Question

For an object that weighs 1000 grams- to get it to levitate 15 cm above a negatively charged plate it requires extra electrons.
1) what charge is required to be added to this object?
2) how many electrons would this be?

the only information given for the plate is that it is negatively charged.

the only information given on the charge of the plate is that it's negatively charged.

Answer 1

Since the charge on plate is not given we assume it to be same as of the object. And object will bear negative charge as well to get repelled from the plate.

Also we assume that the charge and mass are distributed uniformly on the object and on plate as well. With this assumption the object can be of any shape but we can assume all of its charge and mass to be located at centre of gravity.

Part (a)

To levitate the object 15cm above the plate, the electrostatic repulsive force between two is being balanced by the gravitational pull.

i.e. ..... (1)

Where

And

Therefore, from equation (1),

or

Therefore, both plate and object will bear of charge.

Part (b)

We know, .......(2)

Where is number of electrons

And is electronic charge.

Hence, electrons will make   of charge.