CIE A Level Physics
CIE A Level Physics Electric Fields Exam Questions
Free CIE A Level Physics exam questions for Electric Fields, with a mark scheme for every pack. Covers electric field strength, uniform fields between plates, charged particle motion, Coulomb's law, electric potential and electric potential energy.
Electric Fields exam questions
1 question pack 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 CIE A Level Physics Electric Fields
Electric fields is Topic 18 of the Cambridge International AS & A Level Physics (9702) syllabus. It covers fields as regions where a charge experiences a force, uniform fields between parallel plates, and the radial field of a point charge described by Coulomb's law. You calculate field strength, potential and electric potential energy, and analyse the motion of charged particles such as electrons in a uniform field. The topic deliberately parallels Gravitational fields, and it links to Capacitance and Magnetic fields. Examiners often ask you to compare the two field types and to explain the parabolic path of an electron between plates.
What you need to know
- Field strength Electric field strength is force per unit positive charge, E = F/q, in N C^-1 (equivalent to V m^-1). Field lines point from positive to negative.
- Uniform fields Between parallel plates, E = ΔV/Δd. A charged particle entering at right angles to the field experiences a constant force and follows a parabolic path.
- Coulomb's law F = Q1Q2/(4πε0r^2) for point charges, where ε0 = 8.85 × 10^-12 F m^-1. A uniformly charged sphere behaves as a point charge at its centre. Field strength E = Q/(4πε0r^2).
- Electric potential Potential at a point is the work done per unit positive charge in bringing a small test charge from infinity to that point, V = Q/(4πε0r). Field strength is the negative of the potential gradient.
- Electric potential energy Ep = Qq/(4πε0r) for two point charges. The change in potential energy of a charge q moving through p.d. ΔV is qΔV.
- Comparison with gravity Both fields follow inverse square laws. Gravitational forces are always attractive, whereas electric forces can attract or repel, so electric potential can be positive or negative.
Common mistakes in Electric Fields
- Using the plate separation in mm in E = V/d. Convert to metres.
- Saying the field between parallel plates gets weaker further from a plate. The field is uniform, except near the edges.
- Using 1/r^2 for potential. Potential varies as 1/r, field strength as 1/r^2.
- Forgetting that an electron is negatively charged, so the force on it is opposite to the field direction.
- Adding field strengths from two charges as scalars. Field strength is a vector; potential is a scalar and can be added directly.
Try three Electric Fields exam-style questions
1. Two parallel plates 5.0 cm apart have a potential difference of 200 V. Calculate the force on an electron between the plates. (3 marks)
Show the mark scheme answer
E = V/d = 200/0.050 = 4000 V m^-1; F = eE = 1.60 × 10^-19 × 4000; F = 6.4 × 10^-16 N.
2. Calculate the electric potential at a distance of 0.10 m from a point charge of +2.0 nC. (2 marks)
Show the mark scheme answer
V = Q/(4πε0r) = 2.0 × 10^-9/(4π × 8.85 × 10^-12 × 0.10); V = 180 V.
3. An electron enters the region between two parallel plates at right angles to the field. Explain why its path is a parabola. (3 marks)
Show the mark scheme answer
The electric force is constant and perpendicular to the initial velocity; so there is constant acceleration perpendicular to the initial direction; velocity parallel to the plates is constant, so the motion is like a projectile and the path is parabolic.
Electric Fields FAQs
What does Cambridge AS & A Level Physics Electric fields cover?
It covers electric field strength, uniform fields between parallel plates, motion of charged particles, Coulomb's law, electric potential and electric potential energy.
Which paper is Electric fields examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
What equations do I need for Electric fields?
Learn E = F/q, E = ΔV/Δd, F = Q1Q2/(4πε0r^2), E = Q/(4πε0r^2), V = Q/(4πε0r) and Ep = Qq/(4πε0r).
