UNIT
4 – AC AND ELECTROMAGNETIC INDUCTION
2025
1. Figure
3 below shows an ideal transformer. Explain why current flowing through
secondary coil is greater than that in primary coil.
2. Figure
9 shows a metallic rod AB of length l = 3 m moving in an (external uniform
magnetic field, B =(1/π) T which is directed into the plane of this paper
If the position of the rod changes
with time as x = πt, where x is in metre and t is in second, then:
(1) Calculate the motional emf
developed in the rod.
(2) Name the law used to find the
direction of induced current.
3. When
current flowing through a solenoid decreases from 15 A to 0 in 0.2 s, an emf of
30 V is induced in it. Calculate the coefficient of self-inductance of the
solenoid.
4. An
alternating emf E = 5.0 sin (314 t) V is applied to a circuit containing a
resistor connected in series with an unknown component X.
The current in the circuit is found
to be I = 3.0 sin π (314t-π/3) A.
(a) Identify the component X.
(b) Calculate rms value (Irms)
of the current flowing through the circuit.
(c) Find the frequency of the source.
(d) Calculate power factor.
(e) Find the impedance (Z) of the
circuit.
2024
5. (i)
What type of transformer is used in a mobile phone charger?
(ii) Why is the core of a transformer
made of soft iron and not of steel?
6. (a)
A 220V, 50Hz ac source is connected to a coil having coefficient of
self-inductance of 1 H and a resistance of 400Ω. Calculate: (1) the reactance
of the coil. (2) the impedance of the coil. (3) the current flowing through the
coil.
(b) Draw a labelled graph showing
variation of impedance (Z) of a series LCR circuit Vs frequency (f) of the ac
supply. Mark the resonant frequency as f0.
7. (a)When
current flowing through a solenoid decreases from 5A to 0 in 20 milliseconds,
an emf of 500V is induced in it.
(1) What is this phenomenon called?
(2) Calculate coefficient of
self-inductance of the solenoid.
2023
8. What
is meant by “Motional emf”?
9. When
an electric current is passed through a wire or a coil, a magnetic field is
produced. Is the reverse phenomenon possible i.e., can a magnetic field produce
an electric current? Explain with the help of an appropriate example.
10. A
60 Ω resistor, a 1.0 H inductor and a 4mF capacitor are connected in series to
an ac supply generating an emf e = 300 sin (500t) V. Calculate:
(a) impedance of the circuit.
(b) peak value of the current flowing
through the circuit.
(c) phase difference between the
current and the supply voltage.
11. (a)
An ac generator generates an emf which is given by e = 311 sin(240pt)V.
Calculate:
(1) frequency of the emf.
(2) r.m.s. value of the emf.
12. The
primary coil of a transformer has 60 turns whereas its secondary coil has 3000
turns.
(1) If a 220V ac voltage is applied
to the primary coil, how much emf is induced in the secondary coil?
(2) If a current of 5A flows in the
primary coil, how much current will flow in a load in the secondary coil? State
the assumption you have made regarding the transformer, in this calculation.
2022
13. Two
coils P and S are wound on a common core. Their coefficient of mutual
inductance does not depend on:
(a) Number of turns in the coil P.
(b) Number of turns in the coil S.
(c) Material of the core.
(d) Voltage applied to the coil P or
Q.
14. Which
one of the following statements is not true at resonance of a series LCR
circuit?
(a) Reactance of the capacitor is
equal to the reactance of the inductor.
(b) Impedance of the circuit is
minimum
(c) Current in the circuit is minimum
(d) Current is in phase with supply
voltage.
15. A
graph of reactance of capacitor Xc vs frequency (f) of an ac source is
16. Consider
a strong bar magnet NS and a solenoid AB as shown in Figure 3 below. G is a
central zero galvanometer. Both the bar magnet and the solenoid are at rest.
The bar magnet is now moved slowly to
the new position N’S’. The pointer of the galvanometer G
(a) remains undeflected at 0 mark.
(b) shows a constant deflection.
(c) shows a momentary deflection and
comes back to zero.
(d) vibrates simple harmonically
about 0 mark.
2020
17. Write
an expression for force per unit length between two long current carrying
wires,kept parallel to each other, in vacuum and based on that define an
ampere, the SI unit of current.
18. Figure
1 below shows a metallic rod MN of length l = 80 cm, kept in a uniform magnetic
field of flux density B – 0.5T, on two parallel metallic rails P and Q.
Calculate the emf that will be induced between its two ends, when it is moved
towards right with a constant velocity v = 36 km/hr.
19. When
current flowing through one coil changes from 0 Amp to 15 Amp in 0.2 s, an emf
of 750 V is induced in an adjacent coil. Calculate the coefficient of mutual
inductance of the two coils.
20. (i)
A 400W resistor, a 3H inductor and a 5μF capacitor are connected in series to a
220V, 50Hz ac source. Calculate the :
(1) Impedance of the circuit.
(2) Current flowing through the
circuit.
(ii) Draw a labelled graph showing
the variation of impedance (Z) of a series LCR circuit versus frequency (f) of
the ac supply
21. (i)
When an alternating emf e = 310 sin (100πt) V is applied to a series LCR
circuit, current flowing through the circuit can be given by i = 5 sin (100πt +
π/3) A.
(1) What is the phase difference
between the current and the emf ?
(2) Calculate the average power
consumed by the circuit.
(ii) Obtain an expression for the
resonant frequency (fo) of a series LCR circuit.
2019
22. A
transformer is used to step up an alternating emf of 200 V to 440 V. If the
primary coil has 1000 turns, calculate the number of turns in the secondary
coil.
23. An
alternating emf of 200 V, 50 Hz is applied to an L-R circuit, having a
resistance R of 10 W and an inductance L of 0.05 H connected in series.
Calculate:
(1) Impedance.
(2) Current flowing in the circuit.
(3) Draw a labelled graph showing the
variation of inductive reactance (XL) versus frequency (f)
2018
24. An
alternating emf of 220V is applied to a circuit containing a resistor R having
resistance of 160Ω and a capacitor ‘C’ in series. The current is found to lead
the supply voltage by an angle θ = tan–1(3/4).
(i)
Calculate: (1) The capacitive reactance
(2) Impedance of the circuit (3) Current flowing in the circuit
(ii)
If the frequency of the applied emf is 50
Hz, what is the value of the capacitance of the capacitor ‘C’?
25. An
A.C. generator generating an emf of e= 300 sin (100π t) V is connected to a
series combination of 16 μF capacitor, 1H inductor and 100 Ω resistor.
Calculate:
(i) Impedance of the circuit at the
given frequency.
(ii) Resonant frequency ƒ0.
(iii) Power factor at resonance
frequency ƒ0.
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