OxfordAQA International A Level Physics
OxfordAQA International A Level Physics 3.8 Electric Fields and Capacitance Exam Questions
Free OxfordAQA International A Level Physics revision for 3.8 Electric Fields and Capacitance: what you need to know, common mistakes and exam-style questions with answers. Covers coulomb's law, electric field strength, uniform and radial fields, electric potential, capacitance, parallel plate capacitors, dielectrics and energy stored.
Practise 3.8 Electric Fields and Capacitance with past papers
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About OxfordAQA International A Level Physics 3.8 Electric Fields and Capacitance
Electric fields and capacitance is part of Unit 3 (PH03), Fields and their consequences, in the OxfordAQA International A2 year. You apply Coulomb's law to point charges, define electric field strength and electric potential for radial and uniform fields, follow charged particles moving through a uniform field, and then study capacitors, including the role of a dielectric and the energy stored. The section is set out to parallel gravitational fields, so comparison questions are common. Examiners also ask you to calculate the force between charges, the deflection of an electron between plates, the capacitance of parallel plates, and the energy from a Q-V graph.
What you need to know
- 3.8.1 Coulomb's law F = Q1Q2 / (4 pi epsilon0 r^2) for point charges in a vacuum, where epsilon0 = 8.85 x 10^-12 F m-1. Like charges repel and unlike charges attract. For subatomic particles the electric force is far larger than the gravitational force.
- 3.8.2 Field strength E = F / Q, the force per unit positive charge. Radial field: E = Q / (4 pi epsilon0 r^2). Uniform field between parallel plates: E = V / d. A charged particle entering a uniform field at right angles follows a parabolic path, like a projectile.
- 3.8.3 Electric potential V is the work done per unit positive charge in moving it from infinity to the point; V = Q / (4 pi epsilon0 r). It is positive near a positive charge and negative near a negative charge. Work done = Q delta V, and E = -delta V / delta r.
- 3.8.4 Capacitance C = Q / V, measured in farads. For parallel plates C = A epsilon0 epsilonr / d. A dielectric raises the capacitance because its polar molecules align in the field and reduce the field between the plates for a given charge.
- Energy stored E = 1/2 QV = 1/2 CV^2 = 1/2 Q^2 / C, equal to the area under a graph of V against Q.
- Comparing fields Both gravitational and electric forces follow an inverse-square law for point sources, but gravity is always attractive while electric forces can attract or repel.
Common mistakes in 3.8 Electric Fields and Capacitance
- Using E = V / d for a radial field. It only applies to a uniform field between parallel plates.
- Forgetting to square r in Coulomb's law, or using the diameter of a sphere instead of the distance from its centre.
- Thinking a capacitor stores charge in the sense of having net charge. The plates hold equal and opposite charges; it stores energy.
- Calculating energy stored as QV instead of 1/2 QV. Half the energy supplied by the source is dissipated in the circuit while charging.
- Treating potential as a vector. Potential is a scalar, so potentials from several charges are added algebraically.
Try three 3.8 Electric Fields and Capacitance exam-style questions
1. Two point charges of +2.0 nC and +3.0 nC are 0.10 m apart in air. Calculate the force between them. Take 1 / 4 pi epsilon0 = 8.99 x 10^9 N m^2 C-2. (2 marks)
Show the mark scheme answer
F = 8.99 x 10^9 x 2.0 x 10^-9 x 3.0 x 10^-9 / 0.10^2; F = 5.4 x 10^-6 N (repulsive).
2. A 470 microfarad capacitor is charged to 12 V. Calculate the energy stored. (2 marks)
Show the mark scheme answer
E = 1/2 CV^2 = 0.5 x 470 x 10^-6 x 12^2; E = 3.4 x 10^-2 J.
3. Explain why inserting a dielectric between the plates of an isolated charged capacitor increases its capacitance. (3 marks)
Show the mark scheme answer
Polar molecules in the dielectric rotate to align with the field; this produces an opposing field which reduces the resultant field and the pd between the plates; the charge is unchanged so C = Q / V increases.
3.8 Electric Fields and Capacitance FAQs
What does OxfordAQA International A Level Physics 3.8 Electric Fields and Capacitance cover?
It covers Coulomb's law, electric field strength in radial and uniform fields, the motion of charged particles in a field, electric potential, capacitance, dielectrics and the energy stored by a capacitor.
Which unit is 3.8 Electric Fields and Capacitance examined in?
It is examined in Unit 3 (PH03), Fields and their consequences.
What are the key equations for 3.8 Electric Fields and Capacitance?
Learn F = Q1Q2 / 4 pi epsilon0 r^2, E = F / Q, E = V / d, V = Q / 4 pi epsilon0 r, C = Q / V, C = A epsilon0 epsilonr / d and E = 1/2 QV.
