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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 2 Coghlan 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. 3 Coghlan (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 4 Coghlan 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 5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.50 Coghlan 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 13. 6 Coghlan 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. Physics 12 Electricity 7 Coghlan (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