Master Resource Book in JEE Main Physics

D. B. Singh

Chapter 21

Electromagnetic Induction and Alternating Current - all with Video Answers

Educators

Section 1

Round 1

Problem 1

A cylindrical bar magnet is kept along the axis of a circular coil. The magnet is rotated about its axis such that north pole faces the coil. The induced current in the coil
(a) is zero
(b) is clockwise from magnet side
(c) may be clockwise or anti-clockwise
(d) is anti-clockwise from magnet side

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 2

A uniform but time varying magnetic field $B(t)$ exists in a circular region of radius $a$ and is directed into the plane of the paper as shown in figure. The magnitude of induced electric field at point $P$ at $a$ distance $r$ from the centre of the circular region
(a) is zero
(b) decreases as $1 / r$
(c) increases as $r$
(d) decreases as $1 / r^{2}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 3

A jet plane is travelling towards west at a speed of $1800 \mathrm{~km} / \mathrm{h}$. What is the voltage difference developed between the ends of the wing having a span of $25 \mathrm{~m}$, if the earth's magnetic field at the location has a magnitude of $5 \times 10^{-4} \mathrm{~T}$ and the dip angle is $30^{\circ}$. [NCERT]
(a) $2.1 \mathrm{~V}$
(b) $3.1 \mathrm{~V}$
(c) $4.1 \mathrm{~V}$
(d) $5.2 \mathrm{~V}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 4

The wing span of an aeroplane is $36 \mathrm{~m}$. If the plane is flying at $400 \mathrm{kmh}^{-1}$, the emf induced between the wings tips is (Assume $V=4 \times 10^{-5} \mathrm{~T}$ )
(a) $16 \mathrm{~V}$
(b) $1.6 \mathrm{~V}$
(c) $0.16 \mathrm{~V}$
(d) $0.016 \mathrm{~V}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 5

The wing span of an aeroplane is $36 \mathrm{~m}$. If the plane is flying at $400 \mathrm{kmh}^{-1}$, the emf induced between the wings tips is (Assume $V=4 \times 10^{-5} \mathrm{~T}$ )
(a) $16 \mathrm{~V}$
(b) $1.6 \mathrm{~V}$
(c) $0.16 \mathrm{~V}$
(d) $0.016 \mathrm{~V}$

Saman Zulfiqar

Numerade Educator

Problem 6

A square of side $L$ metres lies in the x-y plane in a region. Where the magnetic field is given by $\mathbf{B}=B_{0}(2 \hat{\mathbf{i}}+3 \hat{\mathbf{j}}+4 \hat{\mathbf{k}}) \mathbf{T}$, where $B_{0}$ is constant. The
magnitude of flux passing through the square is [NCERT Exemplar]
(a) $2 B_{0} L^{2} \mathrm{~Wb}$
(b) $3 B_{0} L^{2} \mathrm{~Wb}$
(c) $4 B_{0} L^{2}$ Wb
(d) $\sqrt{29} B_{0} L^{2} \mathrm{~Wb}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 7

A movable wire is moved to the right crossing an anti-clockwise induced current, figure. The direction of magnetic induction in the region $P$ points
(a) to the right
(b) to the left
(c) up the paper
(d) down into the paper

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 8

A movable wire is moved to the right crossing an anti-clockwise induced current, figure. The direction of magnetic induction in the region $P$ points
(a) to the right
(b) to the left
(c) up the paper
(d) down into the paper

Saman Zulfiqar

Numerade Educator

Problem 9

An aeroplane in which the distance between the tips of the wings in $50 \mathrm{~m}$ is flying horizontally with a speed of $360 \mathrm{kmh}^{-1}$ over a place where the vertical component of earth's magnetic field is $2 \times 10^{-4} \mathrm{Wbm}^{-2}$. The potential difference between the tips of the wings would be
(a) $0.1 \mathrm{~V}$
(b) $1.0 \mathrm{~V}$
(c) $0.2 \mathrm{~V}$
(d) $0.01 \mathrm{~V}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 10

