Name
Solutions
Trafalgar High School
Physics
School Assessed Coursework
Electricity
Reading time: 10 minutes
Writing time: 40 minutes
Directions to students
Answer all questions in the spaces provided
Circle the best answer for multiple choice questions
There are 14 questions and 20 marks.
Where necessary, use results calculated in previous questions
Question 1
A current is flowing in a wire as indicated in the diagram above. The direction of the magnetic
field at point X in the diagram is best described as being
A.
in the same direction as the current I.
B.
in the opposite direction to the current I.
C.
into the page.
D.
out of the page.
Questions 2 and 3 refer to the following diagram
Question 2
The diagram above shows a current of 2.0A flowing through a wire of length 2.5m inside a
magnetic field with magnetic flux density of 0.4T. The magnetic field is directed into the page.
What is the magnitude of the force acting on the wire?
F = nBil
= 1  0.4  2  2.5
=2N
Question 3
The direction of the force on the wire in the diagram is best described as being
A.
up the page.
B.
down the page.
C.
into the page.
D.
out of the page.
Questions 4, 5 and 6 refer to the following diagram
Question 4
A small direct current electric motor consists of a rectangular coil of length AB = CD =2.4cm
and width BC = AD = 1.2cm as shown in the diagram above. There are 100 turns of wire in the
coil. A current of 20mA is flowing in the direction given by the arrows. The rectangular coil is
placed between the poles of a magnet with magnetic flux density 0.05 T.
What is the maximum magnetic flux passing through the coil?
The maximum magnetic flux occurs when the plane of the coil is parallel to the ends of the
magnet. 
= B x A x N = 0.05 x 0.024 x 0.012 x 100 = 1.4 x 10-3 Wb.
Question 5
As seen from the commutator end, the coil
A.
rotates clockwise.
B.
rotates anticlockwise.
C.
rotates clockwise and then anticlockwise.
D.
does not rotate.
Using the right hand slap rule, the force on AB is down and the force on CD is up. Hence, the
rotation is anticlockwise. ANS B
Question 6
The torque of an electric motor is T Nm.
If the magnetic flux density (B) and the current (I) passing through the coil of the electric motor
are both doubled while everything else remains the same, what will the new torque of the
motor be?
Torque = F  r
=nBILr
2 2
 Torque
4
Question 7
Which one of the following graphs would best represent the variation of torque with time for
one complete rotation of a simple electric motor?
Questions 8 and 9 refer to the following diagram
Question 8
A loop of wire is contained completely within a magnetic field of magnetic flux density 0.5 T.
The loop of wire has the dimensions 1.5 m by 0.75 m. Calculate the magnetic flux through the
loop.
. = B x A
= 0.5  0.75  1.5
= 0.56 Wb
Question 9
What is the magnitude of the EMF generated in the loop when it is pulled out of the magnetic
field at a constant speed of 0.1 ms-1?
=

 Blv
t
= 0.5  0.75  0.1
= 3.75  10-2 V
Questions 10 and 11 refer to the following diagram
Question 10
The input terminals P and Q are connected to a 50V AC power supply. The output terminals R
and S are connected to an AC voltmeter. Based on this diagram, what prediction can you make
about the voltmeter reading? Give a reason for your answer.
(2 marks)
Question 11
The 50 V AC power supply is replaced by a 50 V DC power supply. What is the new reading
on the voltmeter? Explain your answer.
(2 marks)
Questions 12 and 13 refer to the diagram below which shows the sinusoidal voltage and
current for an electricity supply.
A
D
B
C
Question 12
If the voltage at point A is 200 V, what is the RMS voltage for this supply?
Question 13
The frequency for this supply is 50 Hz. What is the time represented by point D on the graph?
Give your answer in milliseconds (ms).
This question is worth 5 marks. Spend approximately 10 minutes on it.
More marks will be given for a brief, well thought out answer than for a longer, less careful
answer.
500kV
House
Question 14
Although houses in Victoria receive their electric power at 240V, the voltage at which this
power is transmitted from its source is often as high as 500 kV. The total power being
transmitted is 1000MW
Explain how electric power is transmitted over long distances and what steps are taken to keep
power loss to a minimum. In your answer, you should refer to voltage, current and resistance.
One mark for any of the following statements.
Transformers are used to increase the transmission voltage up to 500 kV. At power substations,
the voltage is stepped down again to 65 kV or 10kV and finally to 240 V for domestic use.
Power is transmitted over long distances at high voltages in order to keep the current to as small
a value as possible. The power being transmitted at any one instant is constant and equal to VI.
Hence, if the voltage is high, the current is low.
Power loss in the transmission lines as heat is equal to I2R. Hence, for every factor of 10 by
which the current is reduced, the power loss is reduced by a factor of 100.
The other way of reducing the power loss is by decreasing the resistance of the transmission
system (the wires). This can be done by using wires made from good conducting material and
by using wires of large cross sectional area since the resistance of a wire is directly proportional
to the resistivity of the wire and inversely proportional to its cross sectional area.
In practice, aluminium is chosen as the conductor because it is relatively light and less
expensive than metals such as gold, silver and copper.
There is a limit on how thick the conducting cables can be because of the increase in weight and
the requirement for expensive support structures.
Give a mark for a correct relevant calculation