# Homework Help: Particle in space

1. Jun 18, 2015

### Fluxxx

1. The problem statement, all variables and given/known data
A charged particle, passing through a certain region of space, has a velocity whose magnitude and direction remain constant. If it is known that the external magnetic field is zero everywhere in this region, can you conclude that the external electric field is also zero?

2. Relevant equations
F=ma

3. The attempt at a solution
Constant velocity means no acceleration is occurring. No accelerations means that the sum of the external forces acting on the mass is zero. That can mean either no forces are acting on it, or that forces are acting on it, but they exactly cancel each other.

So in this case we have no magnetic field, but possibly an electric field. There might also be a gravitational field. So it is then possible that the gravitational force and the electric force exactly cancel each other out, so that the sum of the forces are zero. So then answer to the question then is "no".

Would you agree with my reasoning?

2. Jun 18, 2015

### RyanH42

I think the answer is yes.Cause the gravitational force is lower then electromagnetic force.$\vec{F}=Mmg/r^2$.Its a particle so I think the mass of particle will be very low.Then the force will be low.If we consider this situation the anwer will be yes.

3. Jun 18, 2015

### haruspex

But you don't know an upper bound on the other mass.

4. Jun 18, 2015

### RyanH42

I thought that way cause.If we consider sun and an electron the force will be very small like 10-11 ,$r=1$.If we think larger masses we need to thing something like black hole-electron maybe a neutron star electron.But I dont think that neutron star will pull just one electron.Maybe it does I am not sure.

5. Jun 18, 2015

### Wily Willy

I would say you are definitely correct to say no. Cathode ray tube televisions and ink jet printers balance gravity against EM all the time. What I am really curious about is how the moving charged particle is not creating its own magnetic field.

6. Jun 18, 2015

### SammyS

Staff Emeritus
OP says the external magnetic field is zero.

7. Jun 19, 2015

### Fluxxx

This topic has 127 views. Any more thoughts? The reason that I'm asking is that the answer given in my textbook is simply a "Yes.", but no explanation whatsoever is given. Also, if the answer is yes, that would mean that my answer is incorrect, but my answer is based on my reasoning, and I don't see how my reasoning would be incorrect.

8. Jun 19, 2015

### haruspex

I don't think you were intended to invoke other forces, such as gravitation.

9. Jun 20, 2015

### RyanH42

This post and my posts makes the answer.