An aeroplane in which the distance between the tips of the wings in $50 \mathrm{~m}$ is flying horizontally with a speed of $360 \mathrm{kmh}^{-1}$ over a place where the vertical component of earth's magnetic field is $2 \times 10^{-4} \mathrm{Wbm}^{-2}$. The potential difference between the tips of the wings would be
(a) $0.1 \mathrm{~V}$
(b) $1.0 \mathrm{~V}$
(c) $0.2 \mathrm{~V}$
(d) $0.01 \mathrm{~V}$

Saman Zulfiqar

Numerade Educator

Problem 11

When a sheet of metal is placed in a magnetic field, which changes from zero to a maximum value, the induced currents are set up in the direction shown in figure. What is the direction of magnetic field?
(a) Into the plane of the paper
(b) Out of the plane of the paper
(c) West to East
(d) South to North

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 12

vAs shown in the figure, a magnet is moved with a fast speed towards a coil at rest. Due to this induced electromotive force, induced current and induced charge in the coil is $E, I$ and $Q$ respectively. If the speed of the magnet is doubled, the incorrect statement is $\quad$ [NCERT Exemplar]
(a) $E$ increases
(b) $l$ increases
(c) $Q$ remain same
(d) $Q$ increases

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 13

The two rails of a railway track insulated from each other and the ground are connected to a milli-voltmeter. What is the reading of the $\mathrm{mV}$, when a train travels at a speed of $180 \mathrm{kmh}^{-1}$ along the track, given that the horizontal component of earth's magnetic field is $0.2 \times 10^{-4} \mathrm{Wbm}^{-2}$ and the rails are separated by $1 \mathrm{~m}$.(a) $10^{-2} \mathrm{mV}$
(b) $10 \mathrm{mV}$
(c) $10^{2} \mathrm{mV}$
(d) $1 \mathrm{mV}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 14

(a) $10^{-2} \mathrm{mV}$
(b) $10 \mathrm{mV}$
(c) $10^{2} \mathrm{mV}$
(d) $1 \mathrm{mV}$

Prem Bijarniya

Numerade Educator

Problem 15

An axle of truck is $2.5 \mathrm{~m}$ long. If the truck is moving due north at $30 \mathrm{~ms}^{-1}$ at a place where the vertical component of the earth's magnetic field is $90 \mu \mathrm{T}$, the potential difference between the two ends of the axle is
(a) $6.75 \mathrm{mV}$ with west end positive
(b) $6.75 \mathrm{mV}$ with east end positive
(c) $6.75 \mathrm{mV}$ with north end positive
(d) $6.75 \mathrm{mV}$ with south end positive

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 16

A cylindrical bar magnet is rotated about its axis. A wire is connected from the axis and is made to touch the cylindrical surface through a contact. Then [NCERT Exemplar]
(a) a direct current flows in the ammeter $A$.
(b) no current flows through the ammeter $A$.
(c) an altemating sinusoidal current flows through the ammeter $A$ with a time period $T=\frac{2 \pi}{\omega}$.
(d) a time varying non-sinosoidal current flows through the ammeter $A$.

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 17

A circular ring of diameter $20 \mathrm{~cm}$ has a resistance of $0.01 \Omega$. The charge that will flow through the ring if it is turned from a position perpendicular to a uniform magnetic field of $2.0 \mathrm{~T}$ to a position parallel to the field is about
(a) $63 \mathrm{C}$
(b) $0.63 \mathrm{C}$
[c) $6.3 \mathrm{C}$
(d) $0.063 \mathrm{C}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 18

There are two coils $A$ and $B$ as shown in figure. A current starts flowing in $B$ as shown, when $A$ is moved towards $B$ and stops when
A stops moving. The current in $A$ is counterclockwise. $B$ is kept stationary when $A$ moves. We can infer that $\quad$ [NCERT Exemplar]
(a) there is a constant current in the clockwise direction in $A$.
(b) there is a varying curent in $A$.
(c) there is no current in $A$.
(d) there is a constant current in the counterclockwise direction in $A$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 19

There are two coils $A$ and $B$ as shown in figure. A current starts flowing in $B$ as shown, when $A$ is moved towards $B$ and stops when
A stops moving. The current in $A$ is counterclockwise. $B$ is kept stationary when $A$ moves. We can infer that $\quad$ [NCERT Exemplar]
(a) there is a constant current in the clockwise direction in $A$.
(b) there is a varying curent in $A$.
(c) there is no current in $A$.
(d) there is a constant current in the counterclockwise direction in $A$

