Physics M.C.Q (1 Marks)

A step-up transformer has transformation ratio of 3 : 2. What is the voltage in secondary if voltage in primary is 30 V

(a) 45 V

(b) 15 V

(c) 90 V

(d) 300 V

A coil having an area  is placed in a magnetic field which changes from  to  in a time interval t. The e.m.f. induced in the coil will be

(a)

(b)

(c)

(d)

An electron moves along the line AB, which lies in the same plane as a circular loop of conducting wires as shown in the diagram. What will be the direction of current induced if any, in the loop

(a) No current will be induced

(b) The current will be clockwise

(c) The current will be anticlockwise

(d) The current will change direction as the electron passes by

Consider the situation shown in the figure. The wire AB is sliding on the fixed rails with a constant velocity. If the wire AB is replaced by semicircular wire, the magnitude of the induced current will

(a) Increase

(b) Remain the same

(c) Decrease

(d) Increase or decrease depending on whether the semicircle bulges towards the resistance or away from it

A rectangular coil ABCD is rotated anticlockwise with a uniform angular velocity about the axis shown in diagram below. The axis of rotation of the coil as well as the magnetic field B are horizontal. The induced e.m.f. in the coil would be maximum when

(a) The plane of the coil is horizontal

(b) The plane of the coil makes an angle of 45° with the magnetic field

(c) The plane of the coil is at right angles to the magnetic field

(d) The plane of the coil makes an angle of 30° with the magnetic field

Assertion :   The back emf in a dc motor is maximum when the motor has just been switched on.

Reason     : When motor is switched on it has maximum speed.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If the assertion and reason both are false.

Assertion :   An ac generator is based on the phenomenon of self-induction.

Reason     : In single coil, we consider self-induction only.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

Pure inductance of 3.0 H is connected as shown below. The equivalent inductance of the circuit is

(a) 1 H

(b) 2 H

(c) 3 H

(d) 9 H

A solenoid has 2000 turns wound over a length of 0.30 metre. The area of its cross-section is 1.2 . Around its central section, a coil of 300 turns is wound. If an initial current of 2 A in the solenoid is reversed in 0.25 sec, then the e.m.f. induced in the coil is

(a) 6

(b) 4.8

(c) 6

(d) 48 mV

The north and south poles of two identical magnets approach a coil, containing a condenser, with equal speeds from opposite sides. Then

(a) Plate 1 will be negative and plate 2 positive

(b) Plate 1 will be positive and plate 2 negative

(c) Both the plates will be positive

(d) Both the plates will be negative

A conducting square loop of side l and resistance R moves in its plane with a uniform velocity v perpendicular to one of its sides. A magnetic induction B constant in time and space, pointing perpendicular and into the plane at the loop exists everywhere with half the loop outside the field, as shown in figure. The induced e.m.f. is

(a) Zero

(b) RvB

(c) VBl/R

(d) VBl

One conducting U tube can slide inside another as shown in figure, maintaining electrical contacts between the tubes. The magnetic field B is perpendicular to the plane of the figure. If each tube moves towards the other at a constant sped v then the emf induced in the circuit in terms of B, l and v where l is the width of each tube, will be

(a) Zero

(b) 2 Blv

(c) Blv

(d) -Blv

The current carrying wire and the rod AB are in the same plane. The rod moves parallel to the wire with a velocity v. Which one of the following statements is true about induced emf in the rod

(a) End A will be at lower potential with respect to B

(b) A and B will be at the same potential

(c) There will be no induced e.m.f. in the rod

(d) Potential at A will be higher than that at B

An electric potential difference will be induced between the ends of the conductor shown in the diagram, when the conductor moves in the direction

(a)  P

(b) Q

(c)  L

(d) M

At a place the value of horizontal component of the earth's magnetic field H is 3. A metallic rod AB of length 2 m placed in east-west direction, having the end A towards east, falls vertically downward with a constant velocity of 50 m/s. Which end of the rod becomes positively charged and what is the value of induced potential difference between the two ends

