Download Motors and Generators Revision for Trials

Motors and Generators Revision for Trials
1.
Two wires of a 2.0 m long cord supplying power to an electrical appliance are 3.0 mm apart and
carry a DC current of 8.0 A. Determine the magnitude and the direction of the force between
the wires.
2.
A coil of wire such as shown is 9.0 cm wide and is 12.0 cm deep (into the page). It carries a
current of 2.5 A in a magnetic field of 0.5 T.
a) Calculate the force on each wire when the coil is
horizontal.
b) Calculate the force on each wire when the coil is
vertical
c) Calculate the maximum torque.
3.
Students investigating the construction of a small transformer
open up a ‘plugpack’ designed to connect a 4 V mobile phone
battery charger to the power mains. They discover that the
transformer inside has a thin wire on the coils connected to the
power mains and thicker wire on the output to the charger. The
core is made up of thin sheets of iron.
a) Why would the wire on the low voltage output be thicker
than the high voltage input? (1 mark)?
b) Why is the core made from thin sheets of iron? (2 marks)
4. With alternating current (AC) the current
reverses its direction every 50th of a second.
Suppose you applied alternating current to a
DC motor as shown here. Predict what you
think might happen to the motor.
5. The relative motion between a conductor and a magnetic field can create a voltage which in turn
can produce a current in the conductor. Sometimes this induced current is useful while in other
devices it is a nuisance and efforts are mode to reduce it.
a) Name two devices where the induced current is useful and briefly describe how one of these
operates. (3 marks)
b) Name two devices where induced currents could be a problem and briefly describe how the
induced currents could be reduced in one of these. (3 marks)
6. A rectangular coil in an AC generator has dimensions of 12.0  5.0 cm, and is made up of 54
identical loops. Initially it is lying parallel to a uniform magnetic field of 0.22 T directed
horizontally across it, and it can spin about its long axis.
y
B
x
Spin axis
a) State whether the current will flow towards x or towards y when the coil is turned
clockwise in the diagram above.
b) Explain how this device could be used to extract energy from the wind.
c) Identify two structural changes which could be made to this device so as to improve its
performance, and justify one of your statements.
d) This device is turned into a motor by connecting the coil contacts to a 18 V battery pack.
Calculate the current through the coil and hence the initial force acting on the side xy if
the total resistance of the coil is 450 ohms.
7. The number of windings in a transformer’s primary coil is 500 and that
of its secondary coil is 1500. The transformer is connected to a 240 V
domestic power point.
a) Calculate the potential difference across the secondary winding.
b) Identify if this is a step up or a step down transformer
8. A pendulum is oscillating to and fro when a magnet is brought near as
shown.
a) Describe the effect this will have on the motion of the pendulum.
b) Predict the direction in which the eddy currents will flow when the
disc is in the position shown
c) Slits are then cut in the pendulum as shown. Describe the effect this
will have on the motion.
1. F = 8.5 x 10-3 N. For a complete circuit the current must flow to and from an appliance. Thus the
currents are in opposite directions and the wires repel.
2. a) F = BIl = 0.5 × 2.5 × 0.12 = 0.15 N
b) The force is unchanged at 0.15 N, but does not contribute towards the torque.
c) max = nAIB = 1.0 × (0.12 ×0.09) × 2.5 × 0.5 = 1.35 × 10–2 Nm
OR
max = 2Fd = 2 ×0.15 × 0.045 = 1.35 × 10–2 Nm
3.
a) Output has a lower voltage but a higher current and hence requires thicker wire
b) Thin sheets reduce eddy currents in the core, so reduce losses through heating. Iron increases
the magnetic field.
4.
The purpose of the commutator is to reverse the direction of flow of the current in the coil
each half cycle. This keeps the current flowing in the same direction next to each pole of the
magnet. If AC was supplied, this may cancel the effects of the commutator so that the current
reverses direction relative to the magnetic poles. This would cause the coil to oscillate but not to
rotate.
5.
a) Eddy current braking (balances), AC induction motors, induction cooktops, generators.
b) Transformers, AC motors, input leads to audio amplifiers.
AC motors: Any iron exposed to alternating fields is made of thin laminations tending to prevent
circulating currents. The oxide on the iron is a sufficient insulator because the voltage is very
small.
6.
a) When the upper face of the coil is turned clockwise current will flow in the direction x to y
b) The coil could be attached directly to an axle of a wind turbine, so it turns because the blades
are forced to rotate as the wind blows on them. Generating electricity always requires energy to
be put into the system (conservation of energy) – in this case the wind provides that energy.
c) To improve the efficiency of this device the rotor could be forced to rotate in a radial
magnetic field created by curved pole-pieces, because maximum emf is induced for more time as the
rotor is turned through the field. Another improvement would be to increase the number of loops
upwards from 54.
d) V = I R  18 = 450  I  I = 4.0  10-2 A
FB = B I ℓ sin θ  FB = 0.22  4.0  10-2  12.0  10-2 sin 90 newtons (per wire).
There are 54 identical loops, each having a side x y, so FTOTAL = 5.7  10-2 m
7.
a)Vp/Vs = np/ns; Vp = Vs × (np/ns) = 240.0 × (1500.0/500.0) = 720.0 V
c) This is a step-up transformer because the voltage is increased.
8.a) As the metal moves through the magnetic field, eddy currents will be induced whose magnetic
fields will oppose the change (movement) that induced the current. This will cause the metal sheet to
slow down and stop.
b For the position shown in which only half the metal sheet is in the magnetic field, the induced
current is directed to oppose the movement to the right. The current produces a force to the right
thus opposing the motion to the left. Inside the magnetic field this current will be upwards, and it
will complete its circuit outside the magnetic field.
c The slits reduce the area of metal in which eddy currents can be induced, so that a number of
smaller eddy currents are produced. As a result, the induced magnetic field is less and the pendulum
will