Download Question 9 - Vicphysics

ELECTRIC LAWS
The 60-watt light bulb in Sam’s desk lamp has burnt out. There is no spare replacement that operates on
the 240-V supply. However, a 60-watt light bulb is found that operates at 120 V. Sam suggests building the
following circuit in order to use the 120-V light bulb.
Figure 1
Question 1
What is the value of the resistor R that will allow the 120-V light bulb to operate correctly?
Sometimes strings of Christmas-tree lights consist of three groups of globes that are connected as shown in
Figure 3
There are 16 globes in group P. Each of the globes has a voltage of 10 V across it and a current of 0.50 A
flowing through it when the string of lights is operating as designed. The globes in groups Q and R have a
different power rating to those in group P.
The number of globes in groups Q and R is equal. Although they have a different power rating from the
globes in group P, the potential difference across each globe is still 10 V when operating.
Question 2
How many globes are there in group Q?
Question 3
What current is being supplied from the electricity supply to the string of lights?
Assume that the power rating of all globes in Q and R are identical.
Question 4
How much power is dissipated by each globe in groups Q and R?
One of the globes in group Q burns out.
Question 5
Indicate in the box beside each group whether the globes in that group are on or off. If the group is on,
indicate whether the globe is brighter or dimmer compared to when the system is operating correctly.
Question 6
What is the resistance of the thick copper wire?
Question 7
How many electrons enter the copper wire each second? An electron has a charge of 1.60 x 10 -19 C.
A portable electric heater has two settings, 'high' and 'low’. These heating levels are obtained by connecting
two heating elements either in series or in parallel, across the 240- VRMS mains supply. Each element has the
same electrical resistance.
When the heating elements are connected in series, the total power dissipated in them is 960 W.
Question 8
What is the resistance of each element?
Question 9
What is the value of the ratio
total power dissipated in heating elements in series
total power dissipated in heating elements in parallel
A student is provided with several lengths of wire, plus:
a 6-V battery
a 6-W light globe
a switch
a 2-ohm resistor
Question 10
In the space provided draw a circuit that could be connected using all the components, so that the globe
lights up all the time, but becomes brighter when the switch is closed.
A lightning discharge between the ground and a cloud is composed of a series of strokes. The duration of
each lightning stroke is typically 30 microseconds. The power per stroke is 1.0 x 10 12 W.
Question 11
If the voltage between the cloud and the ground is 4.0 x 106 volt, how much charge is transferred in a single
stroke?
MAGNETIC FIELDS
The left-hand side of Figure 1 shows three sources of magnetic fields. Three possible magnetic field patterns
in the shaded planes are shown on the right-hand side.
Question 1
For each of the three sources draw a line linking the source to the magnetic field pattern it produces.
FORCES ON WIRES
A wire carrying an electric current of 3.0 A flowing from Y to Z is placed in a magnetic field of
0.5 T as shown in the diagram below.
X
X
X
X X X
Z
Y
X
wire
X
X
2.0 m
X X X
Question 1
Calculate the force on the wire YZ.
Question 2
What is the direction of the force acting on the wire?
The current in a lightning stroke passes from ground to cloud. The result of this is to generate a magnetic
field in the region of the stroke.
Question 3
Which one of the diagrams (A-C) best indicates the direction of the magnetic field at point X, a distance r
from the lightning stroke? The direction of the current / is shown. The field at X is shown as an arrow.
DC MOTORS
The circuit of a simple DC
electric motor is shown
right. It consists of a
current-carrying coil of 50
turns as the armature. The
coil is square with sides of
5.0 cm. The coil is in a
uniform magnetic field of
strength 0.005T.
A current of 3.0 A flows
through the coil in the
direction shown in above.
Question 1
Calculate the magnitude of
the force exerted on the 50 wires of side P of the coil.
Question 2
When the coil is in the position shown in Figure 5, which of the directions (A–D) below best shows the
direction of the force exerted on side P of the coil?
The ends of the coil are connected to the commutator, as shown, so that it is free to rotate with the coil.
