CLASSIFIED 5 MAGNETISM ELECTROMAGNETIC INDUCTION GENERATOR MOTOR - TRANSFORMER. Mr. Hussam Samir

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1 CLASSIFIED 5 MAGNETISM ELECTROMAGNETIC INDUCTION GENERATOR MOTOR - TRANSFORMER Mr. Hussam Samir

2 EXAMINATION QUESTIONS (5) 1. A wire perpendicular to the page carries an electric current in a direction out of the page. There are four compasses near the wire. Which compass shows the direction of the magnetic field caused by the current? 2. A transformer has 50 turns on its primary coil and 100 turns on its secondary coil. An alternating voltage of 25.0 V is connected across the primary coil. What is the voltage across the secondary coil? A) 12.5 V B) 50.0 V C) 175 V D) 200 V 3. In a cathode-ray tube, a hot tungsten cathode releases particles by thermionic emission. What are these particles? A) α-particles B) electrons C) protons D) tungsten atoms 4. Which line in the table shows whether iron and steel are ferrous or non-ferrous materials? MR. HUSSAM SAMIR Page 1

5. Which statement describes a property of a magnet? A It attracts ferrous materials. B It could have only one pole (north or south). C It points in a random direction when suspended.

3 5. Which statement describes a property of a magnet? A It attracts ferrous materials. B It could have only one pole (north or south). C It points in a random direction when suspended. D It repels non-ferrous materials. 6. Which procedure may be used to demagnetise a steel bar? A cooling it in a freezer B earthing it with a copper wire C placing it in a solenoid carrying a large direct current (d.c.) D striking it repeatedly with a hammer 7. A solenoid carrying a current is used to demagnetise a bar magnet. Which conditions achieve demagnetisation? 8. In the circuit shown, the switches S1 and S2 may be open (off) or closed (on). MR. HUSSAM SAMIR Page 2

4 Which line in the table shows the voltmeter reading for the switch positions given? 9. Fig. 9.1 shows apparatus used to investigate electromagnetic effects around straight wires. Fig. 9.2 is a view looking down on the apparatus shown in Fig (a) A battery is connected to T1 and T2 so that there is a current vertically down the thick wire. On Fig. 9.2, draw three magnetic field lines and indicate, with arrows, the direction of all three. [2] (b) Using a variable resistor, the p.d. between terminals T1 and T2 is gradually reduced. State the effect, if any, that this will have on MR. HUSSAM SAMIR Page 3

5 (i) the strength of the magnetic field,... (ii) the direction of the magnetic field.... (c) The battery is now connected to terminals T3 and T4, as well as to terminals T1 and T2, so that there is a current down both wires. This causes the flexible wire to move. (i) Explain why the flexible wire moves (ii) State the direction of the movement of the flexible wire.... (iii) The battery is replaced by one that delivers a smaller current. State the effect that this will have on the force acting on the flexible wire The diagram shows a coil connected to a battery and a switch. Two unmagnetised iron bars hang freely near opposite ends of the coil. What happens to the iron bars when the switch is closed? A Both X and Y move away from the coil. B Both X and Y move towards the coil. C X moves towards the coil, Y moves away from the coil. D Y moves towards the coil, X moves away from the coil. 10. The device X in this circuit is designed to cut off the electricity supply automatically if too much current flows. What is device X? A) a fuse B) a switch C) a resistor D) an ammeter MR. HUSSAM SAMIR Page 4

11. The wire XY shown in the diagram is connected to a sensitive voltmeter with a centre zero. XY is then moved quickly once through the magnetic field. What is observed on the voltmeter?

