Electromagnetic induction and energy conservation

[rajeesh]

Lenz's law states that the current induced in a coil due to a moving magnet is in a direction such that it opposes the motion of the magnet.

 

Now the question is would the moving magnet experience resistance instantly or does it have to wait for the current to be induced in the coil and the effect of this current to resist its motion.

Since nothing can travel faster than light, it would take some amount of time (however small it may be) for the current to be induced in the coil and the effect of this current to travel back to the coil.

 

So is energy conserved for such small intervals of time?

[swansont]

Lenz's law states that the current induced in a coil due to a moving magnet is in a direction such that it opposes the motion of the magnet.

 

Now the question is would the moving magnet experience resistance instantly or does it have to wait for the current to be induced in the coil and the effect of this current to resist its motion.

Since nothing can travel faster than light, it would take some amount of time (however small it may be) for the current to be induced in the coil and the effect of this current to travel back to the coil.

 

So is energy conserved for such small intervals of time?

The induced current should experience the field immediately, because the field is present where the coil is.

[rajeesh]

The induced current should experience the field immediately, because the field is present where the coil is.

 

 

But how would the current induced in the coil, oppose the moving magnet instantly since there is separation between them?

As the magnet starts moving it would take time t = (distance b/w the magnet and the coil/velocity of light) for the current to be induced in the coil and another equal time t for the effect of this induced current to travel back to the magnet and oppose its motion.

[Ronald Hyde]

But how would the current induced in the coil, oppose the moving magnet instantly since there is separation between them?

As the magnet starts moving it would take time t = (distance b/w the magnet and the coil/velocity of light) for the current to be induced in the coil and another equal time t for the effect of this induced current to travel back to the magnet and oppose its motion.

 

That's only true if the interaction involves 'real' photons such as your flashlight produces. Virtual photons escape that requirement. If you look at is in Feynman's 'sum

over possible future histories' way, the system is looking into the future to see what configurations it can have. This is very much in conflict with our ordinary intuition

of causality, but it is how the world works, every test that challenges it has proven wrong.

[swansont]

But how would the current induced in the coil, oppose the moving magnet instantly since there is separation between them?

As the magnet starts moving it would take time t = (distance b/w the magnet and the coil/velocity of light) for the current to be induced in the coil and another equal time t for the effect of this induced current to travel back to the magnet and oppose its motion.

The coil sees a changing field. The source doesn't matter. That interaction — the response to a changing field — has no separation to it. The coil is sampling the field and sees the change in it.

[rajeesh]

That's only true if the interaction involves 'real' photons such as your flashlight produces. Virtual photons escape that requirement. If you look at is in Feynman's 'sum

over possible future histories' way, the system is looking into the future to see what configurations it can have. This is very much in conflict with our ordinary intuition

of causality, but it is how the world works, every test that challenges it has proven wrong.

 

So the magnet knows that there is a coil, waiting to absorb its energy in the future and hence it starts experiencing resistance instantly as soon as it starts moving.

Now if we assume that there is no coil at the instant when the magnet starts moving, but the decision to place the coil in the field of the moving magnet is taken only after the magnet starts moving but before the effect of the moving magnet has reached the point where the coil is placed. In this case would the magnet experience resistance instantly?

 

If yes would it mean that the magnet is looking into the future and it knows what the decision is going to be.(to place the coil or not)

[John Cuthber]

If I have a coil of superconducting wire with a current flowing through it, it has a stored energy 1/2 LI^2.

That energy is stored in the magnetic field round the magnet.

As far as I can see, the energy is transferred to the magnetic field and then to the other wire.

Energy is conserved because the field acts as a temporary store.

[swansont]

So the magnet knows that there is a coil, waiting to absorb its energy in the future and hence it starts experiencing resistance instantly as soon as it starts moving.

No, that would violate causality.

[rajeesh]

No, that would violate causality.

 

But if the magnet does not experience resistance instantly that would violate the law of conservation of energy as we have a moving magnet (whose energy has not been retarded) and also current in the coil at the instant the effect of the moving magnet reaches the coil.

 

Can a causal interpretation really explain the energy transfer in this case?

[swansont]

But if the magnet does not experience resistance instantly that would violate the law of conservation of energy as we have a moving magnet (whose energy has not been retarded) and also current in the coil at the instant the effect of the moving magnet reaches the coil.

 

Can a causal interpretation really explain the energy transfer in this case?

Magnetic fields contain energy, proportional to B^2.

 

If the coil is moving toward a magnet, it sees an increasing field. So the induced field is opposite of that, reducing the field. IOW, less energy in the field, so energy in current flow doesn't violate conservation of energy.

[rajeesh]

Magnetic fields contain energy, proportional to B^2.

 

If the coil is moving toward a magnet, it sees an increasing field. So the induced field is opposite of that, reducing the field. IOW, less energy in the field, so energy in current flow doesn't violate conservation of energy.

 

But in this case it is the magnet which is moving towards the coil. How will the same argument hold in this case?

[swansont]

But in this case it is the magnet which is moving towards the coil. How will the same argument hold in this case?

It's the same scenario — you can pick any reference frame to analyze it. If it doesn't violate energy conservation in one frame, it doesn't violate it in any frame.

[rajeesh]

It's the same scenario — you can pick any reference frame to analyze it. If it doesn't violate energy conservation in one frame, it doesn't violate it in any frame.

 

Lets take the frame of the moving magnet.

If the magnet starts to move at time t=t1 and lets assume that t0 is the time taken for the flux linking the coil to change due to motion of the magnet.

 

t0 = (distance between the magnet and the coil/velocity of light)

 

Now when exactly does the magnet begin to experience resistance. Is it at t1 or (t1+t0) or (t1+2t0)

 

I think energy is really conserved here but is the entire transaction causal?

