The questions are on the electric field test. Take it under real conditions before reading the mark scheme.
Quick answer: An electric field E = F/q is the force per unit positive charge, a vector drawn with field lines. This mark scheme gives the model answers and the marks for the 8 questions of the AP Physics 2 test on electric field, from force from a field to force on an electron, with the equivalent answers to accept.
Award marks as shown for the relation, the substitution and the final answer with unit. For explanations, accept any wording that states the right physical idea. Do not penalise a repeated unit error more than once.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 10
Question 1 Force from a field (2 marks)
(a) The force per unit positive charge, E = F/q.
(b) F = qE = 5.0 × 10⁻⁹ × 4.0 × 10³ = 2.0 × 10⁻⁵ N.
Marking: 1 mark for each part.
Question 2 Field and potential (3 marks)
E = kQ/r² = (8.99 × 10⁹)(2.0 × 10⁻⁹) / (0.50)² = 72 N/C, directed away from the charge. V = kQ/r = 17.98 / 0.50 = 36 V.
Marking: 1 mark for E; 1 mark for V; 1 mark for the direction.
Question 3 Parallel plates (2 marks)
E = V/d = 250 / 0.0050 = 5.0 × 10⁴ V/m.
Marking: 1 mark for the relation and conversion to metres; 1 mark for the value with unit.
Question 4 Accelerated electron (3 marks)
K = eΔV = 1.60 × 10⁻¹⁹ × 800 = 1.28 × 10⁻¹⁶ J. v = √(2K/m) = √(2 × 1.28 × 10⁻¹⁶ / 9.11 × 10⁻³¹) = 1.7 × 10⁷ m/s.
Marking: 1 mark for K = eΔV; 1 mark for the value; 1 mark for the speed.
Question 5 Moving along an equipotential (3 marks)
(a) The work is zero because A and B have the same potential, so ΔV = 0 and W = qΔV = 0. The field is perpendicular to the path.
(b) Positive: the field pushes a positive charge outward, to lower potential.
Marking: (a) 1 mark for zero, 1 mark for the reason; (b) 1 mark.
Question 6 Inside a conductor (3 marks)
If there were a field, free electrons would move. They keep moving until their own field cancels the applied one, so no field remains inside. Excess charge repels itself to the outer surface. E = 0 inside.
Marking: 1 mark for free charges moving; 1 mark for E = 0 at equilibrium; 1 mark for the charge on the surface.
Question 7 Proton energy change (2 marks)
ΔU = qΔV = 1.60 × 10⁻¹⁹ × 100 = +1.6 × 10⁻¹⁷ J, an increase.
Marking: 1 mark for the value; 1 mark for stating an increase.
Question 8 Force on an electron (2 marks)
The force is to the left, opposite to the field, because the charge is negative. It moves towards higher potential.
Marking: 1 mark for the direction of force; 1 mark for the higher potential.
