# Cuurent in straight wire and direction of magnetic force

1. Jul 26, 2010

### tbaguy

1. The problem statement, all variables and given/known data
The current in a straight wire is traveling from left to right (in the plane of a page).
Right above the wire is a electron which is moving parallel to the wire and also left to right
The question is which direction is the magnetic force above the wire pointing.
Possible answers from Princeton mcat review are: a) up the plane of the page b) into the page c) out of the page d) down the plan of the page

author 's answer is a. up the plane of the page

I think he is wrong and the answer should be d) down the page

Can someone tell if the author is right and why.

2. Relevant equations

left hand rule since there is a electron

3. The attempt at a solution

If you curl your left hand around the wire, your thumb should point to the left for the direction of the current. Your finger, which is the magnetic field should be in the page. therefore the magnetic force which is indicated by palm of your left hand should point down the plane of the page
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 26, 2010

### Swap

no the author is correct. For a negatively charged particle the direction of magnetic force is opposite to the positively charged particle. It will be down only when the particle is negative. F= qbv. here the sign of q also matters...

3. Jul 26, 2010

### Onamor

Why do you use your left hand?
Have you remembered the difference between Current and Conventional Current?

I use my right and agree with their answer:
right hand grip (thumb points right) gives the field at the electron as out of the page;
right hand rule gives force on electron as upwards.

4. Jul 26, 2010

### Swap

I think u should rethink on the current and conventional current. U can assume the electron moving from the left to right as the a conventional current from right to left. Now there are two wires with conventional current passing in the opposite direction. So they will repel . So the electron will move up.