# Suspended in air by adding electrons

1. Jan 22, 2006

### Gramma2005

I am stuck on this Physics problem and desperately need some help.

As a science project, you've invented an "electron pump" that moves electrons from one object to another. To demonstrate your invention, you bolt a small metal plate to the ceiling, connect the pump between the metal plate and yourself, and start pumping electrons from the metal plate to you. How many electrons must be moved from the metal plate to you in order for you to hang suspended in the air 2.10 below the ceiling? Your mass is 60 kg. Hint: Assume that both you and the plate can be modeled as point charges.

I am confused about how I am supposed to know the number of electrons they begin with for the point charge equation. E = K (q / r^2)

Any help would be greatly appreciated!

2. Jan 23, 2006

### durt

The magnitude of the force due to the electric charge will be

$$F = \frac{k Q^2}{r^2}$$

This is equal to your weight so

$$Q = r \sqrt{\frac{m g}{k}}$$

This yields the charge in Coulombs, so you need to convert to the number of electrons given the fact that one electron has a charge of 1.6*10^-19 C.

3. Jan 23, 2006

### Gramma2005

Thanks For The Help

That was the right answer. I was just missing the last part, converting it to electrons.