A Level Physics: Electric Fields — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Electric Fields revision notes

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Questions

1. Define electric field strength. [2]

2. State Coulomb’s law and compare it with Newton’s law of gravitation, giving one similarity and one difference. [4]

3. Two parallel plates 8.0 mm apart have a p.d. of 400 V.

(a) Calculate the electric field strength. [2] (b) Calculate the force on an electron in this field. (e = 1.60 × 10⁻¹⁹ C) [2] (c) Calculate the acceleration of the electron. (m = 9.11 × 10⁻³¹ kg) [2]

4. An electron enters the field of question 3 travelling parallel to the plates.

(a) Describe and explain the shape of its path. [3] (b) (Cross-topic comparison — magnetic fields are covered in Magnetic Fields , not in this resource.) Explain how this compares with the path of an electron entering a uniform magnetic field perpendicular to the field lines. [3]

5. Two point charges of +3.0 nC and −5.0 nC are 0.12 m apart. (1 ÷ 4πε₀ = 8.99 × 10⁹ N m² C⁻²)

(a) Calculate the force between them. [3] (b) State whether it is attractive or repulsive. [1]

6. Compare electric and gravitational fields on: the nature of the force, the sign of the potential, and the SI unit used for field strength in each case. [3]

7. Calculate the electric field strength 0.15 m from a point charge of +4.0 × 10⁻⁶ C. (1 ÷ 4πε₀ = 8.99 × 10⁹ N m² C⁻²) [3]

8. Calculate the electric potential at a point 0.20 m from a point charge of −6.0 × 10⁻⁶ C, and explain the significance of the negative sign in your answer. [3]

9. State the direction convention used for electric field lines, and explain what it means when field lines are drawn closer together in one region than another. [2]

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Answers

1. The force per unit positive charge [1] at a point in the field, E = F ÷ Q [1].

2. The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation [1] [1]. Similarity: both obey an inverse square law [1]. Difference: gravitational force is always attractive, whereas electric force can be attractive or repulsive [1].

3. (a) E = V ÷ d = 400 ÷ 8.0 × 10⁻³ [1] = 5.0 × 10⁴ V m⁻¹ [1]. (b) F = EQ = 5.0 × 10⁴ × 1.60 × 10⁻¹⁹ [1] = 8.0 × 10⁻¹⁵ N [1]. (c) a = F ÷ m = 8.0 × 10⁻¹⁵ ÷ 9.11 × 10⁻³¹ [1] = 8.78 × 10¹⁵ m s⁻² [1].

4. (a) A parabola [1]. The electric force acts in a constant direction, perpendicular to the initial velocity [1], giving uniform acceleration in that direction while the parallel component of velocity is unchanged — exactly like projectile motion [1]. (b) In a magnetic field the path is a circle [1], because the magnetic force is always perpendicular to the velocity [1] and so continually changes direction, acting as a centripetal force rather than a force of fixed direction [1].

5. (a) F = (1 ÷ 4πε₀) × Q₁Q₂ ÷ r² [1] = 8.99 × 10⁹ × (3.0 × 10⁻⁹ × 5.0 × 10⁻⁹) ÷ (0.12)² [1] = 8.99 × 10⁹ × 1.5 × 10⁻¹⁷ ÷ 0.0144 = 9.36 × 10⁻⁶ N [1]. (b) Attractive — the charges are opposite [1].

6. Force: gravitational is always attractive; electric can be attractive or repulsive [1]. Potential: gravitational potential is always negative; electric potential can be positive or negative [1]. Unit: electric field strength is measured in N C⁻¹ (equivalently V m⁻¹); gravitational field strength is measured in N kg⁻¹ [1].

7. E = (1 ÷ 4πε₀) × Q ÷ r² [1] = 8.99 × 10⁹ × (4.0 × 10⁻⁶) ÷ (0.15)² [1] = 1.60 × 10⁶ N C⁻¹ [1].

8. V = (1 ÷ 4πε₀) × Q ÷ r [1] = 8.99 × 10⁹ × (−6.0 × 10⁻⁶) ÷ 0.20 = −2.70 × 10⁵ V [1]. The negative sign shows the potential is negative because the source charge is negative — work would be done by the field (not against it) in bringing a positive test charge from infinity to this point [1].

9. Field lines point in the direction of the force on a positive test charge placed in the field [1]. Lines drawn closer together indicate a stronger field in that region [1].

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Where marks are usually lost

• Forgetting to convert mm to m for plate separation.
• Saying a magnetic field gives a parabolic path.
• Not stating both the similarity and the difference when comparing with gravity.
• Omitting the sign or direction when asked whether a force is attractive.
• Forgetting that electric field strength and electric potential from a point charge use different formulas (E has r², V has r) — mixing them up is a common slip under time pressure.
• Dropping the sign of the source charge when calculating potential, or misreading what a negative potential means physically.

Work through the Electric Fields revision notes alongside these questions: the notes summarise the equations and the radial-versus-uniform field distinction in condensed form, while these questions test whether you can apply the point-charge formulas correctly and interpret the sign of a calculated potential, rather than just recall the equations.