1. | In electric circuits, ‘inductance’ is the property of a circuit which: A. | produces a voltage in a conductor |
B. | causes the circuit current to increase |
C. | remains at a constant value at switch on and switch off |
D. | prevents any opposition to voltage flow |
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2. | Inductors have the effect of storing current as an electromagnetic field whenever the current increases and: A. | storing voltage when the current decreases |
B. | giving it back when the current decreases |
C. | remaining constant when the current decreases |
D. | storing current as resistance when the current decreases |
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3. | Look at the diagram: The following diagram shows the symbol for a: B. | ferrite cored indictor |
D. | epoxy-resin cored inductor |
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4. | An EMF can be induced in a conductor if there is a relative movement between a conductor and: |
5. | Faraday’s Law of electromagnetic induction states that the value of the EMF induced in a circuit depends on the number of conductors in the circuit and the rate of change of the: A. | magnetic flux linking the conductors |
B. | electrostatic flux linking the conductors |
C. | voltage linking the conductors |
D. | magnetic current flowing through the air-gap |
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6. | The following formula can be used to determine the voltage induced in a conductor by a moving magnetic field: In the formula, the ø stands for the: A. | change in magnetic voltage |
B. | magnetic flux in webers |
C. | time period for the magnetic change |
D. | number of conductors cut by the magnetic field |
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7. | A coil of 400 turns is cut by a magnetic field of 0.15 Wb in 4 seconds. The voltage induced in the coil will be: |
8. | The EMF generated in a conductor can be determined using the following formula: In the formula the θ symbol stands for the: A. | flux density of the magnetic field in teslas |
B. | velocity of the conductor in metres per second |
C. | length of the conductors being cut by the magnetic field |
D. | angle at which the magnetic field cuts the conductors |
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9. | A 15 cm conductor is rotated within a magnetic field of a constant density of 0.8 Tesla. If the velocity relative to the field is 12 m per second, then the voltage generated at a cutting angle of 45º will be: |
10. | The inductive effect of Lenz’s law is such that the induced current will appear in such a direction that it: A. | is always in the same direction as the change that produced it |
B. | opposes the change that produced it |
C. | acts to assist the change that produced it |
D. | not induce a voltage in the conductor |
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11. | Magnetic inductance occurs when a magnet: A. | is used as a compass to find direction |
B. | has been demagnetized by a physical force |
C. | induces a magnetic field in another material |
D. | loses its magnetism due to rough handling |
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12. | In an electric circuit, inductance occurs when a current in one circuit causes: A. | a short circuit in an another circuit |
B. | the resistance of the circuit to collapse |
C. | the supply voltage to alternate with another circuit |
D. | another current to be created in that same circuit |
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13. | The value of inductance in an inductor is a function of the size and number of turns in the coil, the magnetic effects of the core and the: A. | flux density at which it is operating |
B. | size of the induction coil |
C. | type of core that the coil is using |
D. | initial resistance of the conductor in the coil |
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14. | Look at the following photo: The photo shows: B. | a laminated iron-cored inductor |
C. | a powered ferrite cored inductor |
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15. | The henry is the inductance of a closed circuit in which an EMF of 1 volt is produced when the electric current flowing in the circuit varies uniformly at the rate of: |
16. | The EMF generated in an inductor can be determined using the following formula: In the formula the L symbol stands for the: A. | length of the coil in meters |
B. | length of the conductors in meters |
C. | amount of time since the last induction |
D. | inductance of the coil in Henry |
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17. | If the current through an inductor of 1.2 H is reduced uniformly from 6 A to 1 A in 0.4s, then the value of the induced EMF will be: |
18. | The current in a 150 turn coil changes from 4 amps to zero amps causing a flux change of 200 mWb. The inductance of the coil is: |
19. | The following formula can be used to determine the inductance of a coil from physical quantities. In the formula the term μo stands for the: A. | zero reference magnetic flux |
D. | cross section area of the coil |
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20. | An air cored coil with 50 turns of wire has been wound over a length of 100 mm on a plastic pipe with an outside diameter of 25 mm. The inductance of coil will be: |
21. | The term ‘self-inductance’ is used when a conductor has a voltage induced in it by: A. | its own magnetic field |
B. | a nearby magnetic field |
C. | its own electromotive force |
D. | a nearby electromotive force |
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22. | When voltage is applied, to an inductor, a magnetic field builds up and in doing so, produces a generated voltage that: A. | assists the voltage producing it |
B. | opposes the applied voltage |
C. | prevents any current flow |
D. | acts to keep the circuit resistive |
