1. May 28, 2005

### shikagami

1. A charged particle moves in a straight line through a certain region of space. The magnetic field in that region
a) has a magnitude of zero
b) has a zero component perpendicular to the particle's velocity
c) has a zero component parallel to the particle's velocity

2. The north-pole end of a bar magnet is held near a stationary positively charged piece of plastic. Is the plastic
a) attracted
b) repelled
c) unaffected by the magnet

The answers are 1) either a or c, 2) c

I don't understand why. Can someone explain.

2. May 28, 2005

### dextercioby

I'd say it's "a" and "b" for #1.

HINTS:1.What's the expression of the magnetic Lorentz force and what kind of
mathematica operation does it contain?
2.For the second problem,what's the direction of the magnetic field...?

Daniel.

3. May 28, 2005

### shikagami

for 1. I don't know what Lorentz force is
2. The direction of the magnetic field should be towards the south-pole end.

4. May 28, 2005

### OlderDan

Lorentz Force is the name of the the force acting on a charged particle in an electric and magnetic field. Only the magnetic field is involved in this problem. Only the component of the field perpendicular to the velocity results in a force. That's why the answer is a or b and not a or c.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html

For the second one, the important thing is that there is no relative motion of the charges and the magnet, so relative to the magnetic field, v is zero.

5. May 29, 2005

### whozum

For number 1, 'b' would be the correct answer. As having 'no magnetic field' is a case of having 'no magnetic field in the perpendicular direction' when considered in context of the question and its answers.

6. May 29, 2005

### OlderDan

Good point. a) might be true, but it is not certainly true. Same can be said about c). b) has to be true

Last edited: May 29, 2005
7. May 29, 2005

### whozum

The lorentz force is one of the first interactions between fields and particles learned in magnetism, it is also a very important one. You should know this one like the back of your hand.

Its also one of the easier ones :)

Good evening folks :)