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Slide 1

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Past Exam Questions
1: Motor Effect…read more

Slide 2

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An electron moves due North in a horizontal plane with
uniform speed. It enters a uniform
magnetic field directed due South in the same plane.
Which one of the following statements
concerning the motion of the electron in the magnetic
field is correct?
A It accelerated due West.
B It slows down to zero speed and then accelerates
due South.
C It continues to move North with its original
D It is accelerated due North.…read more

Slide 3

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· What do we know?
· Electron's moving N A It accelerated due
· Magnetic field's moving N -> S West.
· Not perpendicular, so no force is
B It slows down to zero
applied (F = BIL sin, sin 0 = 0, speed and then
therefore, no force) accelerates due South.
· No change to direction of force, so
can't be A. C It continues to move
· As there is no new force applied North with its original
due to the magnetic field, the speed.
magnitude of the velocity of the
D It is accelerated due
electron doesn't change, so it carries
on moving as it did before.
· So the answer is...…read more

Slide 4

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A wire carrying 3A runs perpendicular to a magnetic field of strength 2x10 -5 T. 4cm of
wire is within the field.
a) Calculate the magnitude of the force on the wire. [2 marks]
b) The wire is rotated so that it runs parallel to the magnetic field. What is the force on
the wire now? Explain your answer. [2 marks]…read more

Slide 5

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· Start by writing out what you know from the question:
· I = 3A
· B = 2x10-5 T
· L = 0.04 m (Remember to convert cm to m)
· What equation should be used?
· F = BIL (ignore the sin in this case as sin90 = 1)
· Now put the numbers in:
· F = 2x10-5 x 3 x 0.04
· F = 2.4x10-6 N (or 2.4µN)…read more

Slide 6

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· This is asking you to say what happens when the wire is parallel to the magnetic
field and explain why.
· What will the force be?
· Using F = BILsin we know what B I and L are already, but has changed to 0,
and sin0 = 0!
· This means that the force will be 0.
· Why?
· Because the current is running parallel to the magnetic field, which suggests that
the magnetic field induced by the wire and the magnetic field itself are
perpendicular (use right-hand grip rule to show where the field is moving in the
wire,) and so have no effect on each other.
· Don't confuse the magnetic field induced by the wire with the current. You use
the angle the CURRENT makes with the magnetic field it's in as for your equation!…read more

Slide 7

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