# Creating Electric Field

1. Jan 28, 2012

### bb.minhtri

Creating Magnetic Field

1. The problem statement, all variables and given/known data

Which of the following that creates a magnetic field?

A. A moving charge
B. An electric field that changes with time.
C. Both of them.

2. Relevant equations

3. The attempt at a solution
A is definitely correct, I wonder if B is correct as well. I can not figure out their relation(electric field and magnetic field). If possible, please provide me a formula.

Thanks a lot for your help:D

Last edited: Jan 28, 2012
2. Jan 28, 2012

### Simon Bridge

When you think back through your course, do you vary the electric field to generate a magnetic one or do you vary the magnetic field to generate an electric one?

I think this is the distinction they are looking for.

3. Jan 28, 2012

### kunguz

I believe B will also create a magnetic field perpendicular to the electric field. Here is my logic:

* When there is an alternating electric field, charges will change position to keep up with the polarity which is same with the condition a.
* As example: Assume you are charging a capacitor with AC; your capacitor will definitely drain current in steady state. But in DC case capacitor will only drain current at the time in between zero voltage on capacitor and DC voltage on capacitor.

You know the formula of the capacitor

I= C* dv/dt

So you know the relation between the current and the magnetic field: Or so to say the maxwell equations:

http://en.wikipedia.org/wiki/Magnetic_field#Maxwell.27s_equations

4. Jan 28, 2012

### Simon Bridge

The presence of a time-changing E does imply something about charges moving and so it would be accompanied by a magnetic field ... does that fit in with the level that the question is being asked?

That's the trick with these things.

5. Jan 28, 2012

### bb.minhtri

Thanks. I think I can get on with that explanation:D The electric field change due to charge movement.