EMAG-2-PRELIMS PDF

1. Reluctance in a magnetic circuit is 6. “The direction of an induced e.m.f is analogous to: always such that it tends to set up a a. Power in an electric circuit current opposing the motion or the b. Current in an electric circuit change of...

1. Reluctance in a magnetic circuit is 6. “The direction of an induced e.m.f is analogous to: always such that it tends to set up a a. Power in an electric circuit current opposing the motion or the b. Current in an electric circuit change of flux responsible for inducing c. Resistance in an electric circuit that e.m.f.”, this is the statement for? d. Voltage in an electric circuit a. Faraday’s law b. Ohms law 2. The magnetic flux around a straight, c. Lenz’s law current-carrying wire: d. Coulomb’s law a. Consists if straight lines parallel to the wire.  COULOMB’S LAW: the magnitude b. Does not vary in strength with of the electrostatic force of distance from the wire. attraction or repulsion between c. Is the strongest near the wire. two point charges is directly d. Gets stronger with the increasing proportional to the product of the distance from the wire. magnitudes of charges and inversely proportional to the 3. A material that can be permanently square of the distance between magnetized is generally said to be: them. a. Ferromagnetic  OHM’S LAW: the electric current b. Magnetic passing through a conductor is c. Electromagnetic directly proportional to the d. Permanently magnetic potential difference across it.  FARADAY’S LAW: any change in 4. A unit of overall magnetic field the magnetic environment of a coil quantity is the: of a wire will case a voltage (emf) a. Maxwell to be induced in the coil. b. Ampere-turn  LENZ’S LAW: in c. Tesla electromagnetism, an induced d. Gauss electric current flows in a direction such that the current opposes the 5. The effect of an air gap in a change that induced it. magnetic circuit is to: a. Reduce the magnetemotive force 7. Faraday’s laws of electromagnetic b. Divide the flux induction are related to: c. Increase the reluctance a. The e.m.f. of a generator d. Reduce the flux density b. The strength of a magnetic field c. The current flowing in a conductor d. The e.m.f. of a chemical cell C. The speed increases as the flux decreases 8. If the flux per pole of a shunt-wound D. The speed can be controlled by a d.c. generator is halved, the diverter generated e.m.f. at constant speed a. Is tripled 12. The speed if a d.c. motor may be b. Is doubled increased by: c. Is halved A. Increasing the armature current d. Remains the same B. Increasing the field current C. Decreasing the applied voltage 9. Ferrites are particularly suited for D. Decreasing the field current high-frequency applications because of their 13. The effect of inserting a resistance a. Low eddy current loss in series with the field winding of a b. High mobility shunt motor is to: c. Low distortion A. Increase the speed of the motor d. High conductivity B. Decrease the armature current C. Increase the magnetic field 10. Remanence is sometimes called… D. Reduce the speed of the motor a. Conductivity b. Retentivity 14. The strength of the magnetic field c. Coercivity around a conductor in air is: d. Permeability A. Directly proportional to the diameter of the conductor  RETENTIVITY or REMANENCE B. Inversely proportional to the voltage or REMANENT MAGNETIZATION on the conductor is a measure of how well the C. Directly proportional to the current substance will memorize the in the conductor magnetism, and become a D. Inversely proportional to the permanent magnet. diameter of the conductor 11. Which of the following statements 15. If no load is attached to the is false for a series-wound d.c. motor? secondary winding of a transformer, A. The speed can be controlled by a what is the current in the primary shunt field regulator winding called? B. The speed decreases with increase A. Excitation current of resistance in the armature circuit B. Direct current C. Stabilizing current D. Magnetizing current 20. Which of the following statements is/are true? 16. A permanent magnet wound most likely be made from: P: hard magnetic material has high A. Copper remanence B. Aluminum Q: hard magnetic material has large C. Brass coercivity D. Steel A. Both P and Q 17. A strong permanent magnet is B. Q only plunged into a coil and left in the coil. C. Neither P nor Q What is the effect produced on the coil D. P only after a short time? A. The coil winding becomes hot 21. Which of the following symbol B. A high voltage is induced