P15 Electromagnetism

P15 Electromagnetism

1. Where on a magnet are the magnetic forces the strongest?

a) the ends
b) the centre
c) the poles
d) the terminals

2. What are the two poles of a magnet called?

a) north (seeking) and south (seeking)
b) positive and negative
c) left-hand and right-hand
d) clockwise and anticlockwise

3. Which phrase describes magnetic poles that will always attract?

a) like poles
b) north poles
c) south poles
d) unlike poles

4. Which phrase describes magnetic poles that will always repel?

a) unlike poles
b) like poles
c) north poles
d) south poles

5. What type of force are magnetic forces?

a) non-contact
b) contact
c) counter
d) inertial

6. Which of these produces its own magnetic field?

a) permanent magnet
b) induced magnet
c) soft iron
d) transformer core

7. Which of the following is the best description of an induced magnet?

a) A magnet that can induce magnetism in another material.
b) A material that becomes magnetic when heated.
c) A material that becomes a magnet when it is placed in a magnetic field.
d) A magnet that has two like poles so it self-repels.

8. What type of force is always present between a permanent magnet and a magnet it has induced?

a) attractive
b) repulsive
c) turning
d) contact

9. What happens to an induced magnet when it is removed from the magnetic field it is in?

a) It remains magnetised.
b) It quickly loses its most or all of its magnetism.
c) It loses it magnetism with a half-life of several days.
d) The magnetism increases to maintain the strength of the field around it.

10. Which list contains a non-magnetic material.

a) steel, cobalt, iron
b) iron, nickel, cobalt
c) iron, steel, zinc
d) steel, iron, nickel

11. What is the best description of a magnetic field?

a) The region inside a magnet where the magnetism is generated.
b) The region around a magnet where a force acts on another magnet.
c) The place where lines of force meet.
d) The area immediately around the poles of a magnet.

12. What type of magnet is formed when a magnetic material is placed in a magnetic field?

a) permanent magnet
b) electro magnet
c) magnetic monopole
d) induced magnet

13. What happens to the strength of the magnetic field around a magnet as the distance from the magnet is increased?

a) it increases
b) it does not change
c) it reverses direction
d) it decreases

14. Which is the best description of the strength of a magnetic field?

a) The maximum force produced by the magnet.
b) A measure of the total area covered by the magnetic field.
c) A measure of the maximum distance that a magnet can act over.
d) The size of the force felt by another magnet placed in the field at a particular point.

15. Which is the best description of the direction of a magnetic field at a particular point?

a) The direction some iron filings would line up in.
b) The direction of the force on a north magnetic pole at that point.
c) The direction of the force on another magnet placed at that point.
d) The direction of the force on a south magnetic pole at that point.

16. In which direction do magnetic field lines run?

a) From the south(seeking) pole of a magnet to the north (seeking) pole.
b) It can vary but is worked out using the left-hand rule.
c) From the strongest pole to the weakest pole.
d) From the north(seeking) pole of a magnet to the south(seeking) pole.

17. What does a magnetic compass contain?

a) a small induced magnet called the needle
b) a concentration of the Earth’s magnetic field
c) a needle made of brass
d) a small bar magnet called the needle

18. In what direction does the needle of a magnetic compass point.

a) Exactly true north.
b) In the direction of the Earth’s magnetic field.
c) Exactly true south.
d) North in the northern hemisphere and south in the southern hemisphere.

19. Where is the Earth’s magnetic field generated?

a) At the poles.
b) In the mantle.
c) In the core.
d) Above the poles where aurora form.

20. Which experiment does the picture show?

a) The effect of plotting compasses on the field around a bar magnet.
b) Using plotting compasses to plot out the shape of the magnetic field around a bar magnet.
c) Lining up plotting compasses so they match the field around the bar magnet.
d) Using plotting compasses to induce magnetism in the bar magnet.

21. What is produced (at GCSE level) around a current carrying wire?

a) An electric field.
b) An electric and magnetic field.
c) An electromagnetic field.
d) A magnetic field.

22. What two factors directly affect the strength of the magnetic field around a conductor.

a) Current and distance from the wire.
b) The thickness of the wire and the distance from it.
c) The length of the wire and its thickness.
d) The potential difference across the wire and the current in the wire.

23. What term is used for an elongated coil of wire as in the diagram?

a) helix
b) magnetic coil
c) solenoid
d) spring coil

24. Which two words describe the magnetic field inside the coil of wire?

a) spiral and strong
b) strong and directional
c) clockwise and uniform
d) strong and uniform

25. What shape is the magnetic field around the coil of wire?

a) similar to a horseshoe magnet
b) helical
c) similar to a bar magnet
d) spiral

26. What can be added to the coil of wire to increase the strength of the magnetic field?

a) Extra field lines.
b) An iron core.
c) A flux capacitor.
d) A magnetic paint.

