KENDRIYA VIDYALAYA SANTRAGACHI

CH:- ELECTROMAGNETIC INDUCTION

VERY SHORT ANSWER QUESTIONS (1 MARK)

  1. Write Faraday’s laws of electromagnetic induction[CBSE (AI) 2009]
  2. State Lenz’s law (1)
  3. Show that Lenz’s law is in accordance with the law of conservation of energy.
  4. When current in a coil changes with time, how is the back emf induced in the coil related to it? [CBSE (AI) 2008]
  5. How does the mutual inductance of a pair of coils change when

(i)distance between the coils is increasedand

(ii)number of turns in the coils is increased? [CBSE (AI) 2013]

  1. Write S.I unit of magnetic flux. Is it a scalar or vector quantity? (1)
  2. The motion of copper plate is damped when it is allowed to oscillate between the two poles of a magnet. What is the cause of this damping? [CBSE (AI) 2013]
  3. The closed loop PQRS is moving into a uniform magnetic field acting at right angles to the plane of the paper as shown. State the direction of the induced current in the loop. [CBSE (AI) 2012]

X X X X X P Q

X X X X X

X X X X X

X X X X X

X X X X X

X X X X X R S

SHORT ANSWER QUESTIONS (2, 3 MARKS)

  1. Two identical loops, one of copper and the other of aluminium, are rotated with the same angular speed in the same magnetic field. Compare (i) the induced emf and (ii) the current produced in the two coils. Justify your answer. [CBSE (AI) 2010]
  2. Define self-inductance of a coil. Obtain an expression for the energy stored in a solenoid of self-inductance L when the current through it grows from zero to I. [CBSE (AI) 2015]
  3. Define the term mutual inductance between the two coils. Obtain the expression for mutual inductance of a pair of long co-axial solenoids each of length l and radii r1and r2.Total number of turns in the two solenoids are N1 and N2 respectively.

[CBSE (AI) 2014, 2009]

LONG ANSWER QUESTIONS (5 MARKS)

Questions that have been repeated two times

LONG ANSWER QUESTIONS (5 MARKS)

  1. What are eddy currents? How are they produced? In what sense eddy currents are considered undesirable in a transformer? How can they be minimized? Give two applications of eddy currents. [CBSE (AI) 2006, 2011]
  2. State the working of a.c. generator with the help of a labelled diagram. The coil of an ac. Generator having N turns, each of area A, is rotated with a constant angular velocity. Deduce the expression for the alternating emf generated in the coil. What is the source of energy generation in this device? [CBSE (AI) 2008C, 2011, 2017]

A horizontal conducting rod 10 m extending from east to west is falling with a speed of 5.0 m/s at right angle to the horizontal component of the Earth’s magnetic field, 0.3 X 10-4 Wbm-2. Find the instantaneous value of the emf induced in the rod. [2017]

Expected Questions for MLL

  1. Write an expression for the energy stored in an inductor of inductance L, when a steady current is passed through it. Is the energy electric or magnetic? (1)
  1. Derive expression for self-inductance of a long air-cored solenoid of length l, cross-sectional area A and having number of turns N.
  1. What do you mean by mutual inductance of two nearby coils? Find an expression for mutual inductance of two co-axial solenoid. (5)

CHAPTER – 6 (ALTERNATING CURRENT)

VERY SHORT ANSWER QUESTIONS (1 MARK)

  1. Mention the two characteristic properties of the material suitable for making core of a transformer. [ CBSE (AI) 2012]
  2. In a series LCR circuit, the voltage across an inductor, a capacitor and a resistor are 30 volt, 30 volt and 60 volt respectively. What is the phase difference between the applied voltage and current in the circuit? [ CBSE (AI) 2007]
  3. The instantaneous current and voltage of an a.c circuit are given by i= 10 sin 314t ampere, v= 50 sin 314t volt. What is the power dissipation in the circuit? [ CBSE (AI) 2008]

SHORT ANSWER QUESTIONS (2, 3 MARKS)

  1. (a) For a given a.c, i= i0sinωt, Show that the average power dissipated in a resistor R over a complete cycle is ½( i0 )2 R.