denotes magnetic field intensity? C. The insulation of the coil burns out A. B D. There is no effect B. mmf C. µ 18. Two bar magnets are placed D. H parallel to each other and about 2 cm apart, such that the south pole of one Where: B = flux density magnet is adjacent to the north pole of mmf = magnetomotive force the other. With this arrangement, the µ = permeability magnets will: A. Repel each other 22. The force between a magnet and B. Have no effect on each other piece of ferromagnetic metal has not C. Attract each other been magnetized: D. Lose their magnetism A. Gets smaller as the magnet gets closer to the metal 19. A changing magnetic field: B. Is never repulsive A. Magnetizes the earth C. Can either be repulsive or attractive B. Results from a steady electric D. Depends on the geomagnetic field current C. Produces a fluctuating electric field 23. For the current-carrying conductor D. Produces an electric current in an lying in the magnetic field shown insulator below, the direction of the current in the conductor is:  proportional to the rate of change of flux that linkages with the coil. 26. With a d.c. motor, the starter resistor: A. Prevent the field current flowing through and damaging the armature A. Same direction as the arrow B. Limits the armature current to a B. Opposite direction of the arrow safe starting value C. Away from the viewer C. Controls the speed of the machine D. Towards the viewer D. Limits the field current to a safe starting value 24. Electromagnetic waves produced 27. A commutator is a device fitted to primarily by heat are called _______. a generator. Its function is: A. Shortwaves A. To prevent sparking when the load B. Microwaves changes C. Infrared rays B. To convey the current to and from D. X-rays the windings C. To convert the a.c. generated into a 25. The magnitude of electromotive d.c. output force (emf) that is induced when a D. To generate a direct current conductor cuts magnetic flux is proportional to its rate. 28. Reluctance in a magnetic circuit is A. Coulomb’s first law analogous to ______ in an electric B. Coulomb’s second law circuit. C. Faraday’s second law A. Power D. Faraday’s first law B. Voltage C. Current  FARADAY’S FIRST LAW OF D. Resistance ELECTROMAGNETIC INDUCTION: whenever a 29. Which of the following statements conductor is placed in a varying about a three-phase induction motor is magnetic field, an electromotive false? force is induced. A. A three-phase supple connected to  FARADAY’S SECOND LAW OF the rotor produces a rotating magnetic ELECTROMAGNETIC field INDUCTION: the magnitude of emf induced is directly B. The rotating magnetic field has a 33. The direction of a magnetic field constant speed and constant within a magnet is magnitude A. Front to back C. The speed of rotation of the B. From north to south magnetic field is called the C. Back to front synchronous speed D. From south to north D. It is essentially a constant speed type machine 30. When the current through the coil of an electromagnet reverses, the 34. What refers to the ratio of A. Directions of the magnetic field magnetic flux density to magnetizing remains unchanged force of a material? B. Magnetic field expands A. Conductivity C. Direction of the magnetic field B. Coercivity reverses C. Permeability D. Magnetic field collapses D. Retentivity 31. When the current through an 35. If the north pole of a permanent inductor is cut in half, the amount of magnet is placed near a piece of soft energy stored in the electromagnetic iron, what is the polarity of the nearest field? induced pole? A. Is quartered A. North B. Doubles B. It could be either north or south C. Does not change C. Impossible to determine D. Quadruples D. South 32. Refer to a three-phase induction 36. An an electromagnet: motor. Which statements are false? A. Will repel other magnetized objects A. The slip speed is the synchronous B. Will either attract or repel speed minus the rotor speed permanent magnets, depending on B. As the rotor is loaded, the slip the polarity decreases C. Will attract only other magnetized C. The frequency of induced rotor objects e.m.f.’s increases with load on the D. Will attract pure, unmagnetized iron D. The torque on the rotor is due to the interaction of magnetic fields 37. The magnetic field due to a D. Increases current-carrying conductor takes the form of: 42. The amount of magnetic field need A. Wavy lines to remove residual magnetism from a B. Rectangles transformer core during each half- C. Concentric circles cycle is called the