Download Practice Electric Power Test

Physics 12 Electricity
1
Coghlan
Electric Power Test – Solutions
Name ___________________________________
1.
Indicate on this diagram the shape and direction of the magnetic field lines around both magnets
(2 marks)
2.
(a) Draw the shape and direction of the magnetic flux lines produced by the solenoid shown below.
(2 marks)
(b) What could happen to the solenoid if it were connected in the circuit with no soft iron core and
not rheostat (variable resistor)?
(1 mark)
The energy conversion of electrical energy to magnetic energy is not 100% efficient and the
accompanying heat energy will start to damage the circuit – melt it.
3.
Where on the Earth's surface would you expect the angle of dip to be (i) 0o and (ii) 90o?
(i) equatorial regions, (ii) polar regions.
(2 marks)
Physics 12 Electricity
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4.
Describe three ways of increasing the magnitude of the magnetic field produced by a solenoid.
Increase current
Increase windings
Increase length
(3 marks)
5.
In each situation below indicate the direction of the force on the conductor. In each case the flow of
conventional current is shown.
(a)
(b)
(c)
(3 marks)
Physics 12 Electricity
6.
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(a) An electron, a proton and a neutron enter a uniform magnetic field at right angles to the field as
shown. Draw on the diagram the path of each particle as it travels across the field.
(3 marks)
(b) Explain why each particle follows the path you have drawn.
(3 marks)
The electron will have a relatively sharp curve upwards because it is carries a negative charge equal
in magnitude to the proton but its mass is far less.
The proton will have a relatively shallow curve downwards because it is carries a positive charge
equal in magnitude to the electron but its mass is far greater.
The neutron carries no charge and so will not be affected by the field.
(c) Does the kinetic energy of the proton change as it enters the magnetic field? Explain
your answer.
No, its speed does not change. The force acting just goes to change its direction – the force is
always at right angles to the velocity and hence has no component in the direction of the proton.
(2 marks)
7.
A sealed transformer has four wire leads coming from it. Without taking the transformer apart
how could you;
(i) find out which wires are paired with which?
Because the primary and secondary coils are wound
independently, there is no electrical connection between them –
there would be infinite resistance between the primary and
secondary coils. An ohm meter measuring resistance would be
able to find the pair of wires with low resistance – this pair
would belong to either the primary or secondary coil.
(ii) find out the ratio of turns on the two coils?
NP = VP
Physics 12 Electricity
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NS
VS
Connecting a potential difference across one of the pairs of coils identified using a resistance meter, and a
voltmeter across the other pair and putting the values into the transformer equation will give the required
ratio.
(2 marks)
8.
Clearly explain the difference between a slip ring and a slit ring in reference to an A.C. generator
and a D.C. generator?
(1 mark)
The generator produces alternating current
because slip rings are used in place of a split - ring
commutator. The slip rings keep a continuous
connection
with the wire around the armature (continued). If a
simple electric motor with a split - ring commutator
is used to generate electricity, you do not get alternating
current – you get direct current.
http://www.physclips.unsw.edu.au/jw/electricmotors.ht
ml
9.
Why is electricity transmitted over long distances at very high voltage e.g. 330 kV?
Electrical Power is transmitted at a very high voltage so that the current in the transmission wires is
as low as possible. This will mean that the electrical energy dissipated as heat in the transmission
wires will be kept to a minimum. P = I2 R
(1 mark)
10.
Tintin is driving his car east with the specially fitted radio aerial in a region where the northerly
component of the earth’s magnetic field is 2.98 x
10-5 T. The vertical height of the aerial is 1.23 m.
Tintin does not know it but his aerial is wired into a
circuit that will cause an explosive device to
detonate if the induced voltage exceeds 1.00 x 10-3
volts. What is the top speed that Tintin and Snowy
can go without being destroyed?
Induced EMF = - Blv
v
= Induced EMF
Bl
=
1.00 x 10-3
2.98 x 10-5 x 1.23
= 27.28 m s-1
= 98.2 km h-1
11.
