7.3 The motor effect and Fleming's left-hand rule
10 exam questions worth 24 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the unit of magnetic flux density?
Choose one answer.
Correct Not quite: the answer is Tesla
Mark scheme
B: Tesla
Model answer
Tesla (B).
In Fleming's left-hand rule, what does the thumb represent?
Choose one answer.
Correct Not quite: the answer is The force
Mark scheme
C: The force
Model answer
The force (C).
Explain why a wire carrying a current in a magnetic field experiences a force.
Mark scheme
the current produces a magnetic field around the wire (1); this interacts with the field of the magnet, causing a force (1)
Model answer
The current creates its own magnetic field around the wire. It interacts with the field of the magnet, and this produces a force on the wire.
State two ways of reversing the direction of the force on a wire in a magnetic field.
Mark scheme
reverse the current (1); reverse the magnetic field / swap the poles of the magnet (1)
Model answer
Reverse the direction of the current, or reverse the magnetic field.
A wire is parallel to a magnetic field. State the size of the force on the wire and explain why.
Mark scheme
zero (1); the force only occurs when the wire cuts across the field lines / is not parallel to the field (1)
Model answer
The force is zero, because the force is only produced when the wire is at an angle to the field.
A wire of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of 0.50 T. Calculate the force on the wire.
Mark scheme
F = B I l (1); 0.50 × 3.0 × 0.20 (1); 0.30 N (1)
Model answer
F = B I l = 0.50 × 3.0 × 0.20 = 0.30 N.
A wire of length 0.080 m experiences a force of 0.12 N when carrying a current of 2.0 A at right angles to a field. Calculate the magnetic flux density.
Mark scheme
B = F ÷ (I l) (1); 0.12 ÷ (2.0 × 0.080) (1); 0.75 T (1)
Model answer
B = F / (I l) = 0.12 / (2.0 × 0.080) = 0.75 T.
The wire in a 0.60 T field has 5.0 cm in the field. A force of 0.045 N is measured. Calculate the current. Give your answer in amperes.
Mark scheme
length = 0.050 m (1); I = F ÷ (B l) (1); 0.045 ÷ (0.60 × 0.050) (1); 1.5 A (1)
Model answer
l = 0.050 m. I = F / (B l) = 0.045 / (0.60 × 0.050) = 1.5 A.
In a diagram, the magnetic field is from left to right and the current is into the page. Use Fleming's left-hand rule to state the direction of the force on the wire.
Mark scheme
force is downwards (1); thumb for the force, first finger for the field (left to right), second finger for the current (into the page) (1)
Model answer
The force is downwards: the first finger points right (field), the second finger points into the page (current), and the thumb points down.
Describe three ways of increasing the force on a wire in a magnetic field, and explain how the equation shows this.
Mark scheme
stronger magnet / bigger B (1); larger current (1); longer length of wire in the field (1); F = B I l so the force is proportional to each of these (1)
Model answer
Use a stronger magnet, increase the current or put a longer length of wire in the field. F = B I l, so the force is directly proportional to B, I and l.
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