MULTIPLE CHOICE QUESTIONS

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1 MULTIPLE CHOICE QUESTIONS (1) In 1831 Faraday in England and hennery in USA observed that an e.m.f is set u in conductor when it moves across a (a) Electric field (b) Magnetic field (c) Gravitational field (d) All of the above () The direction of induced emf or current is found by the use of (a) Faraday s law (b) Lenz s law (c) Amere s law (d) Newton s law (3) The magnitude of motional emf is given by (a) ε = -vbl sinθ (b) ε = -vblcosθ (c) ε = vbltanθ (d) ε = vblsinθ (4) The unit of emf is (a) amere (b) volt (c) Weber (d) tesla (5) The average e.m.f induce in a conducting coil of N loos is equal to the negative of the rate at which the magnetic flux through the coil is changing with time. This statement is (a) Coulomb s Law (b) Faraday s Law of electromagnetic Induction (c) Lenz s Law (d) None of the above. (6) Lenz s law is in accordance with the law of conservation of (a) mass (b) momentum (c) energy (d) charge (7) The henomenon in which a changing current in one coil induces an emf in another coil is called (a) mutual induction. (b) self induction (c) magnetic induction (8) One Henry (H) equals (a) VsA (b) VsA (c) VsA -1 (d) VsA - (9) The mutual inductance (M) can be exressed by (a) (b) (c) s M I M t I M I (d) s (10) If one volt emf is induced in a coil by changing current at a rate of 1 amere er second in the coil, then self inductance is called (a) 1 henry (b) 1volt (c) 1 Weber (d) 1 ohm (11) If the wire is wound on an iron core its flux would be (a) Remain same (b) decreases (c) Increases (d) zero (1) The work done by the battery in moving a small charge Δq against the induced e.m.f is Prof. Muhammad Amin 1 L M t I

2 (a) W= Δq εl. (c) W=1/ Δq εl (13) The energy stored in an inductor is (a) Um 1 LI 1 (c) Um (14) An exression for energy density is given by (b) W= Δq /εl (d) W= εl / Δq (3) For an ideal transformer (a) ower inut = ower outut (b) ower inut < ower outut (c) ower inut > ower outut (4) The loss of ower in transformers can be reduced by (a) using laminated sheets of a core material Prof. Muhammad Amin (b) L I (d) U m U m LI. 1 B 1 B (a) U m (b) Um o o 1 B 1 o (c) Um (d) U m 4 o B (15) A device which converts mechanical energy into electrical energy is called (a) motor (b) inductor (c) transformer (d) current generator (16) The working of A.C generator is based uon the (a) self induction (b) mutual induction (c) electromagnetic induction. (d) all of these (17) The magnitude of back emf, when motor is over loaded (a) increase (b) decrease (c) remain same (d) become zero (18) When motor started then back emf is (a) at its eak value (b) high (c) almost zero (19) A dynamo converts (a) electrical energy into mechanical energy (b) mechanical energy into electrical energy (c) magnetic energy into electrical energy (d) heat energy into electrical energy (0) Transformers work on the rincile of (a) self induction (b) electrostatic induction (c) mutual induction (1) A device which is used to increase or decrease the value of alternating voltage is called (a) A.C generator (b) D.C motor (c) A.C motor (d) Transformer () The equation for the transformer is given by Ns V N s Vs (a) (b) N V Ns V N I s (c) N I s LI (d) both a & c

3 (b) decreasing the resistance of the coils (c) roer couling of rimary and secondary coils (d) all of these (5) In ste down transformer (a) N > Ns (b) N < Ns (c) N = Ns (6) Transformers work on (a)a.c (b) D.C (c) both a and b (7) The mathematical exression for the self inductance is given by (a) Nφ = I (b) NL = φi (c) Nφ = LI (d) Lφ = NI (8) The energy stored er unit volume is called (a) energy density (b)charge density (c) mass density (d)volume density (9) The emf roduced by an Alternating generator is (a) NωABtanθ (b) NωABcosθ (c) NωABsinθ (d) NABsinθ (30) Usually the magnetic field is rovided by (a) bar magnet (b) electromagnet (c) hantom bar magnet (d) all of these (31) In D.C motor the slit rings act as (a) inductor (b) caacitors (c) resistors (d) Commutator (3) The back emf of a motor can be exressed as (a) V IR (b) V IR (c) IR V I V (d) R (33) In an ideal transformer the outut voltage is to the inut voltage (a) less than (b) equal (c) greater than (d) may have any value (34) The magnetic flux linked with a circuit when a unit current flows through it is known as (a) Induced current (b) Induced e.m.f (c) Coefficient of self inductance of conductor (d) none of the above (35) The comonent in a generator which consumes electrical energy is called (a) Caacitor (b) Load (c) Slit rings (d) Commutator (36) If we make the magnetic field stronger, the value of induced current is (a) Decreased (b) Increased (c) Vanished (d) Ket constant (37) Alternating current generators use: (a) Coiled rings (b) Slit rings (c) Sli rings (d) Solenoid rings Prof. Muhammad Amin 3

