# Homework Help: Why wire experiences force in magnetic field, when net charge is zero.

1. Apr 21, 2012

### ZedCar

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

In a current-carrying wire, the total net charge in the metal is zero. Why does the wire experience a force when placed in a magnetic field?

2. Relevant equations

3. The attempt at a solution

The reason for the force is the motion of the electrons, not their charge as such.
i.e. it's a relativistic effect due to their motion.

The current travelling in the wire creates its own magnetic field which interacts with the external one. Depending on the relative angles of the fields, there will be a resulting force acting on the wire.

2. Apr 21, 2012

### tiny-tim

uhh? do magnetic fields interact?

use the force!

3. Apr 22, 2012

### ZedCar

Do you mean, just go with the first part?

i.e. The reason for the force is the motion of the electrons, not their charge. It's a relativistic effect due to their motion.

4. Apr 22, 2012

### tiny-tim

no, i mean the lorentz force!!

(is there any other? )

5. Apr 22, 2012

### Steely Dan

A common misconception people have is that if the net charge is zero, then that implies the net current density is also zero. But if you think about it for a moment, you'll see that this is not true; you can have electrons rushing by static protons at some net drift speed and still have the total number of electrons and protons in any unit of volume in the wire be equal to zero.

Is this the issue here, or are you asking why it is that a net current couples to a magnetic field in the same way that an electric field couples to a net charge?

6. Apr 23, 2012

### ZedCar

Do you mean none of my original answer in the first post is particularly good and the reason is simply due to the Lorentz Force?

7. Apr 23, 2012

### tiny-tim

erm … yes

8. Apr 23, 2012

Okay