6 11. The wire XY shown in the diagram is connected to a sensitive voltmeter with a centre zero. XY is then moved quickly once through the magnetic field. What is observed on the voltmeter? A) The needle moves briefly in one direction and then returns to the centre. B) The needle moves quickly in one direction and stays deflected. C) The needle vibrates rapidly from side to side whilst XY is moving. D) The needle stays still. 12. The diagram shows a transformer connected to a 240 V a.c. supply. What is the potential difference across the secondary coil of the transformer? A) 30 V B) 120 V C) 240 V D) 480 V 13. In order to produce a beam of cathode rays, a heated filament is placed near to an anode in an evacuated glass vessel. What is the type of charge on the anode and why is this charge chosen? 14. A wire passes between the poles of a horseshoe magnet. There is a current in the wire in the direction shown, and this causes a force to act on the wire. MR. HUSSAM SAMIR Page 5

7 Three other arrangements, P, Q and R, of the wire and magnet are set up as shown. Which arrangement or arrangements will cause a force in the same direction as the original arrangement? A P, Q and R B P and Q only C P only D R only 15. Fig. 9.1 is a block diagram of an electrical energy supply system, using the output of a coalfired power station. (a) Suggest one possible way of storing surplus energy when the demand from the consumers falls below the output of the power station.. [1] (b) State why electrical energy is transmitted at high voltage.. [1] (c) A transmission cable of resistance R carries a current I. Write down a formula that gives the power loss in the cable in terms of R and I.. [1] (d) The step-up transformer has 1200 turns on the primary coil. Using the values in Fig. 9.1, calculate the number of turns on its secondary coil. Assume that the transformer has no energy losses. number of turns =... [2] MR. HUSSAM SAMIR Page 6

8 (e) The input to the step-up transformer is 800 kw. Using the values in Fig. 9.1, calculate the current in the transmission cables, assuming that the transformer is 100% efficient. current =... [3] 16. Fig. 9.1 is a sketch of some apparatus, found in a Science museum, which was once used to show how electrical energy can be converted into kinetic energy. When the switch is closed the wheel starts to turn. (a) Explain why the wheel turns when the switch is closed.. [2] (b) On Fig. 9.1, draw an arrow to show the direction of rotation of the wheel. [1] (c) The d.c. motor is another way to convert electrical energy into kinetic energy. In the space below, draw a labelled diagram of a d.c. motor. [3] (d) Describe how the split-ring commutator on an electric motor works.. [2] 17. Electromagnetic induction may be demonstrated using a magnet, a solenoid and other necessary apparatus. (a) Explain what is meant by electromagnetic induction.. [2] (b) In the space below, draw a labelled diagram of the apparatus set up so that electromagnetic induction may be demonstrated. [2] (c) Describe how you would use the apparatus to demonstrate electromagnetic induction. MR. HUSSAM SAMIR Page 7

9 . [2] (d) State two ways of increasing the magnitude of the induced e.m.f. in this experiment [2] 18. Small particles of metal are scattered near a bar magnet to show the pattern of the magnetic field. Which metal is suitable? A) aluminium B) brass C) copper D) Iron 19. A strong electromagnet is used to attract pins. What happens when the current in the coil is halved? A) No pins are attracted. B) Fewer pins are attracted. C) The same number of pins is attracted. D) Many more pins are attracted. 20. Four plotting compasses are placed near a bar magnet. Which plotting compass is shown pointing in the correct direction? 21. A cable in a house is carrying too much current. What is the greatest danger? A) appliances not working B) electric shock C) fire D) low power MR. HUSSAM SAMIR Page 8

10 22. A coil is rotated steadily between the poles of a magnet. The coil is connected to an oscilloscope, which shows a graph of voltage output against time. Which graph shows the voltage output against time? 23. The diagram shows a lamp connected to a transformer. What is the potential difference across the lamp? A) 2.4 V B) 12 V C) 60 V D) 240 V 24. Which materials are suitable for making a permanent magnet and the core of an electromagnet? MR. HUSSAM SAMIR Page 9

25. A mains electrical circuit uses insulated copper cable and the cable overheats. To prevent the cable overheating, how should the cable be changed, and why?

