9 0 1 8 5 1 901850 For Supervisor s Level 1 Physics, 2010 90185 Demonstrate understanding of electricity and magnetism Credits: Five 2.00 pm Thursday 25 November 2010 Check that the National Student Number (NSN) on your admission slip is the same as the number at the top of this page. You should answer ALL the questions in this booklet. For all numerical answers, full working must be shown. The answer should be given with an SI unit. For all describe or explain questions, the answer should be in complete sentences. Formulae you may find useful are provided in the Resource Sheet L1-PHYSR. If you need more space for any answer, use the page(s) provided at the back of this booklet and clearly number the question. Check that this booklet has pages 2 10 in the correct order and that none of these pages is blank. YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION. For Achievement Achievement Criteria Achievement with Merit Achievement with Excellence Identify or describe aspects of phenomena, concepts or principles. Give descriptions or explanations in terms of phenomena, concepts, principles and / or relationships. Give explanations that show clear understanding in terms of phenomena, concepts, principles and / or relationships. Solve straightforward problems. Solve problems. Solve complex problems. Overall Level of Performance (all criteria within a column are met) New Zealand Qualifications Authority, 2010 All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
2 You are advised to spend 50 minutes answering the questions in this booklet. All formulae are provided on the separate Resource Sheet L1-PHYSR. QUESTION ONE: LIGHTNING Lightning can occur when there is charge separation. The diagram above shows a storm cloud above a hill. The bottom of the storm cloud is negatively charged, and the tree and the ground close to the lower end of the cloud become positively charged. (a) (b) On the diagram above draw the distribution of negative charges in the ground. Explain why the ground closer to the bottom of the cloud is positively charged.
(c) Refer to the diagram opposite and state the direction in which charges flow in air during lightning. Explain your answer. Direction: Explanation: 3 During a lightning storm, the voltage between the ground and the storm cloud is 30 000 kv, and a current of 120 000 A flows through the air between the ground and the storm cloud. The cloud is 6.1 km above the ground. (d) Calculate the resistance per kilometre of the air between the storm cloud and the ground. resistance per kilometre = A cell phone tower on a farm is a large metal tube. During a thunderstorm, lightning strikes the tower and a current of 8 000 A flows through it for a time of 2.0 10 4 s. The energy transferred to the metal tube by the lightning is 9.5 10 6 J. (e) Calculate the voltage between the top and the bottom of the tower during the lightning strike. Write your answer in kilovolts. For copyright reasons, this resource cannot be reproduced here. http://opinionatedoldfart.files.wordpress. com/2008/10/telstra_ mobile_phone_tower.jpg voltage = kv
(f) When the current flows through the tower, a magnetic field is created around it. A measuring device in a factory near the tower measures a magnetic field strength of 4.6 10 5 T caused by the lightning. The current through the tower is 8 000 A. The value of μ 0 = 1.26 10 6 Tm A 1 Calculate the distance between the measuring device and the tower. 4 distance = (g) Tall buildings use lightning conductors to protect the building from lightning damage. Describe what a lightning conductor is, and explain how it protects the buildings during a lightning strike. (You may sketch charge distribution in the given diagram to aid your explanation.) Statement: Explanation:
QUESTION TWO: SEED GERMINATORS In cold weather, gardeners use electrically heated boxes to sprout seeds. The soil inside the box is kept warm by three heaters in the base of the box. They are connected as shown in the circuit diagram below. The total current provided by the power supply is 3.5 A. 5 For copyright reasons, this resource cannot be reproduced here. www.homeandgardenchat.com/wp-content/ uploads/2010/02/germination.jpg 3.5 A 18 V A 18 V, 36 W B control unit C (a) The heater A is labelled as 18V, 36W. Show that the current through the heater A is 2.0 A. (Your answer must show the correct formula and working.) (b) The heaters B and C are identical. They are automatically turned on by the control unit when the temperature inside the box falls below a certain level. When the heaters B and C are turned on, the voltage across the control unit is 2.0 V. Calculate the resistance of the heater B. resistance =
6 An electronic circuit contains a thermistor. The thermistor is a temperature sensor. The resistance of a thermistor varies as the temperature of the surroundings changes. The graph below shows the variation of voltage against current for the thermistor in the electronic circuit. Voltage (V) 20 18 16 14 12 10 8 6 4 2 0 0.00 0.20 0.40 0.60 0.80 1.00 Current (A) (c) Use your knowledge of Ohm s law to explain how the resistance of the thermistor varies as the voltage across it increases. State your reasons. (d) The voltage across the thermistor is 10 V. Use the information given in the graph to calculate the power generated in the thermistor. power =
7 (e) Car engines produce large amounts of heat, and are cooled by water. The temperature gauge circuit uses a thermistor to sense the temperature of the cooling water. The sketch below shows a basic wiring diagram for the temperature gauge circuit. + 12 V 365 Ω Temperature gauge (25 Ω) Thermistor In the above circuit, a 365 Ω resistor, a temperature gauge of resistance 25 Ω and a thermistor are connected in series with a 12 V power supply. At a certain temperature, the resistance of the thermistor is 280 Ω. Calculate the voltage across the thermistor at this temperature. voltage =
8 QUESTION THREE: MAGNETIC EFFECTS N S magnet A N S magnet B The diagram shows an overhead view of two identical magnets, A and B, on a table. Magnet A is glued to the table, while magnet B is free to move. Initially magnet B is held down near magnet A. (a) When released, magnet B is repelled away from magnet A even though there is a force of attraction between the north and the south poles of both magnets. Explain why. (b) The diagram below shows a solenoid. Draw lines to show the shape and the direction of the magnetic field formed outside the solenoid. Current
(c) The circuit is switched off so that no current flows through the solenoid. Two iron rods are now placed inside the solenoid, as shown in the diagram. The circuit is then switched on. 9 Iron rods Current Explain, giving reasons, what will happen to the rods when the current flows through the coil of the solenoid.
10 Extra paper for continuation of answers if required. Clearly number the question. Question number
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