27. What is a solenoid with an iron core usually called?

a) A super magnet
b) A magnetron
c) A magneto
d) An electromagnet

28. What rule is used to find the direction of the magnetic field around a wire?

a) Fleming’s Left-hand rule
b) Right hand grip rule.
c) The anti-clockwise rule
d) Newton’s 1st law of magnetism

29. What device can be used to show the magnetic field around a wire carrying an electric current?

a) plotting compass
b) ammeter
c) electroscope
d) magnetron

30. What name is given to the fact that when a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other?

a) electromagnetic induction
b) hall effect
c) motor effect
d) electromotive force

31. What name is given to the rule that is used to find the orientation of the force, field and current in the situation above?

a) Right hand grip rule.
b) The anti-clockwise rule
c) Newton’s law of magnetism
d) Fleming’s Left-hand rule

32. Which of the following describes the orientation of the force, current and field in the rule?

a) They are at 90 degrees to each other.
b) They form the corner of a cube.
c) They are all perpendicular to each other.
d) All the three are valid.

33. Which of these is a factor that affect the size of the force on the conductor (at a right angle to the field) in the motor effect?

a) magnetic flux density
b) current (in the conductor)
c) length (of conductor in field)
d) all three

34. Use the equation Force = Field x Current x Length to calculate the force on a conductor of length 1m in a field of 1 T carrying a current of 1 A.

a) 10 N
b) 3 N
c) 100 N
d) 1 N

35. Use the equation Force = Field x Current x Length to calculate the force on a conductor of length 2m in a field of 3 T carrying a current of 4 A.

a) 12 N
b) 24 N
c) 6 N
d) 3 N

36. Use the equation Force = Field x Current x Length to calculate the length of a conductor with a current of 2 A in a field of 0.5 T that experiences a force of 18 N.

a) 18 m
b) 36 m
c) 9 m
d) 4.5 m

37. Which of the following is the basis of an electric motor?

a) Rotating a wire in a magnetic field produces a turning force.
b) The induced current from a turning coil generates a force that makes it rotate.
c) A coil of wire carrying a current in a magnetic field tends to rotate because of the force on the wire due to the motor effect.
d) An induced magnet is made to spin between the poles of two permanent magnets.

38. Which part of the motor is needed to keep the motor turning once the coil has rotated 90 degrees from the position shown in the diagram?

a) metal or graphite brush contact
b) the N and S pole arrangement
c) the d.c. power supply
d) split-ring commutator

39. Which of the following changes will result in the direction of rotation changing to anticlockwise?

a) Reversing the poles of the magnet.
b) Reversing the poles of the magnet and the polarity of the power supply.
c) Doubling the number of coils.
d) Changing from a metal to a graphite brush.

40. Which of the following changes would all increase the turning force produced by the motor?

a) A bigger magnet, more turns in the coil and larger current.
b) A stronger magnet, fewer turns in the coil and larger current.
c) A stronger magnet, more turns in the coil and larger current.
d) A stronger magnet, more turns in the coil and lower current.

41. Which of the following both rely on the motor effect?

a) microphone and headphones
b) alternator and dynamo
c) dynamo and loudspeaker
d) loudspeaker and headphones

42. Which of the following is the best description of how a moving-coil loudspeaker works?

a) A coil of wire is made to vibrate by the vibrating magnets that are attached to the loudspeaker cone.
b) An alternating current makes a coil of wire, held between the poles of a magnet, vibrate back and forth. The coil is attached to a cone that transfers the vibration to the air.
c) Sound waves are generated by the electromotive force generated in the coil and amplified by the cone.
d) An alternating magnetic field causes the air molecules to become magnetically polarised and alternately attracted and repelled from the coil, thus generating sound waves.

43. If a wire moves relative to a magnetic field (or the field changes), a potential difference is induced across the ends of the wire which can cause a current to flow in the wire. What name is given to this observation?

a) generator effect
b) motor effect
c) magnetic effect
d) dynamo effect

44. In this effect the current generated by moving the wire will produce a magnetic field of its own. Which of the following is the rule for which direction this new field acts in.

a) It adds to the field to make the field stronger (or the change in field larger).
b) It completely cancels out the original field (or change in field).
c) It opposes the original field (or change in field).
d) It forms at 90 degrees to the original field (or change in field).