4 (38) If velocity of a conductor moving through a magnetic field B is made zero, then motional emf is (a) vbl (b) v (c) (d) zero BL BL v (39) The rincile of an alternating current generator is based on (a) Coulomb s law (b) Amere s law (c) Faraday s law (d) Lenz s law (40) The energy density of an inductor is B (a) 0 (b) (c) (41) The inductance is more in self induction in (a) Air cored coil (b) Iron cored (c) Tungsten cored (d) Steel cored o B (4) Induced emf in a coil according to Faraday s Law is exressed as: (a) ε = -N (b) ε = -N N t (c) ε = -N (d) ε = -N t N (43) When the back emf in a circuit is zero, it draws: (a) zero current (b) maximum current (c) minimum current (d) steady average current (44) The ractical alication of the mutual induction henomena is (a) electric motor (b) transformer (c) A.C generator (d) transistor (45) SI units of mutual inductance and self-inductance are: (a) same (d) different (c) no units (46) An inductor may store energy in: (a) its magnetic field (b) its electric field (c) its coil (d) a neighbouring circuit (47) The ratio of average induced emf to the rate of change of current in the coil is called (a) self-inductance (b) mutual inductance (c) self-induction (d) mutual induction (48) The self-inductance of a long solenoid with n turns er unit length is o (a) L na (b) L na o o (c) L o n A (d) L n A (49) Alternating current generator converts which tye of energy into electrical energy? (a) mechanical energy (b) chemical energy (c) solar energy (d) otential energy o B (50) Eddy currents roduced in the core of transformer are resonsible for (a) heat loss (b) ste-u rocess (c) ste-down rocess (d) induction henomenon Prof. Muhammad Amin 4 (d) B μ o

5 (51) When a motor is just started, back emf is almost (a) Maximum (b) Zero (c) Minimum (d) Infinite (5) Turn ratio of a transformer is 50. If 0 V AC is alied to its rimary coil, voltage in the secondary coil will be (a) 44 V (b) 4.4 V (c) 0 V (d) 1100 V (53) The direction of induced current is always so as to oosite the change which causes the current is (a) Faraday s law (b) Lenz s law (c) Ohm s law (d) Kirchoff s 1 st rule (54) If the motor is overloaded, then the magnitude of Back emf. (a) Increases (b) Decreases (c) Constant (d) Becomes zero (55) Which of the following is not resent in A.C. generator (a) Armature (b) Magnet (c) Sli rings (d) Commutator (56) Magnetic otential energy stored in an inductor deends uon (a) under root of the value of current (b) cube root of the value of current (c) cube of the value of current (d) square of the value of current (57) Maximum emf generated in a generator is (a) εo εsin θ (b) ε εo sin θ (c) ε N ω ABsinθ (d) εo N ω AB (58) The SI unit of magnetic induction tesla is equal to (a)na -1 m -1 (b)nam -1 (c)na -1 m (d) NA - m -1 (59) Lenz s law is in accordance with the law of conservation of (a) Momentum (b) Charge (c)angular momentum(d) Energy (60) If magnetic energy density becomes : (a) Four Times (b) Two Times (c) Three Time (d) Six times Prof. Muhammad Amin 5

6 SHORT QUESTIONS 15.1 Does the induced emf in a circuit deend on the resistance of the circuit? Does the induced current deend on the resistance of the circuit? According to Faraday s law, induced emf = N t This shows that induced emf is indeendent of the resistance of the circuit But I R This shows that induced current deends on the resistance of the circuit. Induced current decreases with the increase of R. 15. A square loo of wire is moving through a uniform magnetic field. The normal to the loo is oriented arallel to the magnetic field. Is a emf induced in the loo? Give a reason for your answer. When a square loo of wire is moving through a uniform magnetic field, the normal to the loo is oriented arallel to the magnetic field and induced emf in the loo is zero. Because loo is not cutting the magnetic lines of force so there is no change in magnetic flux. N t N A light metallic ring is released form above into a vertical bar magnet Fig. given, Viewed for above, does the current flow clockwise or anticlockwise in the ring? According to Lenz s law the direction of induced current is such as to oose the cause which roduces it. Hence the face of the ring towards magnet must be North ole and towards the observer which views it from above the south ole. Therefore, current induced as viewed from above is in clockwise direction. Prof. Muhammad Amin 6