Saman Zulfiqar

Numerade Educator

Problem 20

The charge which will flow through a 200 . galvanometer connected to a $400 \Omega$ circular coil 0 1000 turns wound on a wooden stick $20 \mathrm{~mm}$ in diameter, if a magnetic field $B=0.012 \mathrm{~T}$ parallel $\mathrm{t}$ the axis of the stick decreased suddenly to zero is
(a) $6.3 \mu \mathrm{C}$
(b) $63 \mu \mathrm{C}$
(c) $0.63 \mu \mathrm{C}$
(d) $630 \mu \mathrm{C}$

Saman Zulfiqar

Numerade Educator

Problem 21

A square loop of wire of side $5 \mathrm{~cm}$ is lying on a horizontal table. An electromagnet above and to one side of the loop is turned on, causing a uniform magnetic field downwards at an angle of $60^{\circ}$ to the vertical as shown in figure. The magnetic induction is $0.50 \mathrm{~T}$. The average induced emf in the loop, if the field increases from zero to its final value in $0.2 \mathrm{~s}$ is
(a) $5.4 \times 10^{-3} \mathrm{~V}$
(b) $312 \times 10^{-3} \mathrm{~V}$
(c) 0
(d) $0.25 \times 10^{-3} \mathrm{~V}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 22

A coil has an area of $0.05 \mathrm{~cm}^{2}$ and it has 800 turns. is placed perpendicularly in a magnetic field strength $4 \times 10^{-5} \mathrm{~Wb} / \mathrm{m}^{2}$, it is rotated through $90^{\circ} \mathrm{i}$ $0.1 \mathrm{~s}$. The average emf induced in the coil is
(a) $0.016 \mathrm{~V}$
(b) $0.032 \mathrm{~V}$
(c) $0.064 \mathrm{~V}$
(d) $0.029 \mathrm{~V}$

Saman Zulfiqar

Numerade Educator

Problem 24

Two coils of self-inductances $2 \mathrm{mH}$ and $8 \mathrm{mH}$ are placed so close together that the effective flux in one coil is completely linked with the other. The mutual inductance between these coils is
(a) $16 \mathrm{mH}$
(b) $10 \mathrm{mH}$
(c) $4 \mathrm{mH}$
(d) $6 \mathrm{mH}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 25

A coil is wound on a core of rectangular cross-section. If all the linear dimensions of core are increased by a factor 2 and number of turns per unit length of coil remains same, the self-inductance increases by a factor of
(a) 16
(b) 8
(c) 4
(d) 2

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 26

Two coils $X$ and $Y$ are placed in a circuit such that a current changes by $2 \mathrm{~A}$ in coil $X$ and magnetic flux change of $0.4$ Wb occurs in $Y$. The value of mutual inductance of the coils is
(a) $0.8 \mathrm{H}$
(b) $0.2 \mathrm{~Wb}$
(c) $0.2 \mathrm{H}$
(d) $5 \mathrm{H}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 27

Two coils $X$ and $Y$ are placed in a circuit such that a current changes by $2 \mathrm{~A}$ in coil $X$ and magnetic flux change of $0.4$ Wb occurs in $Y$. The value of mutual inductance of the coils is
(a) $0.8 \mathrm{H}$
(b) $0.2 \mathrm{~Wb}$
(c) $0.2 \mathrm{H}$
(d) $5 \mathrm{H}$

Saman Zulfiqar

Numerade Educator

Problem 28

A circular coil of radius $8.0 \mathrm{~cm}$ and 20 turns is rotated about its vertical diameter with an angular speed of $50 \mathrm{rad} / \mathrm{s}$ in a uniform horizontal magnetic field of magnitude $3.0 \times 10^{-2} \mathrm{~T}$. Obtain the maximum and average emf induced in the coil. If the coil forms a closed-loop of resistance $10 \Omega$, calculate the maximum value of current in the coil. Calculate the average power loss due to Joule heating. [NCERT]
(a) $2 \mathrm{~W}$
(b) $0.2 \mathrm{~W}$
[c) $0.49 \mathrm{~W}$
(d) $0.018 \mathrm{~W}$

