# Finding a B-field.

1. Aug 25, 2007

### Bert

given is:
http://img340.imageshack.us/img340/6684/oplosqa0.jpg [Broken]

So if you have a question with a B-field you need any speed given from the particles who entering in this field.
Who can I find this speed? Thanks.

Last edited by a moderator: May 3, 2017
2. Aug 25, 2007

### learningphysics

Get the speed from the kinetic energy 100keV.

Last edited by a moderator: May 3, 2017
3. Aug 25, 2007

### Bert

I try this but I fail so:

$$\frac{1}{2}m . v^2=U$$
then we have
$$1,602 \ . \ 10^{-19} \ . \ 1 . \ 10^5=1,602 \ . \ 10^{-14}= \frac{1}{2} 10^{-27}.v^2$$

so $$v$$ must be $$\sqrt{ \frac{2 \ . \ 1,602 . 10^{-14} } {10 ^{-27} } }$$

is this correct?

4. Aug 25, 2007

### learningphysics

mass of a proton = $$1.67*10^{-27}kg$$. Other than that everything looks good.

5. Aug 25, 2007

Thanks.