# Magnetic field and electric field induce one another forever

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1. Apr 4, 2015

### Joker93

A changing electric field produces magnetic field and vice versa.Does that mean that this process will carry on forever?Think of a circuit with a capacitor.The magnetic field due to the current at a point on the wire(with Ampere-Maxwell law).But current was changing with time,so it also meant that the magnetic field changed.And a changing magnetic field produces an electric field,so we have to go back again from the start with the Ampere law.It seems that this will go on forever.What is the final magnetic and electric field that i have to calculate?

2. Apr 4, 2015

### Staff: Mentor

Well it is called the electromagnetic field after all.

Most mind bending is when you consider changes in inertial reference frame. Viewed from another frame, electric becomes magnetic and magnetic becomes electric.

Yes they are intertwined, but the math is not identical to your circuit analogy. In a photon, the electric and magnetic fields are in phase, not 90 degrees out of phase as intuition and circuit analysis would suggest.

3. Apr 4, 2015

### Joker93

In the example where i have a circuit with battery and capacitor,they do not seem to be oscilating with each other.they just reinforce one another

4. Apr 4, 2015

### Staff: Mentor

Circular reasoning is not the same thing as oscillations. For your circuit to oscillate, there.must also be som inductance. If you apply a DC voltage to an ideal capacitor, with no inductance it will not oscillate and the magnetic field is irrelevant.

5. Apr 4, 2015

### Joker93

With DC current,before steady-state you will have a time changing current.So the induced magnetic field is not irrelevant.

6. Apr 4, 2015

### Staff: Mentor

But a magentic field has no effect on an ideal voltage source or an ideal capacitor. In that sense it is irrelevant.

But non-ideal wiring always has some nonzero inductance and capacitance. Is that what you are thinking of?

7. Apr 4, 2015

### Joker93

No,no.Forget the capacitor or battery.They are just there to give us the time changing current.Suppose you want to calculate the magnetic field using the Ampere-Maxwell law on a point of the wire.Then you find a time changing magnetic field.But that means(from the same law)that you will also have induced electric field that changes with time.And that causes yet another induction of magnetic field and this goes on and on.

8. Apr 13, 2015

### Battlemage!

Don't you also have to consider Lenz' Law here? So wouldn't the induced current and it's corresponding magnetic field produced oppose the original one, so a net field is not infinite?