D. Straight lines radiating outwards A. Demagnetizing field B. Hysteresis field 38. With permeability tuning, moving C. Coercive force the core further into a solenoidal coil: D. Residual force A. Increases the inductance B. Has no effect on the inductance, 43. The magnetic field surrounding a but increases the current-carrying solenoid is capacity of the coil A. Like that of a permanent magnet C. Raises the frequency B. Unable to develop north and south D. Reduces the inductance poles C. Unlike that of a permanent magnet 39. The starting torque of a simple D. One without magnetic flux lines squirrel-cage motor is: A. Increases as rotor current rises B. Low C. Decreases as rotor current rises D. High 40. The slip speed of an induction motor may be defined as the A. Rotor speed + synchronous speed B. Rotor speed - synchronous speed C. Number of pair poles ÷ frequency D. Synchronous speed - rotor speed 44. A substance whose permeability is less than the permeability of free 41. When the speed at which a space is? conductor is moved through a A. Not a magnetic substance magnetic field is increased, the B. Ferromagnetic induced voltage C. Paramagnetic A. Decreases D. Diamagnetic B. Remains constant C. Reaches zero 45. The value of B at H=0 in a D. Runs exactly through the hysteresis curve is called ________ geographic poles A. Porosity B. Remanence 49. The ability of a material to remain C. Coercivity magnetized after the removal of the D. Magnetization magnetizing force is known as A. Reluctance 46. Which of the following statements B. Retentivity is true? C. Hysteresis A. In order to prolong the life of a D. Permeability magnet, store the magnets individually B. The physical motion from the forces 50. The polarity of an induced voltage of the magnetic fields is called is determined by generator action A. The amount of current in the coil C. A coil with 400 turns must provide B. The number of turns in a coil 800 At of magnetizing force. The C. Lenz’s Law amount of current necessary is 0.2 A. D. Motor action D. The larger the area enclosed by the curve in the magnetic hysteresis loop,  Lenz’s law in elextromagnetism the greater the hysteresis loss. states that an induced electric current flows in a direction such 47. A significant advantage, in some that the current opposes the situations, of a toroidal coil over a change that induced it. This law solenoid: determines the polarity of the A. The solenoid cannot carry as much induced voltage described on current Faraday’s law of electromagnetic B. The toroid is easier to tune induction. C. The magnetic flux in a toroid is practically within the core 51. A significant advantage, in some D. The toroid us easier to wind situations, of a toroidal coil over a solenoid is: 48. The geomagnetic field: A. The toroid is easier to tune A. Makes the earth like a huge B. The toroid is easier to wind horseshoe magnet C. The magnetic flux in a toroid is B. Is what makes an electromagnet practically all within the core work C. Is what makes a compass work D. The solenoid cannot carry as much current 55. The induced voltage across a stationary conductor in a stationary magnetic field _____. A. Equals 0 V B. Varies with time C. Depends on the strength of the magnetic field D. Is usually very large 56. The polarity of induced voltage while a field is collapsing is: A. Present only if the force is 52. If the flux density for a given mmf stationary is half that of the magnetic field B. Independent of the force creating strength, what happens to the the field absolute permeability? C. Identical to the force creating the A. Reduced field B. Doubled D. Opposite to the force creating the C. Halved field D. Remains the same 57. Which of the following devices can 53. When the north poles of two bar be used to limit the torque or force magnets are brought close together, applied by an electromechanical there will be device such as a robot? A. A force of attraction A. A back-pressure sensor B. A force of repulsion B. A dynamic transducer C. No force C. An optical shaft encoder D. A downward force D. A capacitive proximity sensor 54. What are the effects of moving a 58. Materials with permeability slightly closed wire loop through a magnetic less than that of free space. field? A. Paramagnetic A. a voltage is induced in the wire B. Anti-magnetic B. All of these choices C. Ferromagnetic C. The polarity across the wire D. Diamagnetic depends on the direction of motion D. A current is induced in the wire. 59. The amount of magnetic field needed to remove residual