Preliminary Course Physics Module 8.3 Electrical Energy in the Home Summative Test. Student Name:

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1 Summative Test Student Name: Date: / / IMPORTANT FORMULAE I = Q/t V = I.R R S = R 1 + R /R P = 1/R 1 + 1/R 2 + P = V.I = I 2.R = V 2 /R Energy = V.I.t E = F/q Part A. Multiple Choice Questions (Circle the letter of the correct answer.) 1. Charged Spheres The force exerted on sphere X by sphere Y has a magnitude of 6.0 N. A third sphere, Z, with a charge 3.0 Q is introduced, as shown in the diagram. The magnitude of the net force on sphere X, due to spheres Y and Z, is: A) 9.0 N B) 12 N C) 18 N D) 24 N 2. The electric field between a positive point charge and a negative point charge is represented by: A) B) C) D)

2 3. An Experiment to Determine the Resistance of a Metal Wire The points plotted on the graph above represent the results obtained from an experiment performed by a student. The best completed graph of this data is: A) B) C) D) 4. The resistance of two different copper wires varies, depending on their: A) diameter only. B) length only. C) length and diameter only. D) length and surrounding insulating material. 5. The resistance of a certain length of wire is R. What is the resistance of a wire of the same material, of length which is twice as long but with half the cross-sectional area? A) R / 4 B) R / 2 C) 2R D) 4R 6. A cell has an e.m.f. of 1.5 V. This means that the cell supplies: A) 1.5 coulombs of charge per second. B) 1.5 amperes per second. C) 1.5 newtons of force per coulomb of electric charge. D) 1.5 joules of electrical energy per coulomb of electrical charge.

3 7. At a particular volume setting, a stereo amplifier applies a maximum voltage of 30.0 V across each of two 8.00 speakers. The effective current in each speaker at this setting is: A) 1.88 A B) 2.65 A C) 3.75 A D) 5.30 A 8. Which one of the following is NOT a reason for connecting the lights of a house in parallel? A) If one globe goes out the others will operate. B) Each globe can operate on the same voltage. C) Less wiring is needed to connect the lights to the mains supply. D) Separate switches can be provided for each globe. 9. Consider the following circuit diagram which shows four bulbs of the same power rating (wattage): If the switch in the above circuit is closed, then: A) all globes would glow brighter B) globes A and B would glow brighter C) globes A and B would glow dimmer. D) all globes would glow the same. 10. Which of the following resistor combinations has the highest resistance? A) B) C) D)

4 11. The diagram shows a popular type of car rear window heater consisting of four identical heating elements. It operates at 12V and carries a current of 4A. What are the voltage across and the current through ONE element? Voltage in volts Current in amperes A) 3 4 B) 3 1 C) 12 1 D) The heating element of an electric iron draws a current of 2.0 A from a 240 V a.c. supply. What is the rate of production of heat in the element in watts? A) 120 B) 480 C) 960 D) The diagram shows two 6 V, 6 W lamps connected to a12 V battery. Which of the following statements s NOT correct? A) The ammeter reads 1.0 A. B) The total power dissipated by the lamps is 12 W. C) The voltmeter reads 6 V. D) If the filament of the lamp Y breaks, the voltmeter reads zero. 14. Which of the following graphs correctly shows the I/V characteristic graph a conductor that obeys Ohm's Law? A) B) C) D)

5 15. A bar magnet is placed under a sheet of glass. Iron filings are shaken onto the glass over the magnet. Which pattern correctly shows the magnetic field? 16. Some factories use dust precipitators in their chimneys to remove airborne pollutants. In one such precipitator a pair of parallel plates is placed in the square chimney with a potential difference of 2 kv between them as shown below. Consider a particle with a small negative charge at rest at one of the three positions, A, O and B, marked on the diagram. The particle will: A) have greatest potential energy at A. B) have greatest potential energy at B. C) have greatest potential energy at O D) have the same potential energy at A and B 17. Which list contains household appliances with similar energy conversions? A) Electric stove, bar radiator, hair dryer. B) Washing machine, electric shaver, radio. C) Stereo, telephone, hair dryer. D) Torch, bar radiator, fan. 18. An uncharged conductor is placed between oppositely charged parallel plates as shown below. Which of the following diagrams correctly represents the field pattern inside the space between the plates? A) B) C) D)

6 19. The diagrams below show the inside of a l3-a electrical plug. Which plug is correctly wired? A) B) C) D) 20. Many household appliances are earthed via the mains plug. Earthing is considered necessary because: A) the earth wire is needed as a spare conductor. B) the appliance needs to be protected against a short circuit. C) static charge built up during use needs to travel into the Earth. D) the electric circuit is not complete (and so AC current will not flow) without it. Part B. Free Response Answer in the spaces provided. 21. The diagram below shows a small light sphere S suspended from A by an insulating thread. It is attracted towards a large positively charged metal sphere L supported on an insulating stand. When the sphere is earthed, the sphere S hangs vertically as shown in Fig. 2. (a) Draw in Fig. 2, the distribution of charges in sphere S. (1mark) (b) Draw (and label) in Fig. 2, all the forces acting on sphere S. (1 mark) (c) Explain why the sphere S hangs vertically when the sphere L is earthed. (2 marks)