Question 3
Explain
• why the commutator must be free to rotate in this manner
• how this is fundamental to the operation of the DC electric motor.
Two students examine a DC motor. They find that it has an armature consisting of a rectangular coil with 50
turns, which is shown below left.
They observe that the armature is in the field of a two-pole magnet,
and can rotate about an axis as shown above. The magnetic field is
produced by the current flowing through field coils. The armature
windings and the field coils are connected in series, so that the
same current flows through each. The current to the armature flows
through a commutator which is not shown. When the motor is
operating, the current flowing is 1.5 A. An enlarged diagram of one
field coil is shown on the left.
Question 4
In which direction must the current flow through the field coil to produce a field as indicated by the arrows?
A. in at X and out at Z
B. in at Z and out at X
C. it is an AC current, so the direction is always changing
D. in either direction, the field direction does not depend on the current direction
With the armature oriented as shown, the magnetic field in the region of side JK is 0.10 T. A current of 1.5 A
flows through the armature.
Question 5
What is the magnitude of the force on the side JK of the armature?
The armature is at rest in the orientation as shown, and the current begins to flow in the direction shown by
the arrows. (from J to M)
Question 6
Which one of the following statements best describes the initial motion of the armature?
A. It will start to rotate anticlockwise.
B. It will start to rotate clockwise.
C. It will start to rotate but the direction cannot be predicted.
D. It will oscillate about the position shown in Figure 4.
Thomas is trying to build a simple motor that will operate on DC current. He first decides to study the
magnetic forces on a current-carrying wire. He places a single loop of wire in a uniform magnetic field, and
connects a battery, as shown in Figure 4.
Use the answer key above to answer the following questions.
Question 7
Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point x?
Question 8
Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point y?
Question 9
Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point z?
Question 10
Which choice (A-I in the answer key) best indicates the direction of the magnetic force on the wire at point
w?
In order to convert the arrangement in
on the right into a motor, Thomas
provides an axis for rotation. He realises
that there must be current flowing
through the coil when it is rotating, so he
attaches a set of slip rings that rotate
with the coil as illustrated in Figure 5.
The coil is initially set with its plane
parallel to the magnetic field as shown,
and the switch is open so that no
current flows.
Question 11
The switch is closed. Which one of the
statements (A-D) best describes the
situation after current begins to How?
A. The coil begins to rotate, but stops
after turning through 90°.
B. The coil begins to rotate and, after
rotating one half turn, rotates back
to its original position. It then
continues to oscillate in this way.
C. The coil does not move.
D. The coil rotates continuously.
Question 12
If the slip rings used by Thomas in the circuit of Figure 5 were replaced with a commutator, which one of the
statements (A-D) best describes the situation after the switch is closed, and the current begins to flow?
A. The coil begins to rotate, but stops after turning through 90°.
B. The coil begins to rotate and, after rotating one half turn, rotates back to its original position. It then
continues to oscillate in this way.
C. The coil does not move. D. The coil rotates continuously.
Two physics students set up a
50-turn coil (JKLM), which is
free to rotate about the axis
shown as the dashed line
below. The loop is placed
between the poles of a magnet,
in a uniform magnetic field of
0.040 T. The current in the coil
is 1.5 A.
Question 13
What is the magnitude of the
magnetic force on side JK
(length = 0.050 m) of the 50turn coil, when oriented as
shown above?
Question 14
What is the magnitude of the magnetic force on side KL (length = 0.040 m) of the 50-turn coil, when oriented
as shown above?
The students now turn off the current and set the coil
at rest, oriented as shown to the right. They then turn
the current on again.
Question 15
What happens to the coil after the current is turned
on? Explain your answer.
A single loop of wire in a uniform
magnetic field is shown below. The
loop can rotate, and is shown at
three different orientations. In each
case there is a current flowing around
the coil from W to X to Y to Z.
Question 16
The magnetic field is 0.10 T, and the
current in the loop is 0.30 A. With the
loop in orientation (a), what is the
magnitude of the force acting on side
WX of the coil? The length of side WX
is 0.030 m. Show your working.
