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CBSE Class 12 Physics Solved Question Paper 2015

Short Answer Type

1. Define capacitor reactance. Write its S.I. units.

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2. What is the electric flux through a cube of side 1 cm which encloses an electric dipole?

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Graph showing the variation of current versus voltage for a material GaAs is shown in the figure. Identify the region of

i) negative resistance

ii) where Ohm’s law is obeyed.

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4. Use