Electric Fields vs. Gravitational Fields

Four positive charges are fixed at the corners of a square as shown.

The total potential at the centre of the square, a distance d from each charge, is (5Q) / (4πε₀d).

Three of the charges have a charge of +Q. What is the magnitude of the fourth charge?

• -7Q/4

• Q

• √2Q

• 2Q

The diagram shows four point charges at the corners of a square of side 2a. What is the electric potential at P, the centre of the square?

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The diagram shows two charges, +4 µC and –16 µC, 120 mm apart. What is the distance from the +4 µC charge to the point between the two charges where the resultant electric potential is zero?

• 24 mm

• 40 mm

• 80 mm

• 96 mm

The electric potential at a distance r from a positive point charge is 45 V. The potential increases to 50 V when the distance from the charge decreases by 1.5 m. What is the value of r?

• 1.3 m

• 1.5 m

• 7.9 m

• 15 m

An electron and a proton are 1.0 × 10^–10 m apart. In the absence of any other charges, what is the electric potential energy of the electron?

• +2.3 × 10^–18J

• –2.3 × 10^–18J

• +2.3 × 10^–8J

• –2.3 × 10^–8J

Two charges, each of + 0.8 nC, are 40 mm apart. Point P is 40 mm from each of the charges.

What is the electric potential at P?

• zero

• 180 V

• 360 V

• 4500 V

When a charge moves between two points in an electric field, or a mass moves between two points in a gravitational field, energy may be transferred.

Which one of the following statements is correct?

• No energy is transferred when the movement is parallel to the direction of the field.

• The energy transferred is independent of the path followed.

• The energy transferred is independent of the start and finish points.

• Energy is transferred when the movement is perpendicular to the field lines.

The diagram shows a negative ion at a point in an electric field, which is represented by the arrowed field lines.

Which one of the following statements correctly describes what happens when the ion is displaced?

When the negative ion is displaced

• to the left the magnitude of the electric force on it decreases.

• to the right its potential energy increases.

• along the line PQ towards Q its potential energy decreases.

• along the line PQ towards P the magnitude of the electric force on it is unchanged.

Two horizontal parallel plate conductors are separated by a distance of 5.0 mm in air. The lower plate is earthed and the potential of the upper plate is +50 V.

Which line, A to D, in the table gives correctly the electric field strength, E, and the potential, V, at a point midway between the plates?

• A

• B

• C

• D

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