Coulomb’s Law problems

Question

1.Determine the factor of change for the electric force if the distance is changed by a factor of 4 and the charges of both particles are changed by a factor of 2

2.A charge of 0.004 C is located 5.0 m from a charge of 0.030 C. What is the electric force experienced by the two charges?

3.A raindrop acquires a charge of 1 x 10^-18 C as it falls. The force of attraction when the raindrop is 3.2 m from the bulb on the end of a car antenna is 2 x 10^-11 N. What is the charge on the end of the car antenna?

Answer 1

For these problems, we use Coulomb's Law, which give the force between 2 electrically-charged objects.

F = k *q1*q2 /r²

k is Coulomb's constant, which is 9*10⁹. Units are Newtons, Meters and Coulombs (the unit of charge)

In any of these problems, you will simply plug in the numbers and solve the equation.

Note the notation for exponents: 9E9 means 9*10⁹ . Hint for working with exponents: first do the basic arithmetic ignoring the exponents, then simply ADD all the exponents. If you are working with a calculator, be careful and check your answers.

For problem involving proportions, simply look at the basic formular to see the effect of changing a parameter. Example, if the distance is doubled, the force is reduced by a factor of 4. All the other terms have a linear relationship---but, if both charges are increased by the same factor, then the force is increased by the square of that factor.

#1: the problem does not state if the parameters are increased or decreased. Let's assume they are increased. If both charge have N times more charge, and the distance is N times larger--the effects cancel (N² / N² = 1)

#2: just plug in the numbers:

F = 9E9 * 4E-3 * 3E-2 / 5² = 4.32E4