The questions are on the electric field test. Take it under real conditions before reading the mark scheme.
Quick answer: An electric field is the region around a charge where other charges feel a force, with strength is E = F / q in N/C. This mark scheme gives the model answers and the marks for the 8 questions of the Physics 11-12 test on electric field, from defining field strength to potential of a point charge, with the equivalent answers to accept.
Marks are awarded for a correct formula, a correct conversion to base units, a clear substitution and a final answer with its unit. Allow answers within rounding error and award method marks if an earlier slip is carried forward consistently.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB D.2
Question 1 Defining field strength (2 marks)
Electric field strength is the force per unit positive charge acting on a small test charge placed at the point, E = F / q. A unit is N/C (or V/m).
Marking: 1 mark for the definition, 1 mark for the unit.
Question 2 Two charged spheres (3 marks)
- F = kQ₁Q₂ / r² = (8.99 × 10⁹ × 5.0 × 10⁻⁹ × 3.0 × 10⁻⁹) / (0.060)² = 3.7 × 10⁻⁵ N.
- It is attractive because the charges have opposite signs.
Marking: (a) 1 mark for the substitution with r in metres, 1 mark for the answer. (b) 1 mark.
Question 3 Changing the distance (2 marks)
The distance triples, so the force falls by a factor of 3² = 9: F = 1.8 × 10⁻³ / 9 = 2.0 × 10⁻⁴ N.
Marking: 1 mark for the inverse square factor, 1 mark for the answer.
Question 4 Field between plates (3 marks)
- E = V / d = 900 / 0.030 = 3.0 × 10⁴ V/m.
- F = qE = 1.5 × 10⁻⁹ × 3.0 × 10⁴ = 4.5 × 10⁻⁵ N.
Marking: (a) 1 mark. (b) 1 mark for the formula, 1 mark for the answer.
Question 5 Field lines (2 marks)
The field at any point has only one direction. If lines crossed, the field would have two directions at the crossing point, which is impossible. Lines that are closer together show a stronger field.
Marking: 1 mark for the single direction argument, 1 mark for spacing meaning strength.
Question 6 Electron gun (3 marks)
- KE = eV = 1.60 × 10⁻¹⁹ × 2500 = 4.0 × 10⁻¹⁶ J.
- v = √(2 KE / m) = √(2 × 4.0 × 10⁻¹⁶ / 9.11 × 10⁻³¹) = 3.0 × 10⁷ m/s.
Marking: (a) 1 mark. (b) 1 mark for the formula, 1 mark for the answer.
Question 7 Potential between plates (2 marks)
The field is uniform, so the potential changes linearly: V = 80 × (1.0 / 4.0) = 20 V at P. The field is E = 80 / 0.040 = 2.0 × 10³ V/m.
Marking: 1 mark for the potential at P, 1 mark for E.
Question 8 Potential of a point charge (3 marks)
- V = kQ / r = (8.99 × 10⁹ × 5.0 × 10⁻⁹) / 0.25 = 180 V ≈ 1.8 × 10² V.
- W = eV = 1.60 × 10⁻¹⁹ × 179.8 = 2.9 × 10⁻¹⁷ J.
Marking: (a) 1 mark for the formula, 1 mark for the answer. (b) 1 mark.