7 15.4 Does the induced emf always act to decreases the magnetic flux through a circuit? No, According to Lenz s law emf induced always ooses the cause which roduces it. Hence when magnetic flux through coil increases, induced emf tries to decrease it and when magnetic flux through coil decreases, induced emf tries to increases it The Fig. shows a coil of wire in the x-y lane with a magnetic field directed along the y-axis. Around which of the three coordinate axes should the coil be rotated in order to generate an emf and a current in the coil? According to Faraday s law, induced emf is only roduced in loo if rate of change of flux takes lace as When the coil is rotated about x-axis the magnetic field lines through the loo changes and hence emf or induced current is roduced, But there will be no induced emf, if coil is rotated about y-axis or z-axis because of no change in flux How would you osition a flat loo of wire in a changing magnetic field so that there is no emf induced in the loo? If a flat loo of wire is laced arallel in changing magnetic field, then no change of flux takes lace and no emf is induced in the loo. BACos N 0 t 15.7 In a certain region the earth s magnetic field oint vertically down. When a lane flies due north, which wingti is ositively charged? As the lane is moving towards north in a region where B is vertically down. Thus if all the fingers of right hand are stretched down keeing thumb erect along the motion of lane i.e. north ward. The alm of right hand indicates the direction of magnetic force on ositive charges towards west. Thus ositive charges in the body of lane moves towards West wing ti making it ositively charged as shown in figure. Prof. Muhammad Amin 7

8 15.8 When an electric motor, such as an electric drill, is being used, does it also act as a generator? If so what is the consequence of this? A running electric motor also acts like a generator. The armature rotates due to which change of magnetic flux occurs. An induced emf called back emf of motor is generated. If the motor is over loaded the back emf induces a large current. As a result the electric motor may burn Show that have the same units. L.H.S Induce emf = = R.H.S W q = J C = volt BA tesla m N T sec Am N m ( Nm J ) Am. sec J Asec = J C sec sec J volts C C A sec Hence L.H.S. = R.H.S Can a D.C motor be turned into a D.C generator? What changes are required to be done? A D.C. Generator can be converted in to D.C. Generator because their construction is same. The armature coil of the motor is couled with some rotating body. Due to rotation, the magnetic flux through the coil changes and as a result, emf is induced at the outut. So by this change, motor is converted into generator. Prof. Muhammad Amin 8

9 15.11 Is it ossible to change both the area of the loo and the magnetic field assing through the loo and still not have an induced emf in the loo? yes, it is ossible to change both area of loo and magnetic field without roducing of emf. As we know N t BA. if magnetic field is increased and area is decreased, such that roduct BA. remains same, then there is no change of flux and therefore no emf is induced. Mathematically Can an electric motor be used to drive an electric generator with the outut from the generator being used to oerate the motor? No an electric motor can not be used to drive an electric generator with the outut from the generator being used to oerate the motor. Because it is the violation of law of conservation of energy. It becomes a self oerating system which works without getting energy from external source A susended magnet is oscillating freely in a horizontal lane. The oscillations are strongly damed when a metal late is laced under the magnet. Exlain why this occurs? (Due to the oscillations of the magnet,) the magnetic flux assing through the metallic late changes. This change of magnetic flux roduces an induced emf and hence induced current or eddy current is roduced in the metal late. The induced current roduces its own magnetic field which always ooses the motion of the magnet. This is according to the Lenz s Law. Hence the oscillatory motion of the bar magnet is damed Four unmarked wires emerge from a transformer. What stes would you take to determine the turns ratio? Two ends of the wire are ends of the rimary coil and the remaining two as the ends of the secondary coil. (i) (ii) (iii) (iv) First we use the ohmmeter to check the continuity of the airs. We searate them into two airs, rimary and secondary. The rimary coil is connected with known voltage (VP) of the A.C. suly and with the hel of voltmeter measure the secondary voltage VS. Find the ratio turns by using the following relation. Vs Ns V N (a) Can a ste-u transformer increase the ower level? (a) In a transformer, there is no transfer of charge from the rimary to the secondary. How is, then the ower transferred? (a) A ste u transformer can not increase the ower level because it is the violation of law of conservation of energy. Power inut is always equal to ower outut. For an ideal transformer ower inut is equal to ower outut. Pin = Pout Prof. Muhammad Amin 9

