# Magnetic fields

1. Apr 4, 2005

### formulajoe

an electron with velocity u = (3,12,-4) x 10^5 experiences no net force at a point in a magnetic field B = ((10,20,30)/ Find E at that point.

I found the magnetic force by u cross B. Set Fe equal to Fm. Divided by q to get E = u cross B. but im not getting the same answer as the back of the book. just need a confirmation.
im getting E = (4.4 x 10^7, -1.3 x 10^7, -6 x 10^6)
the book has the same number for the x direction, its just 4400.

Last edited: Apr 4, 2005
2. Apr 4, 2005

### dextercioby

$$m\vec{0}=q_{e}\left(\vec{E}+\vec{u}\times\vec{B}\right)$$

That's all u need.Use it propertly.

Daniel.

3. Apr 4, 2005

### asrodan

I came up with the same numbers. It's just a simple cross product, the answers in the book must be wrong.

4. Apr 4, 2005

### asrodan

Signs should be reversed though, from bringing E to the other side of the Lorentz force equation.

5. Apr 4, 2005

### dextercioby

Are u sure that $E_{x}$ is not $-4.4\cdot 10^{7} \frac{V}{m}$...?

Daniel.

Last edited: Apr 4, 2005
6. Apr 4, 2005

### formulajoe

what is that formula for?

7. Apr 4, 2005

### dextercioby

What formula?

Daniel.