Magnetism and Electromagnetism
1. A solenoid valve uses magnetism from an electromagnet coil
to actuate a valve mechanism
Fig 2.1
Explain what the circle-and-dot and circle-and-cross symbols
mean, with reference to the right-hand grip rule.
3. What will happen when the pushbutton switch is actuated in
this circuit?
Fig 1.1
a) Essentially, this is an electrically-controlled on/off water
valve. Explain how this system works.
b) In the development of this valve, though the design
engineers discover that the magnetic force produced by
the electromagnet coil is not strong enough to achieve
reliable valve actuation every time. What can be changed
in this solenoid valve design to produce a greater
actuating force?
Fig 3.1
What will happen when the pushbutton switch is actuated in
this circuit?
2. When engineers and physicists draw pictures illustrating the
magnetic field produced by a straight current-carrying wire,
they usually do so using this notation:
Fig 3.2
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4. The pivoted steel arrow from a small compass has become
demagnetised. The coil shown in Fig. 4.1 is to be used to
magnetise it again.
Fig 4.1
a) Describe carefully
(i)
how the coil is used to magnetise the steel arrow,
(ii)
how the polarity of the magnetised needle may be
checked.
b) On Fig. 4.1, draw the magnetic field pattern of the
magnetised needle.
5. Fig. 5.1 shows a simple motor with a rectangular coil that is
free to rotate about an axis A1A2. The coil is connected to a
battery by brushes B1 and B2.
a)
Brush B1 is connected to the positive terminal of the
battery and brush B2 is connected to the negative
terminal of the battery.
(i)
On Fig. 5.1, use an arrow to show the direction of the
conventional current in the coil.
(ii)
State the direction in which the coil rotates, when
viewed from the end closest to the brushes.
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Fig 5.1
b) State what difference, if any, each of the following
changes makes to the rotation of the coil:
(i)
using a battery with a larger potential difference,
(ii)
using a coil with several turns of wire carrying the
same current as in (a),
(iii)
using a stronger magnetic field.
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6. Fig. 6.1 is a diagram of a d.c. motor.
7. Fig. 3.1 shows a light aluminium rod resting between the
poles of a magnet. A current is passed through the rod from
two brass strips connected to a power supply.
Fig. 6.1
(a) (i) State the direction of movement of side AB and of side CD
when the current is in the direction shown in Fig. 6.1.
side AB .............................................. side CD
(ii) Explain the reason for your choices of direction.
(b) When the coil ABCD is vertical, the brushes line up with the
gaps in the split-ring commutator.
The coil rotates past the vertical position. Explain what happens
(i)
to the current in the coil,
(ii)
to the forces on the sides AB and CD of the coil.
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Fig. 3.1
(a) On Fig. 3.1, draw the direction of the current in the rod when
the switch is closed.
(b) State which way the rod moves when the switch is closed.
Give a reason for your answer.
(c) State the effect on the movement of the rod when
(i) the current is increased,
(ii) the current is reversed.
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8. Plotting compasses may be used to plot magnetic fields.
Fig 8.1
Fig 8.2
In Fig. 8.1, four plotting compasses are shown near a wire. There is
no current in the wire and the arrow in each compass points towards
the North.
In Fig. 8.2, the same plotting compasses are shown near a wire in
which there is a current downwards. The current creates a strong
magnetic field near the compasses.
(a)
(i) On Fig. 8.2, draw the direction shown by the arrow in
each compass.
(ii) State where the magnetic field due to the current has its
greatest strength.
(b) Describe how you would use one compass to plot the lines of the
magnetic field around the wire in Fig. 8.2.
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9. Fig. 9.1 shows a coil of wire wrapped around a plastic tube.
Inside the tube are two pieces of soft iron. When the switch
is closed, the compass needles point in the direction of the
magnetic field produced at each position. You may ignore the
magnetic field of the Earth in this question.
Fig 9.1
(a) On Fig. 9.1 mark arrows, in compasses A, B and C, to show
the direction of the magnetic field at each position after the
switch has been closed.
(b) When the switch is closed, the two pieces of soft iron in the
tube become magnets and move.
(i)
On Fig. 9.1, mark the poles formed on each piece of soft
iron.
(ii)
State and explain how the pieces of iron move.
(c) State the effect on the magnetic field of
(i)
reversing the direction of the current,
(ii)
reducing the size of the current.
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