Download P12 Elec Test 2009

Physics 12 Electricity
1
Coghlan
Electric Power Test
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)
Soft iron core
(b) What could happen to the solenoid if it were connected in the circuit with no soft iron core and
no rheostat (variable resistor)?
(1 mark)
3.
Where on the Earth's surface would you expect the angle of dip to be;
(i) 0o
(ii) 90o?
(2 marks)
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4.
List three ways of increasing the magnitude of the magnetic field produced by a solenoid.
(2 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)
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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)
(c) Does the kinetic energy of the proton change as it enters the magnetic field? Explain
your answer.
(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?
(ii) find out the ratio of turns on the two coils?
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(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)
9.
Why is electricity transmitted over long distances at very high voltage e.g. 330 kV
(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?
11.
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)
5.0
0.13
4.5
0.14
4.0
0.16
3.5
0.19
3.0
0.21
2.5
0.25
2.0
0.32
1.5
0.42
1.0
0.64
0.50
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)
Physics 12 Electricity
B
5
B
1
Loop Current
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B
Loop Current
1
(Loop current)2
(b) One of the data points seems not to fit the rest – identify the incorrect data point.
(1 mark)
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)
13.
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)
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(c) Calculate the torque produced by this motor.
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(2 marks)
(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.
(ii) Decrease the current to 2.00 A.
(iii) Use different magnets to produce a 7.00 x 10-1 T field.
(iv) Reduce the friction in the pivots.
14.
Propeller-free ship rules the waves (from STAWA)
Read the article and answer the questions which follow.
Propellerless
Ploughing silently around Kobe harbour near Osaka, Japan's much-vaunted propellerless ship last week
successfully completed its first sea trial. Its sponsors pronounced themselves "very happy" with the
experimental 188 tonne Yamato I, which was driven at 6 knots (11 kilometres per hour) by a
magnetohydrodynamic (MHD) motor that sucks in and ejects water like a jet engine.
MHD motors work on the principle of a linear motor. Two tubes run the length of the ship below the
water line. An electric current is passed through coils of super conducting wire wrapped around the tubes
create an intense magnetic field inside them. At the same time, an electric current is passed through the
water in the tubes. The field and current in the water interact to force the water out of nozzles at the back
of the ship, propelling it forward. The MHD system would be far more energy efficient than propellers,
were it not for the large amounts of energy needed to cool the super conducting coils close to absolute
zero.
“This is a world's first,” says SetsuoTakezawa, director of the project for Japan's Ship and Ocean
Foundation. “We are now in preparation for the next sea trial, which will be a speed test.” Eventually, he
says, an MHD-driven boat should be able to travel at around 100 knots, something a propeller-driven
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vessel could not do because of the turbulence the propeller creates. The super conducting materials used
in the electromagnets will also be improved for the next trials.
There was one minor glitch during the 90 minute cruise. At one point, instruments monitoring the motors
showed the flow of electricity in the two sets of coils beginning to move out of phase. If this got out of
hand it could cause a phenomenon known as "quenching", where resistance builds up in the coils and
causes them to heat up and lose superconductivity Engineers later said that the problem was more with the
indicators than the electromagnets.
The chairman of the Ship and Ocean Foundation, Yohei Sasakawa, says that commercially viable MHD
vessels may not come until early in the 21st century.
Peter Hadfield, Tokyo
(6 marks)
(a) The sponsors claim that the MHD system has advantages. What are they?
(b) The MHD system being tested is not energy efficient. What reason is given for this.
(c) What is needed by the developers if the system is to be far more energy-efficient than
propellers?
(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.
S
Power supply
N
ii. Given the data listed below, determine the
force acting on the salt water between the poles
of the magnet.
Current flow in salt water = 250 A
Salt Water
Conducting
Tube
= 250 A
+
Average magnetic field density = 210 T
Effective length of the conducting salt water =
S
= Power
210 T supply
1.00 m
N
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(e) The developers of the MHD system mentioned in the article were using super conducting wire.
Suggest why they had to do this.
End of Test
Irrigation of the land with seawater desalinated by fusion power is ancient.
It's called 'rain'. -- Michael McClary