Physics Practice Test: Magnetism
- Magnetism
- 10 questions
- 15 minutes
These 10 questions cover magnetic forces, magnetic fields from currents and electromagnetic induction. For the force on a positive charge, point your right-hand fingers along v, curl them toward B, and your thumb gives F; reverse it for an electron.
Tap an option to mark your answer. You can change your mind, or tap the same option again to leave the question blank. The correct answers and explanations appear when you finish the test.Tap an option to answer; tap it again to leave the question blank.
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Question 1
A proton (q = 1.6 × 10−19 C) moves at 2.0 × 106 m/s perpendicular to a 0.50 T magnetic field. What is the magnitude of the magnetic force on it?
Correct answer: D
F = qvB = (1.6 × 10−19)(2.0 × 106)(0.50) = 1.6 × 10−13 N. 3.2 × 10−13 N leaves out the 0.50 T.
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Question 2
A straight wire 0.40 m long carries 3.0 A at right angles to a uniform 0.25 T magnetic field. What is the force on the wire?
Correct answer: C
F = ILB = 3.0 × 0.40 × 0.25 = 0.30 N. 1.2 N multiplies only the current and the length and forgets the field.
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Question 3
An electron moves parallel to the field lines of a uniform magnetic field. What magnetic force acts on it?
Correct answer: B
The magnetic force is qvB sin θ, and for motion parallel to the field θ = 0, so the force is zero. Only the velocity component perpendicular to B feels a force.
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Question 4
A proton moves east through a horizontal magnetic field that points north. In which direction is the magnetic force on the proton?
Correct answer: A
With the right-hand rule, east × north points up (x̂ × ŷ = ẑ). The force is always perpendicular to both v and B, so west and south are impossible; straight down would be the force on an electron.
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Question 5
A charged particle moves in a circle in a uniform magnetic field. If its speed is doubled and nothing else changes, what happens to the radius of the circle?
Correct answer: A
From qvB = mv²/r, the radius is r = mv/(qB), which is proportional to v. The period, 2πm/(qB), is the part that stays the same.
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Question 6
A long straight wire carries a current of 10 A. What is the magnetic field strength 5.0 cm from the wire? (μ0 = 4π × 10−7 T·m/A)
Correct answer: C
B = μ0I/(2πr) = (4π × 10−7)(10)/(2π × 0.050) = 4.0 × 10−5 T. 4.0 × 10−7 T uses 5.0 m instead of 0.050 m.
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Question 7
Two long parallel wires carry currents in the same direction. How do the wires affect each other?
Correct answer: D
Each wire sits in the other's magnetic field, and for currents in the same direction the forces point toward each other. Opposite currents repel, which is the reverse of the rule for electric charges.
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Question 8
The magnetic flux through a 50-turn coil changes by 0.020 Wb in 0.10 s. What is the average induced emf?
Correct answer: B
Faraday's law: emf = NΔΦ/Δt = 50 × 0.020/0.10 = 10 V. 0.20 V is the emf of a single turn.
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Question 9
The north pole of a bar magnet is pushed toward one end of a coil. According to Lenz's law, what magnetic pole does the induced current create at that end of the coil?
Correct answer: B
The induced current opposes the change that causes it, so the coil pushes back against the approaching north pole by forming a north pole of its own. If it attracted the magnet, energy would be created from nothing.
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Question 10
A transformer has 400 turns on its primary coil and 20 turns on its secondary. The primary is connected to 120 V AC. What is the secondary voltage?
Correct answer: D
Vs/Vp = Ns/Np, so Vs = 120 × 20/400 = 6.0 V. 2400 V flips the turns ratio, which would describe a step-up transformer.
Questions you leave blank are not counted as wrong.
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