Below, the arrows indicate possible
directions of the force on side WX of
the loop in the three orientations (a), (b) and (c). The arrows in each orientation are in a plane perpendicular
to the axis of rotation of the loop.
Question 17
For each orientation below, circle the head of the arrow which best represents the direction of the
magnetic force on side WX of the coil. If there is no force on the side, write NF under the diagram.
In Figure 6 to the right, the arrows
indicate possible directions of the
force on side XY for the loop in
orientations (a) and (c) shown in
Figure 4.
Question 18
For each of the two orientations in
Figure 6, circle the head of the arrow
which best represents the direction
of the magnetic force on side XY of
the coil. If there is no force on the
side, write NF under the diagram.
Figure 7 below shows four positions (A, B, C, D) of the coil of a DC motor. The coil can be assumed to be a
single wire which is in a uniform magnetic field parallel to the coil when in the orientation shown in diagram
A.
It rotates in the direction indicated, about the axis which passes through the middle of sides LM and NK. The
coil is attached to a commutator, to which current is passed by brushes (not shown in the figure).
Figure 7
Question 19
For the coil as shown in orientation A of Figure 5, in which direction is the current flowing in the side KL?
Explain your answer.
Side KL of the coil is 0.10 m long, and a magnetic force of 0.60 N acts on it.
Question 20
If the magnetic field has a magnitude of 1.5 T, what is the magnitude of the current in the coil?
Question 21
Consider two cases:
a. The coil is at rest with the orientation shown in diagram A of Figure 5.
b. The coil is at rest with the orientation shown in diagram B of Figure 5.
In the answer book explain what would happen, in each case, if current is allowed to flow in the coil. Your
answer should discuss the forces on each side of the coil, and their net effect.
EMF
A 2.0 m long wire is moved down the page out of the magnetic field (of strength 0.5T ) at a velocity of
-1
2.0 ms as shown in the diagram below.
2.0 m
X
X
X
0.5 T
X X X
Z
Y
X
X
X
X X X
2.0 ms-1
Question 1
Calculate the emf generated in the wire.
Question 2
Describe two different ways in which the emf generated could be doubled.
Question 3
Do induced currents in a circuit oppose or magnify the change in magnetic flux that induces them? Fully
explain your answer in detail.
A square loop of wire of side 0.01 m is moved through a magnetic field.
-4
The maximum magnetic flux generated is 5.0 x 10 Wb.
Question 4
Calculate the magnetic flux density of the magnetic field.
The square loop is moved at a constant speed through the magnetic field in one direction and then back
again through the magnetic field in the reverse direction.
The graph showing the magnetic flux through the coil as a function of time is shown below.
Magnetic
-4
flux through 5.0 x 10
coil (Wb)
Time (s)
Question 5
Draw a graph which shows the variation of emf through the loop against time. Explain the main features of
your graph.
Jackie and Jim are studying electromagnetic induction. They have a small permanent magnet and a coil of
wire wound around a hollow cylinder as shown below.
Jackie moves the magnet through the coil in the direction shown at constant speed.
Question 6
Indicate on the diagram the direction of the induced current that flows in the resistor. Explain the physics
reason for your choice.
They next decide to move the magnet, at a constant speed, all the way through the coil and out the other
side.
Question 7
Which one of the diagrams (A–D) below best shows how the current through the coil varies with time?
The on-off action of a lightning stroke produces an electromagnetic field surrounding the stroke. It is this field
that causes the crackling in your radio or TV during a thunderstorm.
The magnetic field, B, produced by a lightning stroke varies with time as shown in Figure 1.
A small coil is placed perpendicular to the magnetic field, and the induced emf is monitored on an
oscilloscope.
Question 8
Which one of the graphs (A-D) best shows the variation of the emf with time?
Sofia and Max are investigating electromagnetic induction using
a square coil. They place the coil between the poles of a
magnet as shown to the right. The sides of the coil are 0.020 m
long. The unifo
–2 T, and elsewhere in air it is assumed to be zero.
