P.3.7.3.2 Electric field strength

A conducting sphere holding a charge of +10 μC is placed centrally inside a second uncharged conducting sphere.

Which diagram shows the electric field lines for the system?

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A charged spherical conductor has a radius r. An electric field of strength E exists at the surface due to the charge. What is the potential of the spherical conductor?

• r² E

• r E²

• E/r

• r E

Two parallel metal plates of separation a carry equal and opposite charges. Which graph best represents how the electric field strength E varies with the distance x in the space between the two plates?

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An electron moves through a distance of 0.10 m parallel to the field lines of a uniform electric field of strength 2.0 kN C^–1. What is the work done on the electron?

• zero

• 1.6 × 10^–17 J

• 3.2 × 10^–17 J

• 1.6 × 10^–21 J

A parallel-plate capacitor is fully charged and then disconnected from the power supply. A dielectric is then inserted between the plates. Which row correctly identifies the charge on the plates and the electric field strength between the plates?

• Charge: Stays the same

  Electric field strength: Increases

• Charge: Increases

  Electric field strength: Decreases

• Charge: Increases

  Electric field strength: Increases

• Charge: Stays the same

  Electric field strength: Decreases

An electron on the surface of the Earth is placed in an electric field of strength 5000 N C⁻¹. What is the ratio shown for the electron?

• 1.1 × 10⁻¹⁴

• 2.9 × 10⁻¹⁰

• 3.4 × 10⁹

• 9.0 × 10¹³

An electron moving with constant speed enters a uniform electric field at right angles to the direction of the field. What is true about the force acting on the electron?

• It is at right angles to the direction of the field.

• It is in the opposite direction to the direction of the field.

• It causes the electron to continue in the same direction with its speed increasing steadily.

• It causes the electron to continue in the same direction with its speed decreasing steadily.

Two fixed parallel metal plates P and Q are at constant electrical potentials of +100 V and +70 V respectively. A proton travelling from P to Q experiences a force F due to the electric field between P and Q, and a change of potential energy of ΔE_p.

Which line, A to B, in the table gives the direction of F and the value of ΔE_p?

• Direction of F: towards P

  ΔE_p: +30 eV

• Direction of F: towards Q

  ΔE_p: +30 eV

• Direction of F: towards Q

  ΔE_p: -30 eV

• Direction of F: towards P

  ΔE_p: -30 eV

The diagram shows a small negative charge at a point in an electric field, which is represented by the arrowed field lines. Which of the following statements, about what happens when the charge is displaced, is correct?

When the negative charge 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 parallel metal plates are separated by a distance d and have a potential difference V across them. Which expression gives the magnitude of the electrostatic force acting on a charge Q placed midway between the plates?

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