# Homework Help: What statement about magnetic fields interacting with a moving charge is correct?

1. Apr 20, 2016

### Sgtsloth

• Member advised to use the homework template for posts in the homework sections of PF.
What statement about magnetic fields interacting with a moving charge is correct?
A) Magnetic fields can’t change the direction of motion of a moving charge.
B) Magnetic fields can do work on a moving charge.
C) Magnetic fields can accelerate a moving charge.
D) The force on a moving charge in a magnetic field can be in the direction of the magnetic field.
E) Magnetic fields can increase or decrease the kinetic energy of a moving charge.

I know B) isn't correct, since the angle leads to any work being zero., although others I have read online says that it can do work so I don't know what to think.

C) I don't think this can be true, since if you can't do work how can you cause acceleration?

D) is false for sure based on the right hand rule

E) Can KE change without work

2. Apr 20, 2016

### Staff: Mentor

For (C) consider the case of circular motion. Is work being done? Is there acceleration involved?

3. Apr 20, 2016

### Sgtsloth

I known that a varying magnetic field causes a an electric field that can do work, is that what you mean?

4. Apr 20, 2016

### Staff: Mentor

No. I'm saying that you should consider the example of circular motion: an object moving in a circle at a constant speed. Is there acceleration involved?

5. Apr 20, 2016

### Sgtsloth

Constant speed means there is no acceleration, thus C) is false. Am I correct that B is also wrong?

6. Apr 20, 2016

### Sgtsloth

7. Apr 20, 2016

### Staff: Mentor

No, that's incorrect. Is acceleration defined as a rate of change in speed or a rate of change change in velocity? Is velocity constant for circular motion?
Yes.

8. Apr 20, 2016

### Staff: Mentor

Sorry, but to be frank, my schedule is split between volunteer work here and other unrelated matters. All helpers are volunteers. We do what we can.

9. Apr 20, 2016

### Sgtsloth

Thank you

Okay so Velocity changes direction in circular motion.

Let me go through my thinking.

(A) Mag fields CAN change direction, look at the mass spectrometer for example.
(B) We established is incorrect
C) The one in question.
D) Should be false based on the the right hand rule,
E) If no work can be done by a mag field (B), then it shouldn't be able to change a charge's KE.

Am I missing anything?

10. Apr 20, 2016

### Staff: Mentor

Looks like you've covered it.