Question 9
Calculate the magnetic flux through the coil when it is entirely
within the magnetic field, as shown.
Sofia and
Max now
move the
coil from the
position
shown above (entirely inside the magnetic field) to the
position shown left (entirely outside the magnetic field). It
takes 0.040 s to move the coil from the first to the second
position.
Question 10
What is the average emf induced in the coil as it moves
from the first to the second position?
Sofia and Max now move the coil through the magnetic
field, as shown below.
The magnetic flux
through the coil as
it moves from
position (a),
through position
(b), to position (c)
is shown in Figure
8.
Question 11
Circle the letter
(A–D) of the graph below that best shows the emf
induced in the coil as a function of time.
A
B
C
D
Gary and Kate are investigating electromagnetic induction. They have a single wire loop of dimensions
0.030 m long by 0.020 m wide which is placed in a uniform magnetic field. The loop can be rotated by hand
about an axis as shown below in Figure 7. The ends of the loop slide within slip rings so that a measurement
of the emf between the ends of the loop can be made between terminals A and B.
The uniform magnetic field is 0.12 T.
Question 12
What is the value of the magnetic flux through the loop when it is oriented as in Figure 7?
Gary now rotates the coil at a constant rate of 5 rotations per second. Using an oscilloscope they observe
that the voltage between points A and B varies with time as shown in Figure 8.
Figure 8
Kate decides to double the rate of rotation to 10 rotations per second.
Question 13
On Figure 8 above, sketch a graph which shows the variation with time of the voltage between points A and
B at this faster rate of rotation.
A student investigates electromagnetic induction using a single loop coil and an electromagnet as shown
below in Figure 6. The loop is placed between the poles of the electromagnet, perpendicular to the magnetic
field, and connected to an oscilloscope so that any voltage induced in the loop can be measured.
Figure 6
Figure 7
The current in the coils of the electromagnet is reduced to zero and then reversed so that the magnetic field,
B, changes as shown in Figure 7.
Question 14
Which one of the options (A–D) shown below best represents the induced voltage measured on the
oscilloscope?
Question 15
Justify your answer to Question 14, referring to Figures 6 and 7.
To the right is a diagram of a model generator
which a student used to investigate
electromagnetic induction. The student moved the
coil at a slow constant speed and observed the
current using a galvanometer as shown. The
galvanometer has a central zero setting.
One complete cycle for the coil, viewed from
position Z, is shown below.
Question 16
In which one or more positions (A – H) would the magnitude of the magnetic flux through the coil have
been a maximum? (one or more answers)
Question 17
Which one of the following diagrams best shows how the current through the galvanometer varied with time?
The student then modified the generator by
removing the galvanometer and slip rings, and
including a split-ring commutator instead, as
shown right. The way the voltage across the
brushes varied with time was observed using a
cathode ray oscilloscope (C.R.O.).
Question 18
Which one of the waveforms below best
shows what the student observed on the
C.R.O., when the coil was
rotated as before?
The student disconnected
the C.R.O. from the modified
generator as shown below,
in order to operate it as a DC
motor.
To make the DC motor, the
student connected a battery
between.x and y. The coil
now rotated
continuously in the same
direction.
Question 19
Explain how the split-ring commutator enabled
the coil to rotate always in the same direction
Question 20
How should the terminals of the battery be
connected to x and y to make the coil rotate
clockwise as viewed from Z?
RMS and PEAK, TRANSFORMERS, POWER LOSS
A house on a rural block of land 10 km from the main road has been obtaining its electrical power from a
diesel generator. The opportunity has arisen to connect to the state electrical grid.
The owner of the house has two options:
Option 1
Run a 11kV RMS line from the main road to the house and then use a step down
transformer to produce power at 240 V RMS
Option 2
Use a step down transformer at the main road from 11 kV RMS to 240 V RMS and then run
the power to the house.
