# Homework Help: Electron enters uniform magnetic field and takes circular path

1. Apr 24, 2014

### jamesrb

1. The problem statement, all variables and given/known data
A time t = 0 an electron enters a region of uniform magnetic field B = 0.010 T and has kinetic energy of 6.40E-16 J. It goes through a half-circle, exits the field and then accelerates across a gap with a potential difference of 2000 V, increasing in speed. It then hits a collecting cup. At what time t does it hit the cup?

2. Relevant equations
Δt=(1/2)T=(1/2)(2πm/|q|B)
x=v0t+(1/2)at2

3. The attempt at a solution
I can find the time to go through the half circle using the first equation above. It is 1.788E-9s. I need to find the time to get to the cup so I can add the two times up to get the total time.

2. Apr 24, 2014

### jamesrb

I have attached a diagram of the problem.

#### Attached Files:

• ###### ElectronCup.png
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3. Apr 24, 2014

### BiGyElLoWhAt

I think you need to find the radius of the circle using the force of the field on the electron. That will let you get the arc length.

Remember, B fields do no work.

Just out of curiosity, what is T in you're equation ?

One more thing, I don't believe you can solve this without knowing the size of the gap, or at least that it's negligbly small.

4. Apr 24, 2014

### BiGyElLoWhAt

A) What is a Volt?
B) How does a Volt relate to distance and time? (Hint* express a Volt in more fundamental units)

answer these 2 questions and I think you can handle this question.

5. Apr 24, 2014

### jamesrb

The gap from when the electron leaves the B field to the cup is 25 cm, its in the diagram. T is the period. I don't think the radius and arc length are needed.

6. Apr 24, 2014

### BiGyElLoWhAt

Yea I think your equation should work, but I'm sticking to what I said about expressing Volts in fundamental units.