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Q.1 A.1 Q.2 A.2 Q.3 A.3 Q.4 A.4 Q.5 A.5 Q.6 A.6 Q.7 A.7 Q.8 A.8 Q.9 A.9 A conducting loop is placed above a long straight conductor carrying current from left to right. What will be the direction of the induced current in loop if current in conductor is increased? The direction of magnetic field intensity of induced current will be such that it will oppose the magnetic field intensity of conductor. Thus the current in conductor will be in clockwise direction. An artificial satellite with metal surface is always launched in equatorial plane why? This is done because if the satellite moves in equatorial plane flux linked with the satellite is not changing thus the induced emf will be zero. Does Lenz’s law violate the law of conservation of energy? No, it does not violate the law of conservation as the work done to overcome the opposing force of induced emf will be used to generate electrical energy. When a magnet falls through the vertical coil, will its acceleration be different from acceleration due to gravity? Yes, because when the magnet falls through the coil induced emf will be generated which will be oppose the motion of the magnet. Thus acceleration of magnet is less than the acceleration due to gravity. Why the soft iron core is used in dead beat galvanometer? Soft iron core is used in dead beat galvanometer because the eddy currents in the soft iron core will oppose the motion of the coil and it comes to rest instantly. two identical loops one of copper and other of iron are rotated with same angular velocity. In which case will be the induced emf more? Induced emf doesn’t depend upon the nature of material but depends only on the magnetic field, angular velocity and radius of the loop. A lamp is connected in parallel with the coil having large self inductance glows brilliantly before going off. Why? The lamp glows brilliantly before going off due to large induced emf and thus large induced current. A wire kept along north south direction is allowed to fall freely. Will there be induced emf in wire? There will be no induced emf as magnetic flux is not changing. A metal block and a brick of same mass are falling from the same height above the ground. Which will reach the ground earlier and why? brick will reach the ground earlier as induced emf will be there in the metal block which will oppose the motion of the block and its acceleration will be reduced. Sanjay Chopra M.Sc[Hons] Physics Q.10 A.10 Q.11 A.11 Q.12 A.12 Q.13 A.13 Q.14 A.14 Q.15 A.15 Q.16 A.16 Q.17 A.17 Why a thick metal plate oscillating about horizontal axis stops when a strong magnetic field is applied on the plate? This happens because when the magnetic field is switched on the induced emf and thus eddy currents are produced in metal plate due to motion of the plate. The direction of eddy currents will be such that they opposes the motion of the plate and it stops. What are the factors on which the self inductance of coil depends? Self inductance of the coil depends upon the dimensions of the coil, number of turns and medium used as core in coil. how does the self inductance of the coil changes when iron is used as core in place of air? Permeability of iron is much greater than the permeability of air. Thus self inductance with iron core will also increase. How does the mutual inductance between the coils changes when distance between the coils is increased? On increasing the distance the flux linkage between the coil decreases i.e. lesser number of field lines of primary passes through secondary. Thus mutual inductance will decrease. Two similar current carrying coils carry equal currents in same direction. If the loops are brought closer, what will happen to the currents in them? when the loops are brought closer, the magnetic flux linked with the coils increases. Thus induced emf will be produce to oppose this increase and current in coils will decrease. An iron bar falling through hollow region of the cylindrical shell made of copper experiences a retarding force. What can you conclude about the iron bar? The motion of iron bar is retarded because flux linked with it is changes which will induce emf. The induced emf will oppose motion of iron bar. But flux can change only if the iron bar is magnet. How can a space traveler detect the presence of magnetic field on an unknown planet with the help of galvanometer and coil of wire? The traveler can fix coil with galvanometer. If the coil is rotated and galvanometer shows deflection, it implies presence of magnetic field. But if there is no magnetic field then the deflection in galvanometer will not be there. An induced emf has no direction of its own. Comment. Induced emf has no direction of its own as it always opposes the change the flux. Thus direction of induced emf is governed by the cause of change in flux. Q.18 A.18 Q.19 A.19 Q.20 A.20 Q.21 A.21 Q.22 A.22 Q.23 A.23 Q.24 A.24 Q.25 A glass rod of length l moves with velocity v in uniform magnetic field B. what is the emf induced emf in the rod? Induced emf in the rod will be zero as glass is an insulator and no movement of charges takes place in it due to magnetic field. A conductor is placed in magnetic field is moved parallel to the magnetic field B. What is the emf induced in the loop? No, emf is induced in the loop as motional emf is induced if the rod is moved perpendicular to the magnetic field. two coils are being moved out of magnetic field-one coil is moved rapidly and the other slowly. In which case is more work done and why? More work will be done in case of rapidly moving coil. This is because according to Faraday’s law the induced emf is directly proportional to the rate of change of flux linked with the coil. Why the inductance per unit length for solenoid near its center different from inductance per unit length near the ends? Self inductance depends upon the magnetic field of solenoid and field at the center is double of the field at the center of solenoid. Thus self inductance per unit length will also be different. Will an induced current be always produced when there is change in magnetic flux linked with the coil? No, induced current is not essential, whenever flux changes emf is induced but the induced current flows only when the conducting circuit is closed. A closed conducting loop moves normal to the electric field between the plates of large capacitor. Is a current induced in the loop when it is [a]wholly inside the capacitor [b] partially inside the capacitor? No, in both the cases induced current or induced emf will not be there because induced current flows due to the