For both options, the wires connecting from the main road to the house have a total resistance
of 0.050 ohm.
Question 1
What is the peak voltage across the input terminals of the transformer?
Question 2
In Option 2, if there are 10,000 turns in the primary winding of the transformer, how many turns are there in
the secondary winding?
Question 3
When 25 kW of electric power is being drawn from the output terminals of the ideal transformer, calculate,
for Option 1, the RMS current flowing in the transmission wires
Question 4
When 25 kW of electric power is being drawn from the output terminals of the ideal transformer, calculate,
for Option 2, the RMS current flowing in the transmission wires
Question 5
When 25 kW of electric power is being drawn from the output terminals of the ideal transformer, calculate,
for Option 1, the power loss in the transmission wires.
Question 6
When 25 kW of electric power is being drawn from the output terminals of the ideal transformer, calculate,
for Option 2, the power loss in the transmission wires.
The Smith family and the Jones family are farmers near Warragul. Their electricity supply comes more than
100 km, from a power station in the LaTrobe valley. It is carried by transmission lines, at a voltage of 220
kVRMS. Near the town, a switchyard-transformer (T1) steps the voltage down to 10 kVRMS for the local area. A
10-kVRMS line runs to the Jones’ farm, where there is a transformer (T 2) that provides 240 VRMS for the farms.
A 240-VRMS line then runs 2 km to the Smith’s farm. A sketch of the situation is shown below. The
transformers can be considered to be ideal.
Question 7
Explain why the supply from the power station to the local area is chosen to be 220 kV RMS rather than at 240
VRMS. Use numerical estimates to support your answer.
Assume that the input voltage to transformer T1 is 220 kVRMS, and the output is 10 kVRMS.
Question 8
What is the value of the ratio:
number of turns on the primary coil
number of turns on the secondary coil
The supply and the return lines between transformer T2 and the Smith’s farm have a total resistance of
0.0004 ohm m–1.
At a particular time, 20 A of current is being supplied to the Smith’s farm. Assume that the potential at the
secondary for transformer T2 is 240 VRMS.
Question 9
What is the voltage at the Smith’s farm?
It is common practice for the wires in the cables associated with garden lights to carry only low-voltages
(often 12RMS). However it is more efficient to use 240-volt globes in the lights. In order to achieve this, the
circuit shown in Figure 2 is used. At the 240-V supply, the voltage is stepped down using a 240-V to 12-V
transformer, and at the light it is stepped up using a 12-V to 240-V transformer. The wires joining the two
transformers are each many metres long. The transformers can be assumed to be ideal.
Question 10
The light globe is rated at 120 W when connected to a 240-VRMS supply. What current should flow through it
under this condition?
When the system shown in Figure 2 was tested, it was clear that the globe was not operating at the rated
120 W.
Question 11
Explain the reason for this.
When the garden light is operating, the voltage across the input to the transformer that supplies the globe is
10RMS.
Question 12
What is the voltage across the globe?
Under these conditions the current flowing through the long wires is 8.3 A.
Question 13
What current is flowing through the globe?
Question 14
What is the total resistance of the two wires? Remember that the transformers are ideal.
Question 15
Which one or more of the following changes would increase the voltage across the globe?
A. use wires of higher resistance
B. use wires of lower resistance
C. use transformers with ratios of 240:24 and 24:240
D. use transformers with ratios of 240:6 and 6:240
Andreas is having trouble with the reading lamp on his desk. Globes rated at 60 W and designed for an
RMS voltage of 240 V keep burning out. He seeks help from his friend Emma who is a qualified electrician.
They measure the RMS supply voltage and find it to be 264 V.
Question 16
What would the RMS current in one of these 60 W globes be if it was connected to an RMS voltage of 240
V?
Emma and Andreas have different solutions to the problem of the globes burning out. Emma suggests to
solve the problem by stepping down the supply voltage from 264 V to 240 V, using a transformer with 1000
turns in the primary coil as shown in Figure 1.
Figure 1
Question 17
How many turns would there be in the secondary coil, assuming the transformer is ideal?
