# Magnetic Fields- Finding the radius

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1. Apr 17, 2017

### Studentphysics

1. The problem statement, all variables and given/known data
A single ionized uranium ion of mass 6.9 x 10 ^-25 kg is accelerated through a potential difference of 4.4 x 10^5 V.
What is the radius of the path it would take if injected at 90 degrees into 0.47 T uniform magnetic field at this velocity?

3. Attempts

Since centripetal force is equal to magnetic force, I tried using the formula of m(v^2)/r = Bqv

By rearranging this, I was able to isolate for the radius mv/Bq = r

I used these values, in the the formula to find the radius
(4.4 x 10^5 V)(6.9 x 10 ^-25 kg) / (0.47T)(1.6 * 10^-19)

The answer I got was 4.037 m, however this is wrong because the real answer is 4.14m. I'm not sure what I am doing wrong, because the two values seem really close

Thank you

2. Apr 17, 2017

### kuruman

It's probably a round off issue. How did you find v?

3. Apr 17, 2017

### Studentphysics

I used the potential difference of 4.4 x 10^5 V as my V

4. Apr 17, 2017

### kuruman

That's a problem. The "v" in qvB and in mv2/r is a speed, not a potential difference. You need to find the speed from the potential difference. Hint: Use energy conservation.