Physics Test

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Physics Test - 2

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Question 1
3 / -1

A wire of irregular shape turning into a circular shape in a magnetic field which is directed into the paper. The direction of induced current is

A
along abcda

B
along adcba

C
into the plane of the paper

D
out of the plane of the paper

Solution

Solution

When a wire of irregular shape turns into a circular loop, area of the loop tends to increase. Therefore, magnetic flux linked with the loop increases. According to Lenz’s law, the direction of induced current must oppose the change in magnetic flux, for which induced current should flow along adcba.

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Question 2
3 / -1

Two circular coils can be arranged in any of the three situations shown in figure. Their mutual inductance will be,

A
maximum in situation (i).

B
maximum in situation (ii).

C
maximum in situation (iii).

D
the same in all situations.

Solution

Solution

Let the radius of bigger coil be R and radius of smaller coil be r. Let I be the current passing through the bigger coil in anti-clockwise direction.
Magnetic field produced due to circular coil at its centre is vertically upwards.

In case of (i): Magnetic field due to bigger coil and surface vector of smaller coil lies in same direction, i.e., θ = 0°

In case of (ii): Magnetic field due to bigger coil and surface vector of smaller coil perpendicular to each other, i.e., θ = 90°

In case of (iii): No field line passes through the smaller coil since the smaller coil is placed beside bigger coil and B is vertically upwards. Hence, the field at centre of smaller coil will be zero.

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Question 3
3 / -1

A small square loop wire of side l is placed inside a large square loop of side L(L>>l). If the loops are coplanar and their centres coincide, the mutual induction of the system is directly proportional to

A
L/l

B
l/L

C
L²/l

D
l²/L

Solution

Solution

Consider the larger loop to be made up of four rods each of length L, the field at the centre, i.e. at a distance of (L/2) from each rod, will be

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Question 4
3 / -1

A step down transformer, transforms a supply line voltage of 2200 V into 220 V. The primary coil has 5000 turns. The efficiency and power transmitted by the transformer are 90 % and 8 kW respectively. Then the power supplied is

A
9.89 kW

B
8.89 kW

C
88.9 kW

D
889 kW

Solution

Solution

Efficiency of transformer is given by,

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Question 5
3 / -1

Which type of losses do not occur in the transformer?

A
Iron losses

B
Copper losses

C
Mechanical losses

D
Flux leakage
Solution
Solution

In a transformer,
1. Iron losses in actual iron cores, inspite of lamination, eddy current are produced. The magnitude of eddy current may however be small and a part of energy is lost as the heat produced in the iron core.
2. Copper losse in practice, the coils of the transformer possess resistance. So, a part of the energy is lost due to the heat produced in the resistance of the coil.
3. Flux leakage the coupling between the coils is seldom perfect. So, whole of the magnetic flux produced by the primary coil is not linked up with the secondary coil.
And hysteresis loss, humming losses also occur in the transformer.
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Question 6
3 / -1

The magnet in figure rotates as shown on a pivot through its centre. At the instant shown, what are the directions of the induced currents

A
A to B and C to D

B
B to A and C to D

C
A to B and D to C

D
B to A and D to C

Solution

Solution

N pole moves closer to coil CD and S pole moves closer to coil AB on rotation of magnet.

According to Lenz's law, N pole should develop at the end corresponding to C.

∴ Induced current flows from C to D

Also S pole should develop at the end corresponding to B.

∴ Induced current flows from A to B

According to an observer E₁ current will be clockwise in coil−1 and for an observer E^₂ current will be anti-clockwise in coil−2 as shown in figure.

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Question 7
3 / -1

Assertion: A spark occurs sometime, when an electric iron is switched off.

Reason: Sparking is due to large self-induced emf in the circuit during the circuit break

A
Both Assertion and Reason are true and the Reason is correct explanation of the Assertion.

B
Both Assertion and Reason are true, but Reason is not a correct explanation of the Assertion.

C
Assertion is true, but Reason is false.

D
Assertion is false, but Reason is true.

Solution

Solution

When the iron is switched off, then the current starts dropping to zero in circuit, and thus, the magnetic field in the circuit drops. This causes the magnetic flux linked with loop to decrease. Due to self-induction, the current still tries to continue to flow. This causes the spark.

So, both Assertion and Reason are true and Reason is the correct explanation for the Assertion.

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Question 8
3 / -1

For the given circuit, calculate the current which is given by the battery:

(i) just after closing the key.

(ii) long time after closing the key.

A
(i) 2 A (ii) 2.5 A

B
(i) 2.5 A (ii) 2 A

C
(i) 4 A (ii) 3 A

D
None of these

Solution

Solution

JUST AFTER CLOSING THE KEY

Let t = 0 be the time when we need to find the current, i.e., when the switch is just turned on. The inductor opposes the flow of current and behaves as an open circuit. So the current will flow only in the 2 Ω and 3 Ω resistors.

The equivalent resistance of the circuit is,

R_s = 2 + 3 = 5 Ω

So,

LONG TIME AFTER CLOSING THE KEY

After a long time, the inductor does not oppose the flow of current and hence acts as a short circuit.

Now, the equivalent resistance of the circuit is,

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Question 9
3 / -1

The current i in an inductance coil varies with time t according to following graph,

Which one of the following graphs shows the variations of voltage in the coil?

A

B

C

D

Solution

Solution

The potential difference across the inductor is

The slope of the curve is positive and constant for the first half of the time and then negative and a constant value.

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Question 10
3 / -1

A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately

A
30 %

B
50 %

C
90 %

D
10 %

Solution

Solution

The efficiency of transformer

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