Fields is a core part of A-Level Physics. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
V (electric potential)Energy per unit positive charge.
V (grav potential)Energy per unit mass: V = -GM/r.
EquipotentialSurface where potential is constant.
Point charge VV = Q / (4π ε₀ r).
Potential gradientE = -dV/dr in electric field.
Work doneΔW = q ΔV (electric); m ΔV (gravity).
Coulomb's lawF = Q1 Q2 / (4π ε₀ r²).
Newton's lawF = G m1 m2 / r².
Inverse squareForce ∝ 1/r² in both fields.
ε₀Permittivity of free space, ≈ 8.85 × 10⁻¹² F/m.
GUniversal gravitational constant ≈ 6.67 × 10⁻¹¹ N·m²/kg².
PolarityCharges + or -; masses always positive.
FieldRegion where a force acts on a test particle.
gGravitational field strength: g = F / m.
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
In which direction does an electric field point near a positive charge?✗ Radially inwards because positive attracts negative. ✓ Radially outwards — the field is defined by the force on a positive test charge, which is repelled.
What does gravitational field strength at a point represent?✗ The total gravitational force at that point regardless of mass. ✓ The force per unit mass that would act on a test mass placed at that point.
If the distance between two masses doubles, what happens to the gravitational force between them?✗ It halves — gravitational force is inversely proportional to distance. ✓ It quarters — gravitational force is inversely proportional to the square of the distance.
Why is gravitational potential energy near a planet treated as negative?✗ Gravitational potential energy is negative because gravity always points downwards. ✓ Because the zero is defined at infinity; objects bound to the planet have lower (negative) potential energy than at infinity.
What does the spacing of field lines on a diagram indicate?✗ Field lines are equally important regardless of spacing — only the arrows matter. ✓ The strength of the field — closer lines mean a stronger field.
Practise Fields — free games
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