# Charge moving from a wire

1. Jun 18, 2013

### aChordate

1. The problem statement, all variables and given/known data
A charge q+ 4.5 x 10^-9 is located a distance of 7.0 mm to the right of a wire and is moving directly away from the wire with a velocity of v =3.0 x 10^4 m/s, as shown in the drawing (drawing shows charge moving to the right and current moving upwards). The wire carries a current I= 2.2 A. What is the force (magnitude and direction) felt by the charge due to the wire?

2. Relevant equations

B=F/ (|q|vsinθ)

3. The attempt at a solution

According to the right hand rule, the Force would be pointing downward.

2. Jun 19, 2013

### rude man

That is correct.

3. Jun 19, 2013

### aChordate

B=F/ (|q|vsinθ)

B= F / (4.5x10^-9)*(3.0x10^4)*(sin 90)

I am not sure how to find the magnetic field. I would use the current I = 2.2 A? And is sin90 correct?

4. Jun 19, 2013

### rude man

Sin(90) is correct.

5. Jun 19, 2013

### TSny

Are you saying that the force would be in the opposite direction of the current in the wire? I don't think that's correct.

6. Jun 19, 2013

### rude man

The charge is moving in the +x direction and the current is flowing in the +y direction, so at x > 0 the B field is in the -z direction:

v x B =+i x (-k) = +j rats! TSny is right, it flows same direction as the current. OP take note ...

7. Jun 20, 2013

### aChordate

So, If I use ampere's law:

ΔB||*Δl=μ0I

What do I use for Δl? 7.0mm?

8. Jun 20, 2013

### rude man

You should look up ampere's law. No, it's not 7mm.

9. Jun 20, 2013

### aChordate

That's the equation I have in my textbook.

10. Jun 20, 2013

### rude man

I don't think so.
Ampere's law integrates around a closed path, not along a radius.

11. Jun 20, 2013

### aChordate

I don't have radius in the equation, I'm confused.

12. Jun 20, 2013

### rude man

Yes you do. 7mm is the radius of a circle surrounding the wire. You're supposed to integrate around the circle.

BTW that's the problem with your other post so I will not answer that one again until you're clear on this point.