AQA A Level Physics
AQA A Level Physics 3.7 Fields Exam Questions
Free AQA A Level Physics exam questions for 3.7 Fields, with a mark scheme for every pack. Covers gravitational and electric fields, orbits, capacitance, capacitor discharge, magnetic forces on charges and electromagnetic induction.
3.7 Fields exam questions
3 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Physics 3.7 Fields
Fields is AQA section 3.7 and is examined on Paper 2. It starts with the general idea of a field (3.7.1), then covers gravitational fields (3.7.2) and electric fields (3.7.3), which share the same mathematical structure: field strength, potential, inverse square laws and work done moving masses or charges. Capacitance (3.7.4) covers energy stored, dielectrics and exponential charge and discharge. Magnetic fields (3.7.5) covers force on a current and on moving charges, magnetic flux, electromagnetic induction, alternating currents and transformers. Examiners like comparing gravitational and electric fields, orbit calculations, capacitor time constant graphs and the force on a current-carrying wire required practical.
What you need to know
- 3.7.2 Gravitational fields Newton's law F = Gm1m2/r^2, field strength g = F/m = GM/r^2, potential V = -GM/r and g = - delta V / delta r. Work done = m delta V. Equipotentials are perpendicular to field lines.
- 3.7.2 Orbits Gravity provides the centripetal force, giving T^2 proportional to r^3 (Kepler's third law). Geostationary satellites orbit above the equator with a 24 hour period. Escape velocity comes from 1/2 mv^2 = GMm/r.
- 3.7.3 Electric fields Coulomb's law F = Q1Q2/(4 pi epsilon0 r^2). Field strength E = F/Q, for a point charge E = Q/(4 pi epsilon0 r^2), and between parallel plates E = V/d. Potential V = Q/(4 pi epsilon0 r), and charged particles in a uniform field follow parabolic paths.
- 3.7.4 Capacitance C = Q/V, for parallel plates C = A epsilon0 epsilonr / d. Energy stored E = 1/2 QV = 1/2 CV^2 = 1/2 Q^2/C. Dielectric molecules are polarised and increase the capacitance.
- 3.7.4 Charge and discharge Discharge: Q = Q0 e^(-t/RC), and the same for V and I. Time constant RC is the time to fall to 37 percent; time to halve is 0.69RC. Charging a capacitor and plotting ln V against t is a required practical.
- 3.7.5 Magnetic fields Force on a wire F = BIl, on a moving charge F = BQv, causing circular motion with r = mv/BQ, used in cyclotrons. Flemings left-hand rule gives direction.
- 3.7.5 Induction and transformers Flux phi = BA, flux linkage N phi. Faraday's law: emf = N delta phi / delta t; Lenz's law gives the direction. Rotating coil emf = BAN omega sin omega t. Transformers: Ns/Np = Vs/Vp; know rms values and efficiency losses.
Common mistakes in 3.7 Fields
- Forgetting the minus sign in gravitational potential. Gravitational potential is always negative and is zero at infinity.
- Using the distance from the surface instead of the centre of the planet in GM/r^2. r is measured from the centre of mass.
- Saying the time constant is the time for a capacitor to fully discharge. It is the time to fall to 1/e (about 37 percent) of the initial value.
- Stating that a magnetic force does work on a charged particle moving in a circle. The force is perpendicular to the velocity so the speed and kinetic energy stay constant.
- Confusing flux density with flux linkage. B is in tesla, flux is in Wb (BA) and flux linkage is in Wb turns (BAN).
Try three 3.7 Fields exam-style questions
1. A 470 microfarad capacitor discharges through a 10 kilohm resistor. Calculate the time constant and the time for the pd to fall to half its initial value. (3 marks)
Show the mark scheme answer
RC = 470 x 10^-6 x 10 x 10^3 = 4.7 s; V = V0/2 so t = RC ln 2; t = 4.7 x 0.693 = 3.3 s.
2. Show that the orbital period of a satellite is related to its orbital radius by T^2 proportional to r^3. (3 marks)
Show the mark scheme answer
Gravitational force provides the centripetal force: GMm/r^2 = mv^2/r; v = 2 pi r / T so GM/r^2 = 4 pi^2 r / T^2; T^2 = (4 pi^2 / GM) r^3, and 4 pi^2/GM is constant.
3. An electron enters a uniform magnetic field at right angles. Explain why it moves in a circle and how the radius depends on its speed. (3 marks)
Show the mark scheme answer
The force BQv is always perpendicular to the velocity; this provides a centripetal force so the path is circular at constant speed; BQv = mv^2/r gives r = mv/BQ so the radius is proportional to speed.
3.7 Fields FAQs
What does AQA A Level Physics 3.7 Fields cover?
It covers gravitational and electric fields and potentials, orbits, capacitance with charging and discharging, magnetic forces on wires and charges, electromagnetic induction, alternating current and transformers.
Which paper is 3.7 Fields examined in?
Fields is assessed on Paper 2, and the capacitor and magnetic force required practicals can also appear on Paper 3.
How should I revise gravitational and electric fields together?
Make a two-column table comparing force, field strength, potential and work done for masses and charges. The equations have the same inverse square form, but gravitational forces are only attractive while electric forces can repel.