(a) End A, 3	(b) End A, 3 mV
(c) End B, 3	(d) End B, 3 mV

An aluminium ring B faces an electromagnet A. The current I through A can be altered

(a) Whether I increases or decreases, B will not experience any force

(b) If I decrease, A will repel B

(c) If I increases, A will attract B

(d) If I increases, A will repel B

A coil of area  has 500 turns. Magnetic field of   is perpendicular to the coil. The field is reduced to zero in 0.1 second. The induced e.m.f. in the coil is

(a) 1 V

(b) 5 V

(c) 50 V

(d)  Zero

When a magnet is pushed in and out of a circular coil C connected to a very sensitive galvanometer G as shown in the adjoining diagram with a frequency v, then

(a) Constant deflection is observed in the galvanometer

(b) Visible small oscillations will be observed in the galvanometer if v is about 50 Hz

(c) Oscillations in the deflection will be observed clearly if v = 1 or 2 Hz

(d) No variation in the deflection will be seen if v = 1 or 2 Hz

A square coil   area is placed perpendicular to a uniform magnetic field of intensity . The magnetic flux through the coil is

(a) 10 weber

(b)  weber

(c)  weber

(d) 100 weber

In the figure magnetic energy stored in the coil is

(a) Zero

(b) Infinite

(c) 25 joules

(d) None of the above

Quantity that remains unchanged in a transformer is

(a) Voltage

(b) Current

(c) Frequency

(d) None of the above

A coil having an inductance of 0.5 H carries a current which is uniformly varying from zero to 10 ampere in 2 second. The e.m.f. (in volts) generated in the coil is

(a) 10

(b) 5

(c) 2.5

(d) 1.25

The square root of the product of inductance and capacitance has the dimension of

(a) Length

(b) Mass

(c) Time

(d) No dimension

An oscillator circuit consists of an inductance of 0.5mH and a capacitor of 20 μF. The resonant frequency of the circuit is nearly

(a) 15.92 Hz

(b) 159.2 Hz

(c) 1592 Hz

(d) 15910 Hz

A LC circuit is in the state of resonance. If C = 0.1μF and L = 0.25 henry. Neglecting ohmic resistance of circuit what is the frequency of oscillations

(a) 1007 Hz

(b) 100 Hz

(c) 109 Hz

(d) 500 Hz

The time constant of an LR circuit represents the time in which the current in the circuit

(a) Reaches a value equal to about 37% of its final value

(b) Reaches a value equal to about 63% of its final value

(c) Attains a constant value

(d) Attains 50% of the constant value

A solenoid has an inductance of 60 henrys and a resistance of 30 ohms. If it is connected to a 100 volt battery, how long will it take for the current to reach   of its final value

(a) 1 second

(b) 2 seconds

(c) e seconds

(d) 2e seconds

A coil of inductance 40 henry is connected in 0series with a resistance of 8 ohm and the combination is joined to the terminals of a 2 volt battery. The time constant of the circuit is

(a) 40 seconds

(b) 20 seconds

(c) 8 seconds

(d) 5 seconds

A capacitor is fully charged with a battery. Then the battery is removed and coil is connected with the capacitor in parallel, current varies as

(a) Increases monotonically	(b) Decreases monotonically
(c) Zero	(d) Oscillates indefinitely

In an LR-circuit, time constant is that time in which current grows from zero to the value (where  is the steady state current)

(a)

(b)

(c)

(d)

An inductance L and a resistance R are first connected to a battery. After some time the battery is disconnected but L and R remain connected in a closed circuit. Then the current reduces to 37% of its initial value in

(a) RL sec

(b)

(c)

(d)

In L-R circuit, for the case of increasing current, the magnitude of current can be calculated by using the formula

(a)

(b)

(c)

(d)

Two identical induction coils each of inductance L joined in series are placed very close to each other such that the winding direction of one is exactly opposite to that of the other, what is the net inductance

