P.3.7.3.1 Coulomb's Law

The diagram shows the path of an α particle deflected by the nucleus of an atom. Point P on the path is the point of closest approach of the α particle to the nucleus. Which of the following statements about the α particle on this path is correct?

• Its acceleration is zero at P.

• Its kinetic energy is greatest at P.

• Its potential energy is least at P.

• Its speed is least at P.

The diagram shows two particles at distance d apart. One particle has charge +Q and the other –2Q. The two particles exert an electrostatic force of attraction, F, on each other. Each particle is then given an additional charge +Q and their separation is increased to distance 2d. Which of the following gives the force that now acts between the two particles?

• an attractive force of F/4

• a repulsive force of F/4

• an attractive force of F/2

• a repulsive force of F/2

A +2.0 nC point charge X is at a fixed distance from a –1.0 nC point charge Y. The force between the two charges is F. If an additional charge of +2.0 nC is supplied to both X and Y, which line gives the magnitude and direction of the force on X?

• Magnitude: 2 F

  Direction: from X to Y

• Magnitude: 4 F

  Direction: from X to Y

• Magnitude: 2 F

  Direction: from Y to X

• Magnitude: 4 F

  Direction: from Y to X

The gravitational constant, G, is a constant of proportionality in Newton’s law of gravitation. The permittivity of free space, ε₀, is a constant of proportionality in Coulomb’s law.

When comparing the electrostatic force acting on a pair of charged particles to the gravitational force between them, the product ε₀G can appear in the calculation.

Which is a unit for ε₀G?

• C² kg^–2

• C² m^–2

• F kg² N^–1 m^–2

• it has no unit

Two point charges, X and Y, exert a force F on each other when they are at a distance d apart.

When the distance between them is 20 mm, the force they exert on each other is 0.5 F.

What is the distance d?

• 7 mm

• 14 mm

• 15 mm

• 28 mm

Two identical positive point charges, P and Q, separated by a distance r, repel each other with a force F. If r is decreased so that the electrical potential energy of Q is doubled, what is the force of repulsion?

• 0.5 F

• F

• 2F

• 4F

The force between two point charges is F when they are separated by a distance r.

If the separation is increased to 3r, what is the force between the charges?

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A repulsive force F acts between two positive point charges separated by a distance r. What will be the force between them if each charge is doubled and the distance between them is halved?

• F

• 2F

• 4F

• 16F

A repulsive force F acts between two positive point charges separated by a distance r.

What will be the force between them if each charge is doubled and the distance between them is halved?

• F

• 2F

• 4F

• 16F

The repulsive force between two small negative charges separated by a distance r is F. What is the force between the charges when the separation is reduced to r/3 ?

• F/9

• F/3

• 3F

• 9F