Suppose a speck of dust in an electrostatic precipitator has 1.×10^12 protons in it and has a net charge of –5.00 nc (a very large charge for a small speck). how many electrons does it have?

Explanation:

The net charge is given by the sum of the charges of the protons and of the electrons:

where we have

is the net charge

is the number of protons

is the charge of one proton

is the number of electrons

is the charge of one electron

Solving the equation for , we find the number of electrons:

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