# Hey I was wondering why I got this problem wrong on magnetic fields

1. Apr 18, 2012

### chrissy2860

1. The problem statement, all variables and given/known data
An electron and a proton each travel with equal speeds around circular orbits in the same uniform magnetic field ( I tried to attach a diagram...not sure if it worked but I've described it anyways). The field is into the page on the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged:
a.)the electron travels clockwise around the smaller circle and the proton travels counter-clockwise around the larger circle.
b.)the electron travels counter-clockwise around the smaller circle and the proton travels clockwise around the larger circle.
c.)the electron travels clockwise around the larger circle and the proton travels counter-clockwise around the smaller circle.
d.)the electron travels counter-clockwise around the larger circle and the proton travels clockwise around the smaller circle.
e.) the electron travels counter-clockwise around the smaller circle and the proton travels counter-clockwise around the larger circle.

2. Relevant equations
Right hand Rule

3. The attempt at a solution
I answered b because if you use right hand rule then the electron and proton are both going clockwise but the electron is negative so it switches directions and so the electron is going counter-clockwise and the proton is clockwise. He said that the answer is a but I don't understand why since the electron should have made it switch directions. I tried talking to him about it but he just confused me more.

Last edited: Apr 18, 2012
2. Apr 18, 2012

### George Jones

Staff Emeritus
For now, only consider the proton. The force on the proton is directed towards the centre of the circle, so the right-hand rule gives clockwise or counter-clockwise for the proton.

3. Apr 19, 2012

### phyzguy

I think this is incorrect. If the B field is into the page, the proton will always travel counter-clockwise, and the electron always clockwise. Try drawing it out. (a) is the correct answer.

chrissy2860: Look at it this way. If the proton velocity is initially up, the force (vxB, using the right hand rule) is to the left. This will cause the path to curve to the left, which bends the path in a counterclockwise direction. If the velocity is initially down, the force is to the right, which bends the path to the right, which is still in a counterclockwise direction. So the proton always circles counterclockwise. For the electron, everything is reversed, so it always circles clockwise.

Last edited: Apr 19, 2012
4. Apr 19, 2012

### PeterO

Right hand rule will show that if the Field is into the page, the proton will circle in an anti-clockwise direction.
Remember, the proton is not trying to create the field, it is being influenced by an already existing field.
Fingers in the direction of the field, thumb in the direction the proton is moving, "out of the palm" gives the direction of the induced force - and the direction to the centre of the circlular path the proton will follow.

5. Apr 19, 2012

### George Jones

Staff Emeritus
Oops, I meant to end my sentence with "?", not "." I was trying to get chrissy2860 to come with up counter-clockwise for the proton on her own, and thus lead her to (a), i.e., I meant to write