ELECTRIC MOTORS AND ELECTROMAGNETIC INDUCTION
4 marks · NormalA wire is placed horizontally between the poles of a magnet so that it is perpendicular to the magnetic field. The current in the wire is 2.5 A and the magnetic field strength is increased.
State what will happen to the force on the wire as the magnetic field strength increases. Explain your answer.
Describe one change you could make to the wire itself to increase the force further.
How would the direction of the force change if the direction of current in the wire is reversed?
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) The force on the wire increases as the magnetic field strength increases, because force is directly proportional to the magnetic field strength. · must show Force increases with strength, Proportional relationship explained
(b) Increase the length of the wire in the field / increase the current in the wire. · must show Either increase length or current
(c) The force will reverse direction. · must show Direction of force reverses
Must show: Force gets stronger, Change to length/current, Force reverses with current reversal
Common mistakes: Omitting explanation for part (a)
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that force increases with magnetic field strength.
A1 (a) · 1 mark · Explains direct proportionality between field strength and force, or uses correct equation.
B1 (b) · 1 mark · Gives a valid way to increase force: increase length of wire in the field, or increase current.
B1 (c) · 1 mark · States force direction reverses.
Worked solution
a) As the magnetic field strength increases, the force increases because force is directly proportional to field strength (F = BIL).
b) To increase the force, increase the length of wire in the field or the current in the wire.
c) If the current is reversed, the direction of the force acting on the wire will also reverse.