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.