18.3 Coulomb’s Law

For questions 25–27, suppose that the electrostatics force between two charges is F.

What will be the force if the distance between them is halved?

1. 4F
2. 2F
3. F/4
4. F/2

Which of the following is true about the gravitational force between the particles?

1. It will be 3.25×10⁻³⁸ F.
2. It will be 3.25×10³⁸ F.
3. It will be equal to F.
4. It is not possible to determine the gravitational force as the masses of the particles are not given.

1. What is the electrostatic force between two charges of 1 C each, separated by a distance of 0.5 m?
2. How will this force change if the distance is increased to 1 m?

(a) How strong is the attractive force between a glass rod with a $0.700 \mu \text{C}$ charge and a silk cloth with a $–0.600 \mu \text{C}$ charge, which are 12.0 cm apart, using the approximation that they act like point charges? (b) Discuss how the answer to this problem might be affected if the charges are distributed over some area and do not act like point charges.

Two point charges are brought closer together, increasing the force between them by a factor of 25. By what factor was their separation decreased?

Bare free charges do not remain stationary when close together. To illustrate this, calculate the acceleration of two isolated protons separated by 2.00 nm (a typical distance between gas atoms). Explicitly show how you follow the steps in the Problem-Solving Strategy for electrostatics.

(a) By what factor must you change the distance between two point charges to change the force between them by a factor of 10? (b) Explain how the distance can either increase or decrease by this factor and still cause a factor of 10 change in the force.

(a) Common transparent tape becomes charged when pulled from a dispenser. If one piece is placed above another, the repulsive force can be great enough to support the top piece’s weight. Assuming equal point charges (only an approximation), calculate the magnitude of the charge if electrostatic force is great enough to support the weight of a 10.0 mg piece of tape held 1.00 cm above another. (b) D…

A certain five cent coin contains 5.00 g of nickel. What fraction of the nickel atoms’ electrons, removed and placed 1.00 m above it, would support the weight of this coin? The atomic mass of nickel is 58.7, and each nickel atom contains 28 electrons and 28 protons.

Point charges of $5.00 \mu \text{C}$ and $–3.00 \mu \text{C}$ are placed 0.250 m apart. (a) Where can a third charge be placed so that the net force on it is zero? (b) What if both charges are positive?