45. Which of the following are factors that affect the size on the induced potential difference in the generator effect?

a) The strength of the magnetic field.
b) The speed of motion in the field.
c) The resistance of the wire moved in the field.
d) The strength of the magnetic field and the speed of the wire in the field.

46. The diagram shows an alternator and a dynamo.Which produces an alternating current?

a) dynamo
b) alternator
c) both
d) neither

47. Which produces a direct current?

a) dynamo
b) alternator
c) both
d) neither

48. Which relies on the motor effect?

a) dynamo
b) alternator
c) both
d) neither

49. Which relies on the generator effect?

a) dynamo
b) alternator
c) both
d) neither

50. Which has a split-ring commutator?

a) dynamo
b) alternator
c) both
d) neither

51. Which has slip rings?

a) dynamo
b) alternator
c) both
d) neither

52. Which diagram shows the output of an alternator?

a) A
b) B
c) C
d) D

53. Which diagram shows the output of a dynamo?

a) A
b) B
c) C
d) D

54. Which of the following is the best explanation for how a microphone works?

a) Sound waves cause a commutator attached to a coil of wire in a magnetic field to rotate. The moving coil produces an alternating current through the motor effect.
b) An alternating magnetic field causes a diaphragm attached to a coil of wire in a magnetic field to vibrate. The moving coil produces an alternating sound through the generator effect.
c) Sound waves cause a diaphragm attached to a coil of wire in a magnetic field to vibrate. The moving coil produces an alternating current through the generator effect.
d) S-waves cause a voice coil attached to a magnet to vibrate. The moving coil produces an alternating field through the dynamo effect.

55. Which of the following is not a part of a transformer?

a) primary coil
b) secondary coil
c) iron core
d) commutator

56. What property of iron make it an ideal material for its use in transformers?

a) good electrical conductor
b) easily magnetised
c) high density
d) low specific heat capacity

57. The transformer equation Vp/Vs = Np/Ns can be translated into the following sentence.

a) The product of the primary and secondary potential differences is equal to the product of the primary turns and secondary turns.
b) The sum of the primary and secondary potential differences is equal to the sum of the primary turns and secondary turns.
c) The ratio of the primary to secondary potential differences is equal to the ratio of the primary turns to secondary turns.
d) The input efficiency on the primary coil is equal to the output efficiency on the secondary coil.

58. In a step-up transformer the…..

a) primary potential difference is greater than the secondary potential difference.
b) primary number of turns on the coil is greater than the secondary number of turns on the coil.
c) primary current is greater than the secondary current.
d) primary potential difference is less than the secondary potential difference.

59. In a step-down transformer the…..

a) primary potential difference is less than the secondary potential difference.
b) primary potential difference is greater than the secondary potential difference.
c) primary number of turns on the coil is less than the secondary number of turns on the coil.
d) primary current is greater than the secondary current.

60. A transformer has Vp = 12 V, Np = 100 turns and Ns = 200 turns. Calculate Vs

a) 24 V
b) 48 V
c) 6 V
d) 3 V

61. A transformer has Vp = 12 V, Vs = 4 V and Np = 60 turns. Calculate Ns

a) 180 turns
b) 10 turns
c) 20 turns
d) 240 turns

62. A transformer has Vs = 120 V, Np = 100 turns and Ns = 50 turns. Calculate Vp

a) 60 V
b) 50 V
c) 100 V
d) 240 V

63. If a transformer is 100% efficient then input power = output power so primary p.d. x current = secondary p.d. x current since power = potential difference x current. Calculate the primary input power when the primary p.d. is 8 V and and the primary current is 2 A.

a) 16 W
b) 2 W
c) 4 W
d) 24 W

64. Assuming the transformer above is 100% efficient what is secondary or output power?

a) 2W
b) 4W
c) 16W
d) 24W

65. Calculate the secondary p.d. of the transformer above given that the secondary current is 4 A.

a) 64V
b) 8V
c) 32V
d) 4 V

66. Calculate the number of turns on the secondary coil of the transformer above given that the primary coil has 120 turns.

a) 30 turns
b) 60 turns
c) 120 turns
d) 240 turns

67. Which of the following is the best description as to how a transformer works?

a) The alternating current in the primary coil produces a changing magnetic field in the core that induces an alternating current in the secondary coil
b) The alternating current in the primary passes into the core where its p.d. is stepped up or down before leaving via the secondary coil.
c) The core acts as a dynamo that is powered by the current in the primary coil and generates the current for the secondary coil.
d) The iron core hold the primary and secondary coils together to make a low resistance electrical contact to make the transformer have a high efficiency.

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