11 25. A mains electrical circuit uses insulated copper cable and the cable overheats. To prevent the cable overheating, how should the cable be changed, and why? A) Use thicker copper cable which has less resistance. B) Use thicker insulation which stops the heat escaping. C) Use thinner copper cable which has more resistance. D) Use thinner insulation which allows less heat to escape. 26. How is electricity transmitted over large distances and why is it transmitted in this way? 27. Which parts of an a.c. generator slide past each other when the generator is working? A) brushes and coil B) coil and magnets C) magnets and slip rings D) slip rings and brushes 28. A transformer is needed to step down a 240 V a.c. supply to a 12 V a.c. output. (a) In the space below, draw a labelled diagram of a suitable transformer. [3] (b) Explain (i) why the transformer only works on a.c.,......[1] (ii) how the input voltage is changed to an output voltage [2] (c) The output current is 1.5 A. Calculate (i) the power output, power =...[1] (ii) the energy output in 30 s. energy =...[1] MR. HUSSAM SAMIR Page 10

12 29. (a) Fig shows a positively charged plastic rod, a metal plate resting on an insulator, and a lead connected to earth. Describe how the metal plate may be charged by induction....[3] (b) An electrostatic generator sets up a current of 20 ma in a circuit. Calculate (i) the charge flowing through the circuit in 15 s, charge =... (ii) the potential difference across a 10 kω resistor in the circuit. potential difference = A student investigates which end of a magnetic compass needle is attracted to a bar magnet. What does the investigation show? A) Both ends of the compass needle are attracted by the north pole of the magnet. B) Both ends of the compass needle are attracted by the south pole of the magnet. C) One end of the compass needle is attracted by the north pole and the other end by the south pole. D) The compass needle is not attracted by either end of the magnet. 31. When a plastic comb is placed next to a small piece of aluminium foil hanging from a nylon thread, the foil is repelled by the comb. Why is this? A) The comb is charged and the foil is uncharged. B) The comb is uncharged and the foil is charged. C) The comb and the foil have charge of opposite signs. D) The comb and the foil have charge of the same sign. MR. HUSSAM SAMIR Page 11

13 32. A current-carrying coil in a magnetic field experiences a turning effect. How can the turning effect be increased? A) increase the number of turns on the coil B) reduce the size of the current C) reverse the direction of the magnetic field D) use thinner wire for the coil 33. A transformer is to be used to produce a 6 V output from a 24 V input. What are suitable numbers of turns for coil X and for coil Y? 34. A student holds a magnet above a solenoid, which is connected to a centre-zero milli-ammeter as shown Fig MR. HUSSAM SAMIR Page 12

14 (a) The student drops the magnet so that it falls through the solenoid. State and explain what would be observed on the milliammeter (i) as the magnet enters the solenoid,... [2] (ii) as the magnet speeds up inside the solenoid.... [2] (b) As the magnet passes into the coil in part (a), the coil exerts a force on the magnet even though there is no contact between them. (i) State the direction of this force. (ii) Explain how this force is caused. [3] 35. Fig shows the basic parts of a transformer. (a) Use ideas of electromagnetic induction to explain how the input voltage is transformed into an output voltage. Use the three questions below to help you with your answer. What happens in the primary coil? MR. HUSSAM SAMIR Page 13

15 What happens in the core? What happens in the secondary coil?.. [5] (b) State what is needed to make the output voltage higher than the input voltage... [1] (c) The core of this transformer splits along XX and YY. Explain why the transformer would not work if the two halves of the core were separated by about 30 cm... [1] (d) A 100% efficient transformer is used to step up the voltage of a supply from 100 V to 200V. A resistor is connected to the output. The current in the primary coil is 0.4 A. Calculate the current in the secondary coil. current = [2] 36. How many of the following methods could be used to demagnetise a piece of steel? heating it until it is red hot pulling it from a coil that is carrying an alternating current placing it in an east-west direction and hammering it putting it in a coil which is carrying a direct current A) 1 B) 2 C) 3 D) Two rods X and Y look the same. The N pole of a magnet is brought close, in turn, to each end of both rods. The results of these four actions are shown in the table. Which of the rods is a permanent magnet? A) neither of the rods B) both of the rods C) rod X only D) rod Y only MR. HUSSAM SAMIR Page 14