magnetism 62. Magnetic flux can always be from a transformer core during each attributed to: half-cycle is called the A. Aligned atoms A. Residual field B. The geomagnetic field B. Coercive force C. Motion of charged particles C. Hysteresis field D. Ferromagnetic materials D. Demagnetizing force 63. Field exists only when there is 60. What is the difference between A. Current magnetic flux and magnetic flux B. An increasing current density? C. Voltage A. Magnetic flux density is the group D. Decreasing current of lines of force that make up a magnetic field; magnetic flux is the 64. The polarity of the magnetic field flux per area. in a solenoid can be determined using B. Both refer to group of lines of force which of the following? that make up a magnetic field. 1. Grip rule C. Both refer to flux per area. 2. Screw rule D. Magnetic flux is the group of lines 3. Napier’s rule of force that make up a magnetic field; magnetic flux density is the flux per A. 1, 2, and 3 area. B. 1 only C. 2 only 61. The force between a magnet and D. 1 and 2 only a piece of ferromagnetic metal that has not been magnetized:  GRIP RULE: the grip rule states A. Can be either repulsive or attractive that of the coil is gripped with the B. Depends on the geomagnetic field right hand (conventional current C. Get smaller as the magnet gets flow) or left hand (electron current closer to the metal flow), with the fingers pointing in D. Is never repulsive the direction of the current, then the thumb, outstretched parallel to the axis of the solenoid, points on the direction of the magnetic field inside the solenoid.  SCREW RULE: if a normal right- hand thread screw is screwed  along the conductor in the there’s a way to turn-on and off direction of the current (assuming the current flowing, we can say conventional current flow), the that an electromagnet can be direction of rotation of the screw is switched on and off unlike a in the direction of the magnetic permanent magnet. field. 68. What will happen when an 65. _______ bridge is used for inductor’s magnetic field collapses? measuring an unknown inductance in A. Voltage will be induced. terms of a known capacitance and B. Current emf will be high. resistance. C. A new field will expand. A. Hay’s D. Current will double. B. Anderson C. Maxwell’s L/C 69. Which of the following is/are true D. Owen regarding reluctance? P: it is directly proportional to the 66. How should the winding axes of length of magnetic path solenoid inductors be placed to Q: it is directly proportional to the minimize their mutual inductance? permeability of the material A. Parallel to each other B. Interleaved A. Neither P or Q C. At right angles B. P only D. In line C. P and Q D. Q only 67. An advantage of an electromagnet over a permanent magnet is that: A. An electromagnet can be switched on and off B. An electromagnet does not have 70. When the plane of the coil is specific polarity perpendicular to the lines of magnetic C. Permanent magnets must always force, the number of lines of magnetic be cylindrical force passing through the coil will be D. An electromagnet requires no A. Maximum power source B. None of the above C. Minimum  The magnetism present in an D. Zero electromagnet depends on the current flowing, therefore, if 71. A comparison can be made C. To the left between magnetic and electrical D. Downwards quantities. From the following list, match the magnetic quantities with 74. A bar magnet is moved at a their equivalent electrical quantities. steady speed of 1.0 m/s towards a coil 1. Current of wire which is connected to a centre- 2. Reluctance zero galvanometer. The magnet is 3. E.m.f. now withdrawn along the same path at 4. Flux E. m.m.f. 0.5 m/s. the deflection of the 5. Resistance galvanometer is in the: A. Opposite direction as previously, A. A&D; B&F; C&F with the magnitude of the deflection B. A&F; B&D; C&E doubled C. A&B; C&D; E&F B. Same direction as previously, with D. B&F; C&E; D&A the magnitude of the deflection halved C. Opposite direction as previously, 72. If a conductor is horizontal, the with the magnitude of the deflection current flowing from left to right and halved the direction of the surrounding D. Same direction as previously, with magnetic field is from above to below, the magnitude of the deflection the force exerted on the conductor is: doubled A. Away from the viewer B. From left to right 75. Which of the following is false? C. Towards the viewer A. The direction of an induced emf is D. From below to above always such as to oppose