Edexcel A Level Physics
Edexcel A Level Physics Topic 7: Electric and Magnetic Fields Exam Questions
Free Edexcel A Level Physics exam questions for Topic 7: Electric and Magnetic Fields, with a mark scheme for every pack. Covers uniform and radial electric fields, Coulomb's law, capacitors and exponential discharge, magnetic forces, electromagnetic induction and alternating current.
Topic 7: Electric and Magnetic 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 Edexcel A Level Physics Topic 7: Electric and Magnetic Fields
Topic 7: Electric and Magnetic Fields in Edexcel A Level Physics has three strands: electric fields, capacitance, and magnetic fields with alternating current. You work with uniform fields between plates and radial fields from point charges, then capacitors, their stored energy and exponential charge and discharge. The magnetic section covers forces on currents and moving charges, Faraday's and Lenz's laws and r.m.s. values. The core practical uses an oscilloscope or data logger to analyse the p.d. across a charging and discharging capacitor. It is assessed in Paper 1: Advanced Physics I and links strongly with Topic 8 on particle accelerators.
What you need to know
- Electric fields E = F/Q; uniform field E = V/d; radial field E = kQ/r^2 with k = 1/4πε0. Coulomb's law F = kQ1Q2/r^2 and potential V = kQ/r. Equipotentials are perpendicular to field lines.
- Capacitance C = Q/V, energy W = ½QV = ½CV^2 = ½Q^2/C. Discharge: Q = Q0 e^(-t/RC), with time constant RC; ln Q against t is a straight line of gradient -1/RC.
- Magnetic forces F = BIl sin θ for a conductor and F = Bqv sin θ for a moving charge. Fleming's left-hand rule gives direction.
- Magnetic flux Flux Φ = BA cos θ and flux linkage NΦ.
- Electromagnetic induction Faraday's law: ε = -d(NΦ)/dt. Lenz's law: the induced e.m.f. acts to oppose the change producing it, as required by conservation of energy.
- Alternating current Irms = I0/√2 and Vrms = V0/√2. R.m.s. values give the same power as the equivalent direct current.
Common mistakes in Topic 7: Electric and Magnetic Fields
- Using 1/r^2 for electric potential. Potential is kQ/r; field strength is kQ/r^2.
- Using W = QV for the energy stored on a capacitor. It is ½QV.
- Using the wrong hand rule. Use Fleming's left-hand rule for forces on currents.
- Leaving out N when calculating induced e.m.f. in a coil.
- Treating the time constant as the time for complete discharge.
Try three Topic 7: Electric and Magnetic Fields exam-style questions
1. Calculate the force between two charges of +3.0 μC and -2.0 μC separated by 0.15 m. (2 marks)
Show the mark scheme answer
F = kQ1Q2/r^2 = 8.99 × 10^9 × 3.0 × 10^-6 × 2.0 × 10^-6/0.15^2; F = 2.4 N (attractive).
2. A 220 μF capacitor is charged to 9.0 V and discharged through a 47 kΩ resistor. Calculate the time constant and the charge remaining after 15 s. (4 marks)
Show the mark scheme answer
RC = 47 × 10^3 × 220 × 10^-6 = 10.3 s; Q0 = CV = 220 × 10^-6 × 9.0 = 1.98 × 10^-3 C; Q = 1.98 × 10^-3 e^(-15/10.3); Q = 4.6 × 10^-4 C.
3. Use Faraday's and Lenz's laws to explain why a magnet dropped through a copper tube falls slowly. (4 marks)
Show the mark scheme answer
As the magnet falls, the flux linking the tube changes; an e.m.f. is induced so eddy currents flow in the copper; by Lenz's law these currents produce a magnetic field opposing the motion of the magnet; so there is an upward force and the magnet reaches a low terminal velocity.
Topic 7: Electric and Magnetic Fields FAQs
What does Edexcel A Level Physics Topic 7: Electric and Magnetic Fields cover?
It covers uniform and radial electric fields, Coulomb's law, capacitors and exponential discharge, magnetic forces, electromagnetic induction and alternating current.
Which paper is Topic 7 examined in?
It is assessed in Paper 1: Advanced Physics I, and like every topic it can also appear in Paper 3: General and Practical Principles in Physics.
Which core practical is in Electric and Magnetic Fields?
Using an oscilloscope or data logger to display and analyse the potential difference across a capacitor as it charges and discharges through a resistor.