Saman Zulfiqar

Numerade Educator

Problem 29

When current in a coil changes from $2 \mathrm{~A}$ to $-2 \mathrm{~A}$ in $0.05 \mathrm{~s}$, an emf of $8 \mathrm{~V}$ is induced in the coil. The coefficient of self-inductance of the coil is
(a) $0.1 \mathrm{H}$
(b) $0.2 \mathrm{H}$
(c) $0.4 \mathrm{H}$
(d) $0.8 \mathrm{H}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 30

When current in a coil changes from $2 \mathrm{~A}$ to $-2 \mathrm{~A}$ in $0.05 \mathrm{~s}$, an emf of $8 \mathrm{~V}$ is induced in the coil. The coefficient of self-inductance of the coil is
(a) $0.1 \mathrm{H}$
(b) $0.2 \mathrm{H}$
(c) $0.4 \mathrm{H}$
(d) $0.8 \mathrm{H}$

Saman Zulfiqar

Numerade Educator

Problem 31

What is self-inductance of a coil which produces $5 \mathrm{~V}$, when current in it changes from $3 \mathrm{~A}$ to $2 \mathrm{~A}$ in one millisecond?
(a) $5000 \mathrm{H}$
(b) $5 \mathrm{mH}$
(c) $50 \mathrm{H}$
(d) $5 \mathrm{H}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 32

The self inductance $L$ of a solenoid of length $l$ and area of cross-section $A$, with a fixed number of turns $N$ increases as
(a) $l$ and $A$ increase
(b) $l$ decreases and $A$ increases
(c) $l$ increases and $A$ decreases
(d) both $I$ and $A$ decreases

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 33

What is the self-inductance of an air core solenoid $1 \mathrm{~m}$ long, diameter $0.05 \mathrm{~m}$, if it has 500 turns? Take $\pi^{2}=10$
(a) $3.15 \times 10^{-4} \mathrm{H}$
(b) $4.8 \times 10^{-4} \mathrm{H}$
(c) $5 \times 10^{-4} \mathrm{H}$
(d) $6.25 \times 10^{-4} \mathrm{H}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 34

What is the self-inductance of an air core solenoid $1 \mathrm{~m}$ long, diameter $0.05 \mathrm{~m}$, if it has 500 turns? Take $\pi^{2}=10$
(a) $3.15 \times 10^{-4} \mathrm{H}$
(b) $4.8 \times 10^{-4} \mathrm{H}$
(c) $5 \times 10^{-4} \mathrm{H}$
(d) $6.25 \times 10^{-4} \mathrm{H}$

Saman Zulfiqar

Numerade Educator

Problem 35

A coil of wire of certain radius has 100 turns and a self-inductance of $15 \mathrm{mH}$. The self-inductance of a second similar coil of 500 turns will be
(a) $75 \mathrm{mH}$
(b) $375 \mathrm{mH}$
(c) $15 \mathrm{mH}$
(d) None of these

Prem Bijarniya

Numerade Educator

Problem 36

Two circuits have mutual inductance of $0.09 \mathrm{H}$. Average emf induced in the secondary by a change of current from 0 to $20 \mathrm{~A}$ in $0.006 \mathrm{~s}$ in primary will be
(a) $120 \mathrm{~V}$
(b) $200 \mathrm{~V}$
(c) $180 \mathrm{~V}$
(d) $300 \mathrm{~V}$

Saman Zulfiqar

Numerade Educator

Problem 37

If number of turns in primary and secondary coils is increased to two times each, the mutual inductance
(a) becomes 4 times
(b) becomes 2 times
(c) becomes $1 / 4$ times
(d) remains unchanged

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 38

The expression for magnetic induction inside a solenoid of length $L$, carrying a current $i$ and having $N$ number of turns is
(a) $\frac{\mu_{0}}{4 \pi} \frac{N}{L} i$
(b) $\mu_{0} N U$
(c) $\frac{\mu_{0}}{4 \pi} N L$
(d) $\mu_{0} \frac{N^{2}}{L} i$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 39

