# Velocity of an electron hw problem

1. Jan 18, 2010

### Wildcatfan

1. The problem statement, all variables and given/known data
What is the velocity of an electron in the hydrogen atom if the centripetal force is supplied by the Coulomb's force between the electron and proton?

2. Relevant equations
The radius of the electron orbit is 5.29 X 10^-11

3. The attempt at a solution
The answer in the back of the book is 2.19 X 10^6 m/s. I am not really sure how to start this problem since it only gives me the radius. Please help!

2. Jan 18, 2010

### Stonebridge

Do you know the formula for centripetal force F, on a mass m, moving with velocity v, in a circle of radius r? (v is what you must calculate so you must know all the other quantities.)
You need to know the mass of the electron. Is it given?
You also need to know the formula for the coulomb force F. Do you know it?

3. Jan 18, 2010

### rock.freak667

You will also need

$$F=\frac{Q_1Q_2}{4 \pi \epsilon_0 r^2}$$

$$F=\frac{mv^2}{r}$$

Start by first finding the Coloumb's force.

4. Jan 18, 2010

### Wildcatfan

Thanks for the help! I went back in the chapter and found the mass of the electron (don't know why this was not presented in the problem), used Coulomb's law to get the force, then found the velocity solving the square root of force x radius/mass. Thanks again!