(4 marks)
A group of students conducting an experiment to find the magnetic field strength inside a solenoid
through which a current is flowing. They obtained the following results;
Current in solenoid (A) Current in wire loop (A)
Physics 12 Electricity
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5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.50
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0.13
0.14
0.16
0.19
0.21
0.25
0.32
0.42
0.64
1.32
(a) Knowing that the current in the solenoid is directly proportional to field strength, find the
relationship between field strength and current in the wire loop. Sketch the three graphs below
and choose which relationship fits the data best.
(3marks)
B
B
1
Loop Current
Loop Current
B
1
(Loop current)2
(b) One of the data points seems not to fit the rest – identify the incorrect data point.
(1 mark)
3.5 0.19
12.
A coal fired power station transmits its 375 MW electrical supply at a potential difference of 235
kV over power lines with a total resistance of 8.38 . Calculate the actual power loss across the
transmission lines as well as the percentage power loss.
(3 marks)
P
I
P
%
= IV
= P
V
= 375 x 106
235 x 103
= 1596 A
= I2R
= 15962 x 8.38
= 2.135 x 107 W
= 2.135 x 107 x 100
375 x 106
= 5.69 %
Physics 12 Electricity
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Stacey is required to build a simple DC electric motor as part of a laboratory investigation. She
winds a rectangular coil of 20 turns of length 0.0500 m and width 0.0400 m and pivots it so that it
can pivot around an axle. The diagram shows the set up.
At the instant shown the coil is lying parallel to the plane of the magnetic field. The field strength is
a uniform 6.00 x 10-1 T. A current of 3.50 A is passing through the coil in direction ABCD.
(a) On side CD of the coil indicate the direction of the force by drawing an arrow on the diagram.
(1 mark)
(b) Calculate the magnitude of this force.
(1 mark)
F = NIlB
= 20 x 3.5 x 0.05 x 6 x 10-1
= 2.1 N
(c) Calculate the torque produced by this motor.
(2 marks)
τ = Fsx2
= 2.1 x 0.02 x 2
= 0.084 N m ACW
(d) Stacey wants to make the coil turn faster. Which of the following alterations to her design
would be likely to accomplish this? For each alteration explain why the coil will turn faster or not.
(3 marks)
(i) Replace the original coil with a new coil with 30 turns.
Faster because there will be 1.5 times more force.
(ii) Decrease the current to 2.00 A.
Slower
(iii) Use different magnets to produce a 7.00 x 10-1 T field.
Faster, slight increase in B results in more force.
(iv) Reduce the friction in the pivots.
Faster, decreases forces opposing rotation.
14.
(6 marks)
(a) The sponsors claim that the MHD system has advantages. What are they?
More energy efficient than propellers.
Less drag due to turbulence caused by propellers.
Fewer moving parts.
(b) The MHD system being tested is not energy efficient. What reason is given for this.
Too much energy needed to cool the super conducting coils close to absolute zero.
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(c) What is needed by the developers if the system is to be far more energy-efficient than
propellers?
To develop super conductors that operate at ambient temperatures so that energy is not wasted in
cooling.
(d) A simplified diagram of a possible
MHD system is drawn below.
i. Will the salt water move to the
right or the left? Explain.
The water would move to the left – the
field is down, the current is into the
diagram so the left hand rule gives the
force to the left.
ii. Given the data listed below,
determine the force acting on the salt
water between the poles of the magnet.
S
Power supply
N
Salt Water
Conducting
Tube
+
S
Power supply
N
Current flow in salt water = 250 A
= 250 A
Average magnetic field density
= 210 T
Effective length of the conducting salt water
F
= IlB
= 250 x 1.00 x 210
= 5.2 x 104 N
= 1.00 m.
(e) The developers of the MHD system mentioned in the article were using super conducting wire.
Suggest why they had to do this.
A sufficiently large current has to be generated that does not result in the conducting wire melting.
To achieve this a very low resistance wire has to be used – a super conducting wire has virtually no
resistance and hence does not heat up and fail.
End of Test
Irrigation of the land with seawater desalinated by fusion power is ancient.
It's called 'rain'. -- Michael McClary