(a) L2

(b) 2L

(c) L/2

(d) Zero

The resistance and inductance of series circuit are 5 and 20H respectively. At the instant of closing the switch, the current is increasing at the rate 4A - s. The supply voltage is

(a) 20 V

(b) 80 V

(c) 120 V

(d) 100 V

The current through choke coil increases form zero to 6A in 0.3 seconds and an induced e.m.f. of 30 V is produced. The inductance of the coil of choke is

(a) 5 H

(b) 2.5 H

(c) 1.5 H

(d) 2 H

The current in a coil decreases from 1 A to 0.2 A. In 10sec. Calculate the coefficient of self inductance. If induced emf is 0.4 volt.

(a) 5 H

(b)  3 H

(c)  4 H

(d) 2 H

Two circular coils have their centres at the same point. The mutual inductance between them will be maximum when their axes

(a) Are parallel to each other	(b) Are at 60o to each other
(c) Are at 45o to each other	(d) Are perpendicular to each other

Assertion :   Acceleration of a magnet falling through a long solenoid decreases.

Reason     : The induced current produced in a circuit always flow in such direction that it opposes the change or the cause the produced it.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If the assertion and reason both are false.

Assertion   :   The quantity L/R possesses dimensions of time.

Reason       : To reduce the rate of increases of current through a solenoid should increase the time constant (L/R).

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If the assertion and reason both are false.

Why the current does not rise immediately in a circuit containing inductance

(a) Because of induced emf	(b) Because of high voltage drop
(c) Because of low power consumption	(d) Because of Joule heating

A solenoid of length l metre has self-inductance L henry. If number of turns are doubled, its self inductance

(a) Remains same	(b) Becomes 2L henry
(c) Becomes 4L henry	(d) Becomes   henry

If a change in current of 0.01 A in one coil produces a change in magnetic flux of 1.2  in the other coil, then the mutual inductance of the two coils in henries is

(a) 0

(b) 0.5

(c) 1.2

(d) 3

A coil of resistance 10 W and an inductance 5H is connected to a 100 volt battery. Then energy stored in the coil is

(a) 125 erg

(b) 125 J

(c) 250 erg

(d) 250 J

An average induced e.m.f. of 1V appears in a coil when the current in it is changed from 10A in one direction to 10 A in opposite direction in 0.5 sec. Self-inductance of the coil is

(a) 25 mH

(b) 50 mH

(c) 75 mH

(d) 100 mH

A coil resistance 20W and inductance 5H is connected with a 100V battery. Energy stored in the coil will be

(a) 41.5 J

(b) 62.50 J

(c) 125 J

(d) 250 J

When the current change from + 2A to – 2A in 0.05 second, an e.m.f. of 8 V is induced in a coil. The coefficient of self-induction of the coil is

(a) 0.1 H

(b) 0.2 H

(c) 0.4 H

(d) 0.8 H

Two coils are placed close to each other. The mutual inductance of the pair of coils depends upon

(a) The currents in the two coils

(b) The rates at which currents are changing in the two coils

(c) Relative position and orientation of the two coils

(d) The materials of the wires of the coils

An air core solenoid has 1000 turns and is one metre long. Its cross-sectional area is 10 cm². Its self inductance is

(a) 0.1256 mH

(b) 12.56 mH

(c) 1.256 mH

(d) 125.6 mH

A wire cd of length l and mass m is sliding without friction on conducting rails ax and by as shown. The vertical rails are connected to each other with a resistance R between a and b. A uniform magnetic field B is applied perpendicular to the plane abcd such that cd moves with a constant velocity of

(a)

(b)

(c)

(d)

Two circuits have mutual inductance of 0.1 H. What average e.m.f. is induced in one circuit when the current in the other circuit changes from 0 to 20 A in 0.02 s

(a) 240 V

(b) 230 V

(c) 100 V

(d) 300 V