# Energy in magnetic field

1. Jul 12, 2006

### quasar987

Hi all, I have a question.

Suppose you have a device that makes a current pass through a linear "rod":

I -->
(+)---------------------(-)

Now say you start diminishing the current at a rate -dI/dt. Does an induced electric field appears in the direction of the current?

2. Jul 12, 2006

### tommyers

Hi,

By device do you mean a current source? i.e. a battery or such like?

The electric field will appear radially outwards from the 'rod'.

Regards

Tom

3. Jul 12, 2006

### quasar987

I was thinking more along the lines of a long straw and someone hired to blow charges in it.

When we have a current loop of inductance L and you diminish the current at a rate of -dI/dt, the magnetic flux thorugh the loop changes, and that induces an electric field which acts as an electromotive force of magnitude LdI/dt. Does the analogue happens with a linear curent? I.e. is there an induced electric field in the direction of the current?

Last edited: Jul 12, 2006
4. Jul 12, 2006

### pallidin

The answer is no. The induced current from the collapsing magnetic field is OPPOSITE of the direction of the electrical current in the wire which created the field. This is called inductive impeadance.

5. Jul 13, 2006

### abc33333333

1) I produces a B ( except I=0 )
2) Dimishing I ( that means too : dimishing B ) pruduces an E
at EVERY point in space, specially nearly at once near the current.

BUT REMEMBER : Charges can also be Photons ( QED ) without charge.

Appendix:
I said, WeizĂ¤cker's Theory is wrong, but the American Mr. Jearl Walker says, he was right.

6. Jul 13, 2006

### quasar987

Thanks abc33333333, the contrary would have surprised me. In fact, your confirmation gave me the courage to find exactly why it is true from the maxwell's equations. From the geometry of the electrostatic field of the straight wire and the geometric definition of curl, I found that when B changes, the electric field must have a component in the direction parallel (or antiparallel) to the current, and that component must be bigger near the wire.

I was gonna follow that question with one concerning the energy in the magnetic field but I found a satisfying answer.

7. Jul 13, 2006

### lalbatros

quasar987

That's why inductance in a circuit create larger sparks when the circuit is cut.
The induced emf opposes the current reduction, fortunately for our world.

Michel

8. Jul 13, 2006

### quasar987

why do you say "fortunately" ?