10 (b) The rimary and secondary coils are made u of insulated coer wire. There is no electrical connection between rimary and secondary coils. But the two coils are wound on same iron core. Therefore the coils are magnetically linked through core. When magnetic flux changes in the rimary, it also changes in the secondary coil. As a result emf is induced in the secondary coil. That is why ower is transformed from rimary coil to secondary coil When the rimary of a transformer is connected to a.c mains the current in it (a) Is very small if the secondary circuit is oen, but (b) Increases when the secondary circuit is closed. Exlain these facts. (a) If the secondary circuit is oen, the outut ower used is zero. thus a very small amount of current is drawn by the transformer A.C. main. (b) When the secondary circuit is closed, the outut increases. To maintain the inut ower equal to outut ower, the current draw by the transformer from the A.C. main also increases. ADDITIONAL SHORT QUESTIONS (1) How is the efficiency of transformer increased? The efficiency of a transformer can be increased by reducing the ower losses. It can be done by taking the following stes. (i) Loo Area:- Core is assembled from the laminated sheets of a material whose hysteresis loo area is very small. (ii) Insulation:- The insulation between the sheets of core should be erfect to sto eddy currents. (iii) Resistance of rimary and secondary coils:- The material of wires of rimary and secondary coils must be such that their electrical resistance is very small as may be ossible. (iv) Flux Couling:- Primary and secondary coils should be wound in such a way that flux couling between them is maximum. () Discuss the deendence of inductance on current? What other factors effect the inductance of coil? Self Inductance is given by N A o L l Hence, inductance does not deend uon current, but it deends uon (i) Area of coil (ii) Length of the coil (iii) Material of core (iv) Number of turns of the coil (3) Define self inductance, on which factors does it deend. Self inductance, L, is defined as the ratio of the induced e.m.f to the rate of change of current in the same coil. It deends uon (i) The number of turns of the coil. (ii) The cross-sectional area of coil. Prof. Muhammad Amin 10

11 (iii)and the core material. Its equations given as L = m n o AL (4) Differentiate between mutual induction and mutual inductance. Mutual Induction: (i) The henomenon in which a changing current in one coil induces an emf in another coil is called mutual induction. (ii) It is a rocess of induction Mutual Inductance: (i) It is defined as the ratio of average emf induced in the secondary to the time rate of change of current in the rimary. (ii) It is the constant of roortionality. Its unit is Henry. (5) What is back motor effect in generators. In generator, coil is rotated by some external means in a magnetic field and current is induced in the coil. Now the magnetic field of magnet exerts equal and oosite forces on the two sides of current carrying coil. These forces roduce a counter torque that ooses the rotational motion of the coil. This effect is called back motor effect in generators. (6) State Faraday s law and Lenz s law. Faraday s law: The average emf induced in a conducting coil of N loos is equal to the negative of the rate at which the magnetic flux through the coil is changing with time. Lenz s law: The direction of the induced current is always so as to oose the change which causes the current. (7) Using the relation for mutual inductance, show that S.I unit of mutual inductance is VsA -1.What is the common name of this unit? Relation for Mutual Inductance M is M Es I t P So S.I unit of mutual inductance is V s V A / s A 1 V.A s Common name: Common name for VA -1 s is henry denoted by the symbol H. H = VA -1 s (8) Define Henry. It is a unit of inductance defined searately for self inductance and mutual inductance as follows: Self inductance of a coil is called one hennery if induced emf of One volt is roduced in it when current through it is changing at the rate of one amere er second. i.e. 1H = 1VA -1 s And Mutual inductance of a air of coils is said to be one henry if induced emf of one volt is roduced is secondary when current in rimary coil is changing at the rate of one amere er second. i.e. 1 H = 1 VA -1 s Prof. Muhammad Amin 11

PHYS 1442 Section 004 Lecture #15

PHYS 1442 Section 004 Lecture #15 Monday March 17, 2014 Dr. Andrew Brandt Chapter 21 Generator Transformer Inductance 3/17/2014 1 PHYS 1442-004, Dr. Andrew Brandt Announcements HW8 on Ch 21-22 will be