(b) A light bulb is rated at 100 watt for a 220 volt a.c supply. Calculate the resistance of the bulb. [CBSE (AI) 2013]

  1. State the principle of working of a transformer. Can a transformer be used to step up or step down a d.c voltage? Justify your answer [CBSE (AI) 2011]
  2. How is the large scale transmission of electric energy over long distances done with the use of transformers? [CBSE (AI) 2012]
  3. An a.c voltage V= V0sin wt is applied across a

(a)Series RC circuit in which capacitive reactance is a times the resistance of the circuit.

(b)Series RL circuit in which inductive impedance is ‘b’ times the resistance in the circuit.

Find the value of power factor of the circuit in each case.

  1. An AC source of voltage V= Vmsin wt is applied across a series LCR circuit. Draw the phasor diagrams for the circuit, when

(i)Capacitive reactance exceeds the inductive reactance.

(ii)Inductive reactance exceeds capacitive reactance.

[CBSE (AI) 2008C]

  1. A voltage V= Vosin wt is applied to a series LCR circuit. Derive the expression for the average power dissipated over a cycle. Under what condition is (i) no power dissipated even though the current flows through the circuit. (ii) Maximum power dissipated in the circuit. [CBSE (AI) 2014]
  2. You are given three circuit elements X,Y and Z. When the element X is connected across an a.c. source of a given voltage, the current and the voltage are in the same phase. When the element Y is connected in series with X across the source, voltage is ahead of the current in phase by π/4. But the current is ahead of the voltage in phase by π/4 when Z is connected in series with X across the source. Identify the circuit elements X, Y and Z. When all the three elements are connected in series across the same source, determine the impedance of the circuit. Draw a plot of the current versus the frequency of the applied source and mention the significance of this plot.[CBSE (AI) 2015]

LONG ANSWER QUESTIONS (5 marks)

  1. Define the term capacitive reactance. Show graphically the variation of capacitive reactance with frequency of applied alternating voltage.

An ac voltage V= Vosinwt is applied across a pure capacitor of capacitance C. Find an expression for current flowing through it. Show mathematically the current flowing through it leads the applied voltage by angle (π/2). [CBSE (AI) 2008C]

  1. State the condition for resonance to occur in series LCR a.c circuit and derive an expression for resonant frequency. Draw a plot showing the variation of the peak current with the frequency of the a.c source used. Define quality factor Q of the circuit.[CBSE (AI) 2008]
  2. Draw a schematic diagram of a step-up transformer. Explain its working principle. Deduce the expression for the secondary to primary voltage in terms of the number of turns in the two coils. In an ideal transformer, how is the ratio related to the currents in the two coils? How is the transformer used in large scale transmission and distribution of electrical energy over long distances?
  3. A device X is connected to an ac source V = Vo sinwt. The variation of voltage, current and power in one cycle is as shown in the following graph:

YBAC

2π

(a)Identify the device X.

(b)Which of the curves A, B and C represent the voltage, current and power consumed in the circuit? Justify your answer.

(c)How does its impedance vary with frequency of the AC source? Show graphically.

(d)Obtain an expression for the current in the circuit and its phase relation with ac voltage. (2017)

Expected questions for MLL

  1. What is the phase difference between the voltages across the inductor and a capacitor in an AC circuit? (1)
  2. What is phase difference between voltage and current in a LCR series circuit at resonance? (1)
  3. The peak value of e.m.f. in ac is E0. Write its (i) rms (ii)average value over a complete cycle.[CBSE (F) 2011] (1)
  4. An electrical element X when connected to an alternating voltage source, has a current through it leading the voltage by (π/2)rad. Identify X and write an expression for its reactance . (1)
  5. What will be the effect on inductive reactance and capacitive reactance if frequency of AC source increased? (1)
  6. What is wattless current?[CBSE (DELHI) 2011] (1)
  7. Define power factor? State the conditions under which it is maximum and minimum.[CBSE (DELHI) 2010] (2)
  8. An air cored coil L and a bulb B are connected in series to the AC mains. The bulb closed with brightness. How would the glow of the bulb change if an iron rod were inserted in the coil? give reasons in support of your answer. (2)
  9. Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency. (2)