changing magnetic flux and not due the changing electric flux. Why birds fly off a high tension wire when current is switched on? When current begins to increase from zero to maximum value, the magnetic lines of force and thus magnetic flux through the body of bird changes. The change in flux induces current in the body of bird which produces repulsive force and birds fly off. An electron moves in circle with uniform speed in stationary magnetic field normal to the plane of the circle. If the field is made to increase with time, will electron speed up or speed down? Will it continue to revolve in the same circle? Sanjay Chopra M.Sc[Hons] Physics A.25 Q.26 A.26 Q.27 A.27 Q.28 A.28 Q.29 A.29 Q.30 A.30 Q.31 A.31 Q.32 A.32 Using Lenz’s law it is easy to see that the electron speeds up is B increases with time. The electron will not stay in same circular orbit because the speed of electron may not increase in proportion to the magnetic field intensity. A copper ring is held horizontal and a bar magnet is made to fall through the ring with the length along the axis of the ring. Will the acceleration of the falling magnet be equal to greater than or less than g? When the magnet approaches the ring, flux through the ring changes. Induced emf is produced which will oppose the cause of change in flux. As the cause is motion of magnet therefore its motion is opposed and less than g. A small resistor is usually put in parallel with the current carrying coil of an electromagnet. What purpose does this serve? When the current in the coil of large electromagnet is suddenly switched off, flux changes from large value to zero in short time. Thus large induced emf is produced. A low resistor provides conducting path through which current can flow thus reducing risks of sparks. Why the oscillations of copper disc in magnetic field are highly damped? The eddy currents are produced in the copper disc which will oppose the motion of the disc thus its motion is highly damped. Why does capacitor locks dc while ac is allowed to pass through it? Capacitive reactance i.e. resistance offered by the capacitor to the flow of current through it is inversely proportional to frequency I,e, Xc = 1/C. For dc = 0 , thus capacitive reactance is infinity and dc gets blocked. Why ac is more dangerous than dc of the same voltage? AC is more dangerous than dc of the same voltage because peak value of ac is 2 times the rms value. Thus 220V ac will have a peak value of 310V. What are capacitive and inductive reactance and what is phase difference between them? Capacitive reactance is the resistance offered by the capacitor to the flow of current through it and inductive reactance is resistance offered by inductor to the flow of ac through it. Phase difference between inductive and capacitive reactance is 1800. Which is best method of reducing current in an ac circuit and why? the best method of reducing current in an a circuit making use of choke coil. If the choke coil is assumed to be resistance less, then there is no dissipation of energy. Q.33 Why cannot we use ordinary moving coil galvanometer for measuring ac currents and voltages? A.33 Because it measures the average value of emf/current and the average value of ac is zero. Therefore it cannot measure ac current and voltages. Q.34 Can we use 20c/s for lightning purposes? A.34 yes 20c/s can be used for lightning purposes. The fluctuations in the current will be so rapid[40 times/s] that the bulb will appear to glow continuously. Q.35 Is any power consumed by ideal choke coil? A.35 Power consumed by a device is directly proportional to the power factor. For choke coil power factor cos , for choke coil =900, thus power factor is zero and power consumed is zero. Q.36 Why alternating current measuring instruments has a non-linear scale? A.36 Because these instruments are based on heating effects of currents and heat generated is proportional to I2. Q.37 What is the significance of time constant in an LR circuit? A.37 The time constant of LR circuit tells us how fast or slow the growth or decay of current takes place in an LR circuit. Q.38 In any ac circuit, is the applied voltage equal to the algebraic sum of instantaneous voltages across different circuit elements? Is the same true for rms voltages also? A.38 Yes applied instantaneous voltage is equal to algebraic sum of instantaneous voltages across circuit elements. No, this is not true for rms values because such voltages across different circuit elements are not in same phase. Q.39 Can a capacitor of suitable capacitance be used in place of choke coil? A.39 Yes, because average power consumed b capacitor also in one complete cycle of ac is zero. Q.40 The dc and ac both can be measured by hot wire instrument. Why? A.40 Hot wire instruments are based on the principle of generation of heat, which is proportional to the square of current flowing through it. Q.41 A lamp is connected in series with a capacitor. Predict your observations for dc an ac connections. What happens in each case if the capacity is reduced? A.41 Capacitor blocks dc therefore bulb will not glow even if the capacity is reduced if it connected to dc. In ac connections on decreasing C capacitive reactance increases thus the intensity of glow deceases. Q.42 An electric heater is used with ac and dc turn by turn. If the potential difference is kept Sanjay Chopra M.Sc[Hons] Physics A.42 Q.43 A.43 Q.44 A.44 Q.45 A.45 Q.46 A.46 Q.47 constant in the two cases. Will the heat produced be same in two cases? An electric heater is in the form of coil or inductor with inductive reactance L. For dc inductive reactance is zero. Thus current flow in the case of dc is more than the current flow in the case of ac. Thus heat produced in the case of ac will be less than dc. At very high frequency of ac, capacitor behaves as pure conductor. Why? The capacitive reactance is inversely proportional to the frequency of ac. Thus for very high frequency the capacitive reactance reduces to zero and it behaves as a pure conductor. What is the minimum value of power factor? When does this occur? Minimum value of power factor is zero and it occurs for purely inductive or capacitive circuit. What is the phase difference between the voltages across L and C in series LCR circuit connected to ac source? Potential difference across L and C in series LCR circuit is 180. Power factor can be improved by using capacitor of appropriate value in the circuit. Explain. Power factor in LCR circuit is R/Z. When capacitance is changed for one particular value of C, inductive and capacitive reactance is equal, then Z=R and power factor becomes maximum.