The expression for magnetic induction inside a solenoid of length $L$, carrying a current $i$ and having $N$ number of turns is
(a) $\frac{\mu_{0}}{4 \pi} \frac{N}{L} i$
(b) $\mu_{0} N U$
(c) $\frac{\mu_{0}}{4 \pi} N L$
(d) $\mu_{0} \frac{N^{2}}{L} i$

Saman Zulfiqar

Numerade Educator

Problem 40

A current of $10 \mathrm{~A}$ in the primary coil of a circuit is reduced to zero. If the coefficient of mutual inductance is $3 \mathrm{H}$ and emf induced in secondary coil is $30 \mathrm{kV}$, time taken for the change of current is
(a) $10^{3} \mathrm{~s}$
(b) $10^{2} \mathrm{~s}$
(c) $10^{-3} \mathrm{~s}$
(d) $10^{-2} \mathrm{~s}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 41

A $1.0 \mathrm{~m}$ long metallic rod is rotated with an angular frequency of $400 \mathrm{rad} / \mathrm{s}$ about an axis normal to the rod passing through its one end. The other end of the rod is in contact with a circular metallic ring. $\mathrm{A}$ constant and uniform magnetic field of $0.5$ 'T parallel to the axis exists everywhere. Calculate the emf developed between the centre and the ring. [NCERT]
(a) $95 \mathrm{~V}$
(b) $85 \mathrm{~V}$
(al ian y

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 42

A coil of inductance $0.2 \mathrm{H}$ and $1.0 \mathrm{~W}$ resistance is connected to a $90 \mathrm{~V}$ source. At what rate will the current in the coil grow at the instant the coil is connected to the source?
(a) $450 \mathrm{As}^{-1}$
(b) $4.5 \mathrm{As}^{-1}$
(c) $45 \mathrm{As}^{-1}$
(d) $0.45 \mathrm{As}^{-1}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 43

If the rms current in a $50 \mathrm{~Hz}$ AC circuit is $5 \mathrm{~A}$, the value of the current $1 / 300$ seconds after its value \mathrm{\{} b e c o m e s ~ z e r o ~ i s ~
[NCERT Exemplar]
(a) $5 \sqrt{2} \mathrm{~A}$
(b) $5 \sqrt{3 / 2} \mathrm{~A}$
(c) $5 / 6 \mathrm{~A}$
(d) $5 / \sqrt{2} \mathrm{~A}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 44

Two inductors of inductance $L$ each are connected in series with opposite magnetic fluxes. What is the resultant inductance? (Ignore mutual inductance)
(a) Zero
(b) $\underline{L}$
(c) $2 L$
(d) $3 L$

Saman Zulfiqar

Numerade Educator

Problem 45

The number of turns of primary and secondary coils of a transformer are 5 and 10 respectively and mutual inductance of the transformer is $25 \mathrm{H}$. Now, number of turns in primary and secondary are made 10 and 5 respectively. Mutual inductance of transformer will be
(a) $25 \mathrm{H}$
(b) $12.5 \mathrm{H}$
[c) $50 \mathrm{H}$
(d) 6. $25 \mathrm{H}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 46

A uniformly wound solenoidal coil of self-inductance $1.8 \times 10^{-4} \mathrm{H}$ and resistance $6 \Omega$ is broken up into two identical coils. These identical coils are then connected in parallel across a $12 \mathrm{~V}$ battery of negligible resistance. The time constant of the current in the circuit and the steady state current through battery is
(a) $3 \times 10^{-5} \mathrm{~s}, 8 \mathrm{~A}$
(b) $1.5 \times 10^{-5}$ s, $8 \mathrm{~A}$
(c) $0.75 \times 10^{-4} s, 4 \mathrm{~A}$
(d) $6 \times 10^{-5} \mathrm{~s}, 2 \mathrm{~A}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 47

A uniformly wound solenoidal coil of self-inductance $1.8 \times 10^{-4} \mathrm{H}$ and resistance $6 \Omega$ is broken up into two identical coils. These identical coils are then connected in parallel across a $12 \mathrm{~V}$ battery of negligible resistance. The time constant of the current in the circuit and the steady state current through battery is
(a) $3 \times 10^{-5} \mathrm{~s}, 8 \mathrm{~A}$
(b) $1.5 \times 10^{-5}$ s, $8 \mathrm{~A}$
(c) $0.75 \times 10^{-4} s, 4 \mathrm{~A}$
(d) $6 \times 10^{-5} \mathrm{~s}, 2 \mathrm{~A}$

Saman Zulfiqar

Numerade Educator

Problem 48

When a voltage measuring device is connected to $\mathrm{AC}$ mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar]
(a) input voltage cannot be AC voltage, but a DC voltage.
(b) maximum input voltage is $220 \mathrm{~V}$
(c) the meter reads not $v$ but $<v^{2}>$ and is calibrated to read $\sqrt{\left\langle v^{2}\right\rangle}$
(d) the pointer of the meter is stuck by some mechanical defect.