the effect E. producing it 73. For the current-carrying conductor B. The induced emf in any circuit is lying in the magnetic fields shown proportional to the rate of change of below, the direction of the force on the the magnetic flux linking the circuit conductors is: C. An induced emf is set up whenever the magnetic field linking that circuit changes D. Flemming’s left-hand rule or Lenz’s law may be used to determine the direction of an induced emf A. Upwards 76. The amount of magnetic field B. To the right needed to remove residual magnetism from an transformer core during each B. Diamagnetic half cycle is called the C. Hall effect A. Demagnetizing field D. Permeability B. Hysteresis field C. Coercive force  In electromagnetism, permeability D. Residual force refers to the state or quality of being permeable. It is the 77. The unit of magnetic flux is the measure of the response of a A. Weber medium to a magnetic field, B. Ampere-turn expressed as the ratio of the C. Tesla magnetic flux density in the D. Gauss medium to the field strength. 78. A ferromagnetic core is placed in 82. What is the name for a non- an inductor mainly to: metallic material that has the A. Increase the current carrying ferromagnetic properties of iron? capacity A. Toroid B. Increase the inductance B. Solenoid C. Limit the current C. Ferrite D. Reduce the inductance D. Lodestone 79. A permmanent magnet is a key  Ferrites are ceramic-like materials component of with magnetic properties that are A. a capacitive pressure sensor useful in many types of electronic B. An elastomer pressure sensor devices. C. A piezoelectric microphone D. A dynamic microphone 83. Electric motors works on the principle of 80. The phenomenon of perfect A. Couple diamagnetism is called ______. B. Capacitors A. Diamagnetic effect C. Friction B. Meissner effect D. Torque C. Zero Kelvin effect D. Superconductivity 84. In d.c. generators, iron losses are made up of: 81. The ability of a material to A. Hysteresis and friction losses concentrate magnetic flux is called its B. Hysteresis, eddy current and A. Induction copper losses C. Hysteresis and eddy current losses B. Strong magnets D. Hysteresis, eddy current and brush C. Weak magnets contact losses D. Temporary magnets 85. Which of the following statements  Materials with good retentivity are about a three-phase squirrel-cage permanent magnets because they induction motor is false? can retain magnetism even when A. A three-phase supply is connected no external magnetic field is to its stator present. B. A magnetic flux which alternates is produced 89. Permeance is the inverse C. It is cheep, robust and requires little equivalent of which electrical term? or no skilled maintenance A. Voltage D. It has no external electrical B. Conductance connections to its rotor C. Current D. Resistance 86. The slip speed of an induction 90. What is the unit for relative motor depends upon: permeability? A. Eddy currents A. No unit B. Armature current B. H/m C. Mechanical load C. H^2/m D. Supply voltage D. H-m 87. The slip speed of an induction  Relative permitivity is a unitless motor: quantity since it is a ratio of A. Is 100 per cent until the rotor permeability and absolute moves and then falls to a low value permeability that are both B. Is 100 per cent until the rotor expressed in H/m. moves and then decreases slightly C. Is zero until the rotor moves and 91. The main reason why Ni-Zn then rises to 100 per cent ferrites are used for audio and T.V. D. Is zero until the rotor moves and transformers is that then rises slightly A. They are easy to fabricate by rolling B. They are totally free from pores and 88. Materials having good retentivity voids are? C. They have a high electrical A. Permanent magnets resistivity D. They have large saturation C. Maxwell - Wien magnetization D. De Sauty 92. A force of repulsion exists between two ____ magnetic poles. A. Unlike B. Positive C. Like D. Negative 93. When the current through an inductor decreases, the amount of energy stored in the electromagnetic field? A. Doubles B. Decreases C. Increases D. Remains the same 94. If the cross-sectional area of a magnetic field increases, but the flux remains the same, the flux density A. Increases B. Remains the same C. Doubles D. Decreases 95. For a given wirewound core, an increase in current through the coil A. Reverses the flux lines B. Decreases the flux density C. Causes no change in flux density D. Increases the flux density 96. Anderson bridge is a modification of ______ bridge. A. Owen B. Hay’s