Andreas suggests the problem could be solved by connecting an appropriate second globe (globe X in
Figure 2) in series with the 60 W globe.
Figure 2
Question 18
What would be the power used in globe X?
Joe and Jan are installing two low-voltage lights in their garden. The lights are supplied from a transformer
that has an output RMS voltage of 12 V, and is connected to the 240 V household supply.
Question 19
What is the value of the ratio number of turns on the primary coil
number of turns on the secondary coil
Each light is designed to operate at an RMS voltage of 12 V, and has a resistance of 18 ohm, which does
not depend on temperature.
Question 20
What is the power dissipated in such a light when operated at an RMS voltage of 12 V?
Joe and Jan now connect light 1 to the transformer using two wires, each 16.0 m long, as shown in Figure 2.
Each wire has a resistance of 0.050 ohm per metre.
Question 21
What is the RMS voltage across light I? Show your working.
They now connect light 2 directly across the secondary of the transformer as shown in Figure 3.
Joe and Jan thought that with the circuit in Figure 3, the two lights would be equally bright. In fact light 2 is
brighter than light 1.
Question 22
In the space below explain why this is so. Your answer should also include
• the value of the current flowing through light 2
• the value of the current flowing through light 1.
Light 1 is now unplugged.
Question 23
Which of the statements below (A-D) best describes the change in the brightness of light 2?
A. light 2 gets less bright
B. light 2 gets brighter
C. light 2 gets much brighter and bums out
D. light 2 does not change in brightness
A power station in the Latrobe Valley generates electric power at an RMS voltage of 20 kV (20 000 V). The
switchyard transformer steps up the voltage to an RMS value of 500 kV (500 000 V) for transmission to
other parts of Victoria. On the outskirts of large towns a local-area transformer steps down the voltage for
local transmission. The circuit in Figure 9 shows how the power station and the two transformers are
connected.
Figure 9
The RMS current in the secondary of the switchyard transformer is 300 A. Assume the two transformers are
ideal.
Question 24
What is the RMS current in the primary of the switchyard transformer?
The total resistance
Question 25
How much power is lost in the transmission lines?
Question 26
What is the RMS voltage across the primary of the local-area transformer? Give your answer to three
significant figures and make sure you show your working clearly.
A 4.0-kW electric generator is used to supply power to an electric motor which is located some distance
away.
The RMS voltage at the output of the generator is 400 V, and the RMS current supplied to the motor is 10 A.
The total resistance of the two cables, AC and BD, is 3.0 W. The situation is shown below in Figure 2.
Question 27
How much electric power is dissipated in the cables?
An engineer suggests that by connecting a transformer (T1 ) at the generator output, and another (T2 ) at the
motor input, a larger fraction of the generated power could be transferred to the motor. The plan is shown
below in Figure 3.
Question 28
Explain why this procedure increases the power transferred to the motor. Your answer should clearly show
the physics involved. Indicate what type of transformer (step-up or step-down) should be used in each
location.
After installation of the transformers, the RMS current in the transmission wires is 0.5 A. Assume the
transformers are ideal.
Question 29
What is the RMS voltage between the terminals A and B in Figure 3 now?
Question 30
How many turns of wire are in the primary of transformer T 1 if the secondary consists of 5000 turns? Show
your working.
An electrician has imported an electric light globe that is designed to operate with an RMS voltage of 110 V.
When operating at this voltage it has a resistance of 55 ohm. The globe has a power rating of 220 W. Two
methods are considered that will allow the light globe to be used with a mains supply with an RMS voltage of
240 V.
In method 1 a resistor of 65 ohm is placed as shown below in Figure 1, so that when used with an RMS
supply voltage of 240 V, the RMS voltage across the light globe is 110 V.
Figure 1
In method 2 a transformer is used to convert the RMS voltage of 240 V to an RMS voltage of 110 V, as
shown below in Figure 2. Assume the transformer is ideal.
Figure 2
There are 1440 turns on the primary coil of the transformer.