Saman Zulfiqar

Numerade Educator

Problem 49

When a voltage measuring device is connected to $\mathrm{AC}$ mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar]
(a) input voltage cannot be AC voltage, but a DC voltage.
(b) maximum input voltage is $220 \mathrm{~V}$
(c) the meter reads not $v$ but $<v^{2}>$ and is calibrated to read $\sqrt{\left\langle v^{2}\right\rangle}$
(d) the pointer of the meter is stuck by some mechanical defect.

Saman Zulfiqar

Numerade Educator

Problem 50

A $60 \mu \mathrm{F}$ capacitor is connected to a $110 \mathrm{~V}, 60 \mathrm{~Hz} \mathrm{AC}$ supply. Determine the $\mathrm{rms}$ value of the current in the circuit.
(a) $2.5 \mathrm{~A}$
(b) $2.1 \mathrm{~A}$
(c) $3.1 \mathrm{~A}$
(d) $3.5 \mathrm{~A}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 51

A $60 \mu \mathrm{F}$ capacitor is connected to a $110 \mathrm{~V}, 60 \mathrm{~Hz} \mathrm{AC}$ supply. Determine the $\mathrm{rms}$ value of the current in the circuit.
(a) $2.5 \mathrm{~A}$
(b) $2.1 \mathrm{~A}$
(c) $3.1 \mathrm{~A}$
(d) $3.5 \mathrm{~A}$

Saman Zulfiqar

Numerade Educator

Problem 52

In step-up transformer, relation between number of turns in primary $\left(N_{p}\right)$ and number of turns in secondary ( $N_{s}$ ) coils is
(a) $N_{s}>N_{p}$
(b) $N_{p}>N_{s}$
(c) $N_{s}=N_{p}$
(d) $N=2 N$,

Saman Zulfiqar

Numerade Educator

Problem 53

The turns ratio of transformer is given as $2: 3 .$ If the current passing through the primary coil is $3 \mathrm{~A}$. Find the current through the load resistance.
(a) $4.5 \mathrm{~A}$
(b) $1.5 \mathrm{~A}$
(c) $2 \mathrm{~A}$
(d) $\mid \mathrm{A}$
The $\bar{n}$ tuxe 4

Saman Zulfiqar

Numerade Educator

Problem 54

The primary winding of a transformer has 200 turns and its secondary winding has 50 turns, If the current in the secondary winding is $40 \mathrm{~A}$, the current in the primary is
(a) $10 \mathrm{~A}$
(b) $80 \mathrm{~A}$
(c) $160 \mathrm{~A}$
(d) $800 \mathrm{~A}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 55

The number of turns in a secondary coil is twice the number of turns in primary. A leclanche cell of $1.5 \mathrm{~V}$ is connected across the primary. The voltage across secondary is
(a) $1.5 \mathrm{~V}$
(b) $3.0 \mathrm{~V}$
(c) $240 \mathrm{~V}$
(d) zero

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 56

A step-up transformer is used on a $120 \mathrm{~V}$ line to provide a potential difference of $2400 \mathrm{~V}$. If the primary coil has 75 turns, the number of turns in the secondary coil is
(a) 150
(b) 1200
(c) 1500
(d) 1575

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 57

The ratio of turns in primary and secondary coils of a transformer is $1: 20 .$ The ratio of currents in primary and secondary coils will be
(a) $1: 20$
(b) $20: 1$
(c) $1: 400$
(d) $400: 1$
A

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 58

The ratio of turns in primary and secondary coils of a transformer is $1: 20 .$ The ratio of currents in primary and secondary coils will be
(a) $1: 20$
(b) $20: 1$
(c) $1: 400$
(d) $400: 1$
A