7 22. In an experiment to measure the resistivity of nichrome the following readings were obtained for a length of nichrome wire. a) Explain why several values for the micrometer reading were taken. (2 marks) b) The reading on the micrometer when it was fully closed was 0.02 mm. Taking this into account, calculate the diameter of the wire. Hence calculate a value for the resistivity of nichrome. (3 marks) c) The three quantities measured in this experiment are resistance, length and diameter. Which of these three quantities is measured the least accurately? Explain your answer and give one way in which the accuracy of this quantity could be improved. (3 marks)

8 23. The diagram below shows part of a DC circuit. The current through the 3.0 resistor is 2.0 A. a) What is the potential difference across the 3.0 resistor? (1 mark) b) What is the potential difference across the 6.0 resistor? (1 mark) c) What is the value of the current I 2? (1 mark) d) What is the value of the current I? (1 mark) e) What is the potential difference across the XY? (1 mark) f) What is the effective resistance between X and Y? (1 mark) 24. An electric hairdryer is connected, through a correctly wired 3-pin plug, to a mains supply socket. Explain briefly: a) why it is dangerous to handle the hairdryer with wet hands. (3 marks) b) why the fuse is connected to the live wire in the plug. (2 marks)

9 25. The diagram shows part of a lighting circuit on a 240 V AC household electrical system. a) Which is the live wire, A or B? (1 mark) b) Add to the diagram, the correct and safe position of: (i) a switch S 1, which controls lamp X only; (1 mark) (ii) a switch S 2, which controls both lamp Y and Z only. (2 marks) c) Each lamp has a power of 60 W. Calculate the current through one lamp when it is switched on. (2 marks) d) A 5.0-amp fuse is normally used in a household lighting circuit. Use your answer from part c) to justify why this is usually sufficient. (2 marks) 26. Study the circuit shown below. a) Calculate the readings of the voltmeter V and the ammeter A in the circuit diagram. (2 marks)

10 Question 26 (continued) b) Find the power dissipated in the circuit. (1 mark) c) Explain, without detailed calculation, whether the reading of the voltmeter will increase or decrease if the 3 resistor is now changed to a 6 resistor. (2 marks) d) Explain why voltmeters should have as high a resistance as possible but ammeters as low a resistance as possible? (2 marks) 27. The diagram below shows the circuit of a domestic electrical installation. a) Complete the diagram to connect the following: (i) a lighting circuit consisting of three 100 W, 240 V lamps operated by a switch and protected by a 5 A fuse (3 marks) (ii) a power circuit consisting of two power sockets (shown) and protected by a 30 A mains fuse. (3 marks)

11 Question 27 (continued) b) For billing purposes, a meter reader from Singapore Power takes the reading from the electric meter, located at the 'mains box '. What physical quantity does the meter measure? (1 mark) Q28. A teacher provides his students with the equipment shown below. The teacher tells the students that one of the boxes contains two resistors that are placed in parallel and the other box contains two resistors that are placed in series. All four resistors are identical, and the boxes cannot be opened. Students can connect any pieces of the equipment by attaching alligator clips to their respective posts. They may use some or all of this equipment, but they cannot use any additional equipment. Design an experiment that would allow the students to determine which box contains the resistors in series and which box contains the resistors in parallel. Your design should include: clearly labelled schematic diagrams showing the placement of the individual resistors inside the boxes meter placement and measurements that must be taken a description of the analysis that must be done to determine the placement of the resistors in each box

12 29. The diagram below shows the plugs of several electrical appliances connected to an adaptor. a) It can be dangerous to connect too many appliances (through adapters) to the one G.P.O. ( power point ). Using principles of physics, explain why this is so. (3 marks) Q30. A student sets up a circuit to investigate the strength of an electromagnet as shown in the diagram below. When the student closes the switch, the compass needle turns slightly towards the coil. a) Why does the compass needle move when the switch is closed? (1 mark) b) The student then adjusts the variable resistor. The compass needle turns more towards the coil. Why does the compass needle move more after adjusting the variable resistor? (1 mark)

13 Question 30 (continued) c) To protect the coil, the student adds a fuse to the circuit, as shown in the diagram below. This time, when he closes the switch, the fuse 'blows', but the compass needle once again turns towards the coil. This is not what he expected. Why did the fuse 'blow'? (1 mark) d) Why did the needle move after the fuse had 'blown'? (1 mark) 31. A solenoid with an iron core and a bar magnet are placed as shown in the diagram below. The effect of the Earth's magnetic field can be neglected and the needle of two compasses P and Q point in the directions as indicated. a) State the polarities of A and B. (2 marks) b) Indicate: (i) the neutral point using a cross 'X' (1 mark) (ii) the current direction in the solenoid (1 mark) c) State and explain what happens to the neutral point if the iron core is taken out of the solenoid. (2 marks)