PhysicsHelp.co.uk

Gravitational and electric field questions

Distinguish field strength from potential; use centre distances, charge signs and circular orbits.

Practise

Point-charge field

A point charge is -6 μC. Find the magnitude of electric field strength 0.2 m from it in vacuum. Use k = 8.99 × 10⁹ N m² C⁻². Give your answer to 3 significant figures.

Treat the given values as exact unless an uncertainty is stated.

Use decimals or e notation, e.g. 4.2e-3. Enter the unit separately.

Hint

Use E = k|Q|/r². Convert μC to C; magnitude is positive.

Worked solution
  1. E = 8.99 × 10⁹ × 6 × 10⁻⁶/0.2² N C⁻¹.
  2. The field points towards the source charge.
  3. Answer: 1.35 × 106 N C⁻¹ (3 significant figures).
Another question
Question type: Mixed practice
Report a problem

The question reference is attached automatically. Please leave out names and contact details.

Measure radius from the centre

Outside a spherical planet, g = GM/r². For height h above a planet of radius R, use r = R + h. Gravitational field strength points towards the planet; its magnitude is positive. In a circular orbit, gravity provides centripetal force, giving speed √(GM/r).

Field strength and potential are different quantities

Gravitational potential is energy per unit mass: V = −GM/r, taking zero at infinity. It is measured in J kg⁻¹. Gravitational field strength is force per unit mass, measured in N kg⁻¹. Do not use the inverse-square field equation to calculate potential.

Keep the charge sign

For a point charge in vacuum, field magnitude E = k|Q|/r² and signed potential V = kQ/r. Field points away from positive charge and towards negative charge. Between parallel plates, E = ΔV/d away from the positive plate, ignoring edge effects. Force F = qE reverses direction for a negative charge.

What this practice covers

Selected spherical gravitational fields, circular orbital speeds, point-charge electric fields and potentials, and forces in uniform fields. No field superposition, magnetic fields, escape speed, equipotential plotting or charged-particle trajectories are included.

A-level relevance

AQA 7408: 3.7.2.1–4; 3.7.3.2–3 (selected); OCR A H556: 5.4.1–4; 6.2.1–4 (selected). Tasks cover the selected skills listed above.

AQA specification · OCR A specification

Related practice