The questions are on the magnetic fields test. Take it under real conditions before reading the mark scheme.
Quick answer: A magnetic field exerts a force on moving charges and on current-carrying wires. This mark scheme gives the model answers and the marks for the 8 questions of the Physics 11-12 test on magnetic fields, from defining the tesla to force on a wire at an angle, with the equivalent answers to accept.
Award marks for method as well as for the final answer. Accept answers within rounding and follow through an earlier slip. A missing or wrong unit loses one mark only once in each question.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB D.3
Question 1 Defining the tesla (2 marks)
One tesla is the flux density that gives a force of 1 N on a 1 m length of wire carrying 1 A perpendicular to the field.
Marking: 1 mark for force, current and length; 1 mark for perpendicular to the field.
Question 2 Force on a rail wire (3 marks)
a) F = BIL = 0.30 × 5.0 × 0.60 = 0.90 N.
b) Zero: no moving charges, no magnetic force.
Marking: a) 1 mark for substitution, 1 mark for answer with unit. b) 1 mark.
Question 3 Direction of a path (3 marks)
a) The force on a positive charge is F = qv × B. With v to the right and B out of the page, the force points down the page, perpendicular to v, so the path bends downwards.
b) The speed is unchanged.
Marking: a) 1 mark for the direction rule, 1 mark for force perpendicular to velocity. b) 1 mark.
Question 4 Radius of a proton (3 marks)
r = mv/(qB) = 1.67 × 10⁻²⁷ × 2.5 × 10⁶ / (1.60 × 10⁻¹⁹ × 0.35) = 7.5 cm.
Marking: 1 mark for formula, 1 mark for substitution, 1 mark for answer.
Question 5 Why circular? (2 marks)
The force is perpendicular to the velocity, so it provides a centripetal force of constant size and the speed is constant. A constant-size force that always acts at right angles to the motion gives a circular path.
Marking: 1 mark for perpendicular force and centripetal role, 1 mark for constant size giving a circle.
Question 6 Velocity selector (3 marks)
a) v = E/B = 6.0 × 10³ / 0.025 = 2.4 × 10⁵ m/s.
b) No: q cancels from qE = qvB.
Marking: a) 1 mark for formula, 1 mark for answer. b) 1 mark.
Question 7 Comparing proton and electron (2 marks)
The proton has the larger radius. r = mv/(qB), the charges are equal in size, and the proton’s mass is about 1800 times the electron’s.
Marking: 1 mark for proton, 1 mark for the mass argument.
Question 8 Force on a wire at an angle (2 marks)
F = BIL sin θ = 0.15 × 8.0 × 0.30 × sin 40° = 0.23 N.
Marking: 1 mark for the sine, 1 mark for the answer.
