# Magnetic fields and a stream of protons

1. Mar 14, 2005

### TickleMeElma

I have tried everything to solve this problem, and I just cannot come to a conclusion. If anyone finds it comes easy to them, help will be greatly appreciated. :)

Here goes:
A straight stream of protons passes a given point in space at a rate of 1.5x10^9 protons/s. What magnetic field do they produce 2.0m from the beam?

Thanks so much!!

2. Mar 14, 2005

### Gokul43201

Staff Emeritus
What are some of the things you've tried ?

3. Mar 14, 2005

### TickleMeElma

Ok, so I am not sure if this is right, but 1.5x10^9 is also the current, I. So the other variables that are given are r and the permeability of free space constant, 4(pie)x10^-7. So I plugged all that into the equation for B, the magnetic field, which is
(perm. constant)xI/2(pie)r

thanks :)

4. Mar 14, 2005

### Gamma

This is not the current. This is the number of proton crossing this point per second. Use the fact that I = dQ/dt to find the current. Here Q is the total charge that cross this particular point.

5. Mar 14, 2005

### TickleMeElma

Thanks, that's what I did. I multiplied the total charge of a proton by the number of those crossing that point per second.