Question 31
How many turns are on the secondary coil of the transformer?
Question 32
What is the RMS current in the primary coil of the transformer?
Question 33
What is the power supplied from the mains for the two different methods?
A generator at a farmhouse supplies electricity to two motors in a workshop, which is several kilometres from
the house. The RMS voltage at the generator is 230 V. When the two motors are operating, the RMS voltage
at the workshop is 220 V. When operating, the RMS current in motor 1 is 13 A and that in motor 2 is 10 A.
Figure 3 below shows the generator connected to the workshop, but the wiring to the motors has been
omitted.
Figure 3
Question 34
On the copy of Figure 3 in the answer book, show the wiring to the two motors.
Question 35
How much power is supplied by the generator when the two motors are running?
Question 36
How much electric power is lost in the cables which connect the generator and the workshop? Show your
working.
ENERGY AND POWER
Figure 9 below shows a graph of the average daily usage of electricity in a house. The vertical scale of the
graph is in units of kilowatt-hour (kW h).
Figure 9
The unit kW h used on the graph is rarely used in physics.
Question 1
Circle the letter (A–D) that corresponds to the unit that could be used, with a different scale, on the vertical
axis instead of kW h.
A. J
B. J s
C. J s-1
D. W
The average daily usage for the 3-month period September–October–November 1996 is 16 kW h. All this
electricity was used to heat the house with an electric heater with a power consumption of 2000 W.
Question 2
How many hours each day, on average, was the heater used?
The electricity bill for a household usually shows the average daily usage of electric energy for one year as
shown in Figure 4 below.
Figure 4
Question 3
Estimate the total electric energy (in joule) used by the household in the three months of June, July and
August 1996.
The cost of supply is 12 cents per kWh.
Question 4
How much does it cost to operate a 2.4-kW heater for 8 hours?
It is now possible to buy an electrical safety unit that tums off the elecaical current in an appliance if a current
flows in the earth wire. The power lead of the appliance plugs into a box containing the unit. The box in turn
plugs into a power point.
Figure 5 above shows a clothes dryer connected to the mains supply via a safety unit. The iron ring is an
essential part of the safety unit. The active and neutral wires run through the ring. The clothes dryer has an
electrical fault, a connection between the active wire and the earthed metal case of the dryer.
Question 5
The magnitude of the current in the earth wire is 2 A, and the magnitude of the current in the neutral wire is
5 A (see Figure 5). What is the magnitude of the current in the active wire?
Question 6
After the clothes dryer is repaired, there is no current in the earth wire when the dryer is operating correctly.
In this case, which one of the following statements best describes the situation in the iron ring?
A. There is an alternating magnetic flux and an alternating current in the ring.
B. There is an altemating magnetic flux and zero current in the ring.
C. There is zero magnetic flux and zero current in the ring.
D. There is zero magnetic flux and an alternating current in the ring.
E. There is a constant non-zero magnetic flux and a constant non-zero current in the ring.
Figure 6 is a sketch of a single phase electricity supply that the SEC installed for a farmer whose farmhouse
was off the main road.
Before the electrician connected the secondary of the transformer to the house circuit, the RMS voltage
across the secondary coil was rFasured to be 240 V. The transformer, which we can assume to be 100
percent efficient, had a ratio of primary tums to secondary turns equal to 45.0. When the installation was
complete, the electrician conducted a load test at the maximum designed load for the supply. This involved
tuming on most of the electrical appliances in the house. During this load test, the electrician measured an
RMS voltage of 230 V across the secondary coil of the transformer, and an RMS current of 52.5 A in the
secondary coil.
Question 7
During the load test, estimate the power use (in W) in the house.
Question 8
During the load test described above, what would be the RMS voltage (in V) across the primary coil?
Question 9
During the load test described on page 19, what would be the RMS current (in A) through the primary coil?
Question 10
During the load test described on page I 9, what was the power loss (in W) in the wires from the I I 000 V
(RMS) supply at the main road to the transforrner near the house?