16 38. Which component can store energy for use in time delay circuits? A) capacitor B) potentiometer C) resistor D) thermistor 39. (a) An engine on a model railway needs a 6 V a.c. supply. A mains supply of 240 V a.c. is available. (i) In the space below, draw a labelled diagram of a transformer suitable for producing the required supply voltage. (ii) Suggest suitable numbers of turns for the coils. [4] (b) The power needed for this model engine is 12W. Calculate the current taken from the mains when just this engine is in use, assuming that the transformer is 100% efficient. current =.. [2] (c) Explain why transformers will only work when connected to an a.c. supply.... [2] 40. Electromagnetic induction can be demonstrated using a solenoid, a magnet, a sensitive ammeter and connecting wire. (a) In the space below, draw a labelled diagram of the apparatus set up to demonstrate electromagnetic induction. [2] (b) State one way of using the apparatus to produce an induced current....[1] (c) Explain why your method produces an induced current....[2] (d) Without changing the apparatus, state what must be done to produce (i) an induced current in the opposite direction to the original current, (ii) a larger induced current The diagrams show a straight wire carrying a current into the paper. MR. HUSSAM SAMIR Page 15

17 Which diagram shows the magnetic field pattern due to this current? 42. A magnet is suspended from a spring so that it can move freely inside a coil which is connected to a sensitive centre-zero ammeter. What does the ammeter show when the magnet vibrates slowly up and down? A) a reading constantly changing from left to right and right to left B) a steady reading to the left C) a steady reading to the right D) a steady zero reading 43. A student sets up the apparatus shown in order to make a relay. Which metal should be used to make the core? A) aluminium B) copper C) iron D) Steel 44. Fig and Fig show two views of a vertical wire carrying a current up through a MR. HUSSAM SAMIR Page 16

18 horizontal card. Points P and Q are marked on the card. (a) On Fig. 10.2, (i) draw a complete magnetic field line (line of force) through P and indicate its direction with an arrow, (ii) draw an arrow through Q to indicate the direction in which a compass placed at Q would point. (b) State the effect on the direction in which compass Q points of (i) increasing the current in the wire, (ii) reversing the direction of the current in the wire. (c) Fig shows the view from above of another vertical wire carrying a current up through a horizontal card. A cm grid is marked on the card. Point W is 1 cm vertically above the top surface of the card. State the magnetic field strength at S, T and W in terms of the magnetic field strength at R. Use one of the alternatives, weaker, same strength or stronger for each answer. at S... at T... at W... [3] 45. A steel bar is magnetised by stroking it several times with the south pole of a magnet, as shown. MR. HUSSAM SAMIR Page 17

19 Which poles are formed at X and Y? 46. The diagram shows a solenoid connected to a sensitive voltmeter. Which of the following would give a zero reading on the voltmeter? A) holding the magnet stationary inside the solenoid B) moving the magnet away from the solenoid C) moving the magnet towards the solenoid D) moving the solenoid towards the magnet 47. A steel ball on a horizontal wooden table rolls near the north pole of a bar magnet that is lying on the table. Which diagram shows the most likely path of the ball, as seen from above the table? 48. How is electricity transmitted over large distances and why is it transmitted in this way? MR. HUSSAM SAMIR Page 18

1 shows an arrangement that could be used for making an electromagnet or a permanent magnet.

20 49. The diagram shows a transformer with an alternating voltage of 100 V applied to the primary coil. What is the voltage produced across the secondary coil? A) 50 V B) 100V C) 200V D) 8000V 50. Fig. 7.1 shows an arrangement that could be used for making an electromagnet or a permanent magnet. Two bars of the same size are also available, one made of iron and the other of steel. (a) (i) State which bar should be used to make a permanent magnet. (ii) Describe how the apparatus would be used to make a permanent magnet. MR. HUSSAM SAMIR Page 19