Saman Zulfiqar

Numerade Educator

Problem 59

A low-loss transformer has $230 \mathrm{~V}$ applied to the primary and gives $4.6 \mathrm{~V}$ in the secondary. Secondary is connected to a load, which draws $5 \mathrm{~A}$ of current. The current (in ampere) in the primary is
(a) $0.1$
(b) $1.0$
(c) 10
(d) 250

Prem Bijarniya

Numerade Educator

Problem 60

$n$ an induction coil, the coefficient of mutual nductance is $4 \mathrm{H}$. If current of $5 \mathrm{~A}$ in the primary coil cut-off $i 1 / 1500 \mathrm{~s}$, the emf at the terminals of the econdary coil will be
(a) $15 \mathrm{kV}$
(b) $60 \mathrm{kV}$
(c) $10 \mathrm{kV}$
(d) $30 \mathrm{kV}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 61

$n$ an induction coil, the coefficient of mutual nductance is $4 \mathrm{H}$. If current of $5 \mathrm{~A}$ in the primary coil cut-off $i 1 / 1500 \mathrm{~s}$, the emf at the terminals of the econdary coil will be
(a) $15 \mathrm{kV}$
(b) $60 \mathrm{kV}$
(c) $10 \mathrm{kV}$
(d) $30 \mathrm{kV}$

Saman Zulfiqar

Numerade Educator

Problem 62

A transformer is used to light $140 \mathrm{~W}, 24 \mathrm{~V}$ lamp from $240 \mathrm{~V}$ AC mains, The current in the mains is $0.7 \mathrm{~A}$. The efficiency of transformer is nearest to
(a) 909
(b) 8096
(c) 7096
(d) $60 \%$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 63

The reduce the resonant frequency in an $L-C-R$ series circuit with a generator $\quad$ [NCERT Exemplar]
(a) the generator frequency should be reduced
(b) another capacitor should be added in parallel to the first
[c) the iron core of the inductor should be removed
(d) dielectric in the capacitor should be removed

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 64

The reduce the resonant frequency in an $L-C-R$ series circuit with a generator $\quad$ [NCERT Exemplar]
(a) the generator frequency should be reduced
(b) another capacitor should be added in parallel to the first
[c) the iron core of the inductor should be removed
(d) dielectric in the capacitor should be removed

Saman Zulfiqar

Numerade Educator

Problem 65

A transformer is having 2100 turns in primary and 4200 turns in secondary. An AC source of $120 \mathrm{~V}, 10 \mathrm{~A}$ is connected to its primary. The secondary voltage and current are
(a) $240 \mathrm{~V}, 5 \mathrm{~A}$
(b) $120 \mathrm{~V}, 10 \mathrm{~A}$
(c) $240 \mathrm{~V}, 10 \mathrm{~A}$
(d) $120 \mathrm{~V}, 20 \mathrm{~A}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 66

Which of the following combinations should be selected for better tuning of an $L-C \cdot R$ circuit used for communication ? $\quad$ [NCERT Exemplar]
(a) $R=20 \Omega, L=1.5 \mathrm{H}, \mathrm{C}=35 \mu \mathrm{F}$
(b) $R=25 \Omega, L=2.5 \mathrm{H}, C=45 \mu \mathrm{F}$
(c) $R=15 \Omega, L=3.5 \mathrm{H}, C=30 \mu \mathrm{F}$
(d) $R=25 \Omega, L=15 \mathrm{H}, C=45 \mathrm{\muF}$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 67

A motor having an armature of resistance $2 \Omega$ is lesigned to operate at $220 \mathrm{~V}$ mains. At full speed, itA motor having an armature of resistance $2 \Omega$ is lesigned to operate at $220 \mathrm{~V}$ mains. At full speed, it

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator

Problem 68

A transformer is used to light a $100 \mathrm{~W}-110 \mathrm{~V}$ lamp from $220 \mathrm{~V}$ mains. If main current is $0.5 \mathrm{~A}$, efficiency of transformer is
(a) $90 \%$
(b) 9596
(c) 9696
(d) $99 \%$

Varsha Aggarwal

Varsha Aggarwal

Numerade Educator