux production, is because of current flow. Voltage causes current flow, and current flow exhibits a magnetic field when it flows, the field occurs around the charge flow. So now imagine you have a magnetic field established, (as well as an electric one but we need not discuss it) so now faradays induction law... states that with relative motion between a conductor, and a magnetic field, we induce a voltage in said conductor. Now the induced voltage causes a SECONDARY current to flow.. it flows opposite the original current due to laws that flemmings hand rules illustrate.

With no current, therefore no field, there is no induced EMF.

Im not sure but I think you might not realize this:

Two circuits, that are close physically, can induce voltage in one ANOTHER... because of the fields produces.. this is very true when the circuits are in harness' or liquatite or are in close proximity, and often, the conductors are wrapped with a shield that is grounded, so the magnetic fields cut the shield, induce the current in that instead, and the current flows to ground rather than mess with other signals.

 

[FOIWATER]

well, there has to be a FIELD, a permanent magnet as you said will exhibit one, but in an electric circuit, you need current flow to establish a magnetic field. Which is what we are talking about here.

 

[jsmith613]

well, there has to be a FIELD, a permanent magnet as you said will exhibit one, but in an electric circuit, you need current flow to establish a magnetic field. Which is what we are talking about here.

no take my example post #1
a car moves in the Earth's magnetic field (NO circuit OR current). an EMF is induced in the aerial.

how does this therefore fit..how does an opposite mag field get produced

 

[FOIWATER]

didn't see that post.. yeah

 

[sophiecentaur]

In that case there is a change in flux in the observational frame of the car. This is indistinguishable from the current in an electromagnet being changed - so an EMF will be induced.

 

[FOIWATER]

as some one here said, think of a battery!

It isn't a complete circuit but once connected, current will flow from and to end.

there still exists POTENTIAL for current to flow

 

[jsmith613]

In that case there is a change in flux in the observational frame of the car. This is indistinguishable from the current in an electromagnet being changed - so an EMF will be induced.

I am confused? are you saying the EMF and current in this situation are the same??

 

[BruceW]

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.

 

[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 dependant. 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 :)