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23. | The inclusion of an iron core within a coil: A. | prevents any build up of a magnetic field when current flow |
B. | decreases the field strength produced for a given current flow |
C. | increases the field strength produced for a given current flow |
D. | increases the voltage flow produced for a given current flow |
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24. | With an inductor, the value of any induced voltage depends on the: A. | resistance of the circuit |
B. | frequency of the applied voltage |
C. | amount of time the current is steady |
D. | rate of change of flux linkages |
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25. | The following diagram illustrates the relative directions and values of induced voltage during ‘circuit make’ and ‘circuit break: The very high self-induced voltage occurring at ‘switch off’ is due to the: A. | collapsing magnetic field |
B. | increasing supply voltage |
C. | current commencing to build up |
D. | voltage and current locking into synchronism |
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26. | When opening highly inductive circuits, it is necessary to use a bypass circuit through which the high self induced voltage may be discharged. This bypass circuit is called a: C. | magnetic release circuit |
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27. | Any two coils sharing a part of a magnetic field will experience: A. | an increase of supply voltage |
C. | mutual inductance to some extent |
D. | some change in the number of turns |
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28. | When inductors are placed in series, the total value of the inductance is determined by: B. | the inverse of the sum of the inverse inductance values |
C. | combining the values in parallel |
D. | addition of the individual inductor values |
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29. | The voltage across the inductor rises almost instantaneously to a maximum when switched on: A. | but the current flow takes some time to reach its maximum value |
B. | and will then decrease to zero instantly |
C. | and the current flow rises at the same rate as the applied voltage |
D. | and reduces to zero instantly the supply is switched off |
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30. | An inductor is therefore said to have a time constant τ = tau) which is found from the formula: In the formula the symbol ‘L’ stands for the: A. | length of the coil in meters |
B. | inductance of the coil in Henry |
C. | length of time the current flows |
D. | resistance of the circuit in ohms |
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31. | The following diagram shows a time constant curve applied to the rise of current in an inductive circuit: Three time-constants after switch on, the current will be: A. | 63.2% of its final value |
B. | 86.4% of its final value |
C. | 98.2% of its final value |
D. | 99.3% of its final value |
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32. | A 0.75 H choke has an internal resistance of 20 Ω. The time constant of the choke is: |
33. | The energy that can be stored in the electromagnetic field around an inductor can be calculated from the following formula: In the formula the term ‘I’ stands for: A. | current flowing in the inductor |
B. | intensity of the magnetic field |
C. | interval between the time constants |
D. | coil inductance in henrys |
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34. | A 6 H electromagnet with an internal resistance of 40 Ω has a current flowing through it of 5A. The energy stored in its fully charged magnetic field is: |
35. | Care must be taken when working with large inductors as the energy in an inductor can be discharged very fast. This will result in the: A. | build up of coil turns |
C. | generation of a very high voltage |
D. | lines of magnetic force becoming stationary |
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36. | Coils often have a magnetic core, such as laminated iron or ferrite added to: C. | assist in mounting the coil |
D. | increase the inductance |
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37. | Toroidal cores of ceramic materials are popular in many areas of electronics and electrical engineering mainly because: A. | very high value inductors are available in small physical sizes |
B. | very low value inductors are available in large physical sizes |
C. | they are not suitable for both a.c and d.c. applications |
D. | they can be manufactured without using any windings |
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38. | Look at the following diagram: The diagram illustrates the principle of operation of a Kettering ignition system. The operation of this system is based on, which is based on: C. | Piezo-electric principles |
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39. | If the insulation on an inductor breaks down, it can present two common problems-shorted turns and: |
40. | Broken conductors can occur in inductors, usually due to: C. | the current being too small |
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41. | The insulation resistance of inductors with more than one coil should be tested between coils and an insulation value greater than: A. | 1 MΩ should be expected |
B. | 2 MΩ should be expected |
C. | 5 MΩ should be expected |
D. | 10 MΩ should be expected |
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42. | The resistance of inductors can be measured to ensure that the coil has continuity. This test ensures that the: A. | current will continue through to the Earthing medium |
B. | resistance of the coil is the correct value |
C. | insulation resistance of the coil is low enough |
D. | applied voltage will not be connected to the frame |
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43. | Before putting into service, relays using induction coils, should be given an insulation resistance test, a continuity test and: |