21 (iii) Suggest one reason why the circuit contains an ammeter and a variable resistor. 51. Fig. 8.1 shows a long straight wire between the poles of a permanent magnet. It is connected through a switch to a battery so that, when the switch is closed, there is a steady current in the wire. (a) State the direction of the magnetic field between the poles of the magnet....[1] (b) The wire is free to move. The current is switched on so that its direction is into the page. (i) State the direction of movement of the wire. (ii) Explain how you reached your answer to (b)(i). (c) This experiment is the basis of an electric motor. Describe two changes to the arrangement shown in Fig. 8.1 that would enable continuous rotation to take place. change 1... change [2] 52. When electricity is transmitted over long distances, energy is wasted. How can the wasted energy be kept as small as possible? A) Keep the current in the transmission lines as large as possible. B) Keep the power supplied to the transmission lines as large as possible. C) Keep the resistance of the transmission lines as large as possible. D) Keep the voltage supplied to the transmission lines as large as possible. 53. The diagram shows a transformer. MR. HUSSAM SAMIR Page 20

22 What is the voltmeter reading? A) 1.2V B) 12 V C) 120V D) 1200V 54. (a) Fig. 9.1 shows a wire, held between the poles of a magnet, carrying a current in the direction of the arrow. (i) On Fig. 9.1, draw an arrow, labelled F, to show the direction of the force acting on the wire. [1] (ii) Explain why the force F acts on the wire [1] (iii) The directions of the current and the magnetic field are both reversed. State the effect on the force F.... [1] (b) Fig. 9.2 shows a negatively charged particle travelling, in a vacuum, into a region where a MR. HUSSAM SAMIR Page 21

23 magnetic field acts. The magnetic field, shown by the crosses, is acting into the paper. (i) Draw an arrow, labelled F, to show the direction of the force on the particle at point P where it enters the field. (ii) Describe the path of the particle as it continues to move through the magnetic field.... [2] 55. Which test could be used to find which end of a magnet is the north pole? A putting it near a compass needle B putting it near a ferrous metal C putting it near a non-ferrous metal D putting it near a steel spoon 56. In two separate experiments, a magnet is brought near to an unmagnetised iron bar. This causes the bar to become magnetised. Which magnetic poles are induced at X and at Y? MR. HUSSAM SAMIR Page 22

24 57. (a) In Fig. 8.1, a magnet is moving towards one end of a solenoid connected to a sensitive centrezero meter. During this movement a current is induced in the solenoid. Suggest three possible changes to the system in Fig. 8.1 that would increase the induced current [3] (b) Fig. 8.2 shows a transformer. P is the primary coil. S is the secondary coil. The coils are wound on an iron core. P has 200 turns and S has 800 turns. The e.m.f. induced across S is 24 V. The current in S is 0.50 A. The transformer operates with 100% efficiency. Calculate (i) the voltage of the supply to P, voltage =... [2] (ii) the current in P. current =... [2] MR. HUSSAM SAMIR Page 23

25 58. In the laboratory demonstration shown in Fig. 11.1, a copper rod rolls at a steady speed down the sloping parallel copper rails. The rails are in the region of a strong magnetic field that acts vertically downwards. (a) Explain why the voltmeter shows a deflection [2] (b) State, with reasons, the effect on the voltmeter deflection of the following changes: (i) increasing the strength of the magnetic field, deflection... reason (ii) slightly increasing the slope of the copper rails, deflection... reason (iii) changing the direction of the magnetic field so it is parallel to the copper rails and directed down the slope. deflection... reason [4] MR. HUSSAM SAMIR Page 24

26 59. Four nails, A, B, C and D, are tested to find which makes the strongest permanent magnet. One of the nails is placed against a bar magnet and the number of paper clips which the nail can support is recorded. The bar magnet is then removed and the number of paper clips remaining attached to the nail is recorded. Each nail is tested in turn. Which nail becomes the strongest permanent magnet? 60. The diagram shows a simple transformer. From which material should the core be made? A aluminium B copper C iron D steel MR. HUSSAM SAMIR Page 25

1 A strong electromagnet is used to attract pins. core. current. coil. pins. What happens when the current in the coil is halved?

1 A strong electromagnet is used to attract pins. core. current. coil. pins. What happens when the current in the coil is halved? 1 strong electromagnet is used to attract pins. current core pins coil What happens when the current in the coil is halved? No pins are attracted. Some pins are attracted, but not as many. The same number