[swansont]

I suspect the delay will be 2t0

[EMField]

Because the field is composed of a dielectric and already has the energy stored, it simply needs polarized to transmit this energy.

 

 

http://en.wikipedia.org/wiki/Dielectric

The study of dielectric properties is concerned with the storage and dissipation of electric and magnetic energy in materials.

 

The electric field is already present. The nanosecond the magnet begins to move it begins interacting with this field, which is why no delay is observed. The energy is already stored in this field, hence the CMB.

[rajeesh]

I suspect the delay will be 2t0

 

But if the delay is 2t0, what is the state of the system at (t1+t0), we have the magnet which has not lost any energy and the coil with the current induced in it which is extra energy.

[swansont]

But if the delay is 2t0, what is the state of the system at (t1+t0), we have the magnet which has not lost any energy and the coil with the current induced in it which is extra energy.

As I explained before, the current in the coil comes from the energy in the B field, which has decreased.

[Ronald Hyde]

If I have a coil of superconducting wire with a current flowing through it, it has a stored energy 1/2 LI^2.

That energy is stored in the magnetic field round the magnet.

As far as I can see, the energy is transferred to the magnetic field and then to the other wire.

Energy is conserved because the field acts as a temporary store.

The Idea that the magnetic field 'contains energy' is somewhat of a paradox, because there are no quantities that

change with time associated with a constant magnetic field. The magnetic field is B, the curl of the vector potential, and the

current density is the curl of B. All of those are space-like. However if we make something change with time, the current in

a coil for instance, because the current is made up of conserved charges, and we can 'switch off' the current, energy will be

released and can be used to heat up a resistor, for example. All this makes the question of where the energy is in an electomagnet

very interesting. We can think of a 'charged' capacitor or inductor as being an excited state of the uncharged circuit element, and

and as such an excited state, it has extra energy, but when we ask 'where is that energy' things get a little complicated..

[rajeesh]

The Idea that the magnetic field 'contains energy' is somewhat of a paradox, because there are no quantities that

change with time associated with a constant magnetic field. The magnetic field is B, the curl of the vector potential, and the

current density is the curl of B. All of those are space-like. However if we make something change with time, the current in

a coil for instance, because the current is made up of conserved charges, and we can 'switch off' the current, energy will be

released and can be used to heat up a resistor, for example. All this makes the question of where the energy is in an electomagnet

very interesting. We can think of a 'charged' capacitor or inductor as being an excited state of the uncharged circuit element, and

and as such an excited state, it has extra energy, but when we ask 'where is that energy' things get a little complicated..

 

question is - can we really explain the entire transaction without involving quantum mechanics or virtual photons coming from the future

Edited August 27, 2012 by rajeesh

[rajeesh]

That's only true if the interaction involves 'real' photons such as your flashlight produces. Virtual photons escape that requirement. If you look at is in Feynman's 'sum

over possible future histories' way, the system is looking into the future to see what configurations it can have. This is very much in conflict with our ordinary intuition

of causality, but it is how the world works, every test that challenges it has proven wrong.

 

 

Could you please provide more details or references on the virtual photons.

Does the system really look into the future, looks like it will violate causality but can it really happen?

[Hemal Pansuriya]

eather causality is violated or energy conservation is violated . Both can not be hold simultaneously in this situation.

since there can not be instantaneous interaction between magnet and coil. so there should be violation of energy conservation.

Refer to my question: https://physics.stackexchange.com/questions/345924/violation-of-energy-conservation-in-retarded-interaction-of-magnet-and-coil

Edited July 28, 2017 by Hemal Pansuriya

[studiot]

On 27/08/2012 at 7:10 PM, rajeesh said:

 

question is - can we really explain the entire transaction without involving quantum mechanics or virtual photons coming from the future

Yes by the use of a virtual work analysis, Faraday's, Lenz's and Neuman's laws can be deduced from conservation of energy and a virtual work perturbation.

[Strange]

3 hours ago, Hemal Pansuriya said:

eather causality is violated or energy conservation is violated . Both can not be hold simultaneously in this situation.

since there can not be instantaneous interaction between magnet and coil. so there should be violation of energy conservation.

Refer to my question: https://physics.stackexchange.com/questions/345924/violation-of-energy-conservation-in-retarded-interaction-of-magnet-and-coil

The answers there explain why you are wrong. Why did you ignore them and just repeat the same thing?

(I have suggested that the moderators split this off, probably to the Speculations forum as you seem uninterested in the answers.) Edit: I see you are making the same point as the OP, so I withdraw that last bit...

Edited July 28, 2017 by Strange

[studiot]

On 26/08/2012 at 8:31 AM, rajeesh said:

Lenz's law states that the current induced in a coil due to a moving magnet is in a direction such that it opposes the motion of the magnet.

 

Now the question is would the moving magnet experience resistance instantly or does it have to wait for the current to be induced in the coil and the effect of this current to resist its motion.

Since nothing can travel faster than light, it would take some amount of time (however small it may be) for the current to be induced in the coil and the effect of this current to travel back to the coil.

 

So is energy conserved for such small intervals of time?

This is a misapplication of the Law of Conservation of Energy.

COE (the First Law) applies to changes in state function energies, due to inputs or outputs to a system summed over some parameter of the process eg time, at the end of a process.

It cannot be applied directly during a process.

Edited July 28, 2017 by studiot

[swansont]

6 hours ago, Hemal Pansuriya said:

eather causality is violated or energy conservation is violated . Both can not be hold simultaneously in this situation.

since there can not be instantaneous interaction between magnet and coil. so there should be violation of energy conservation.

Refer to my question: https://physics.stackexchange.com/questions/345924/violation-of-energy-conservation-in-retarded-interaction-of-magnet-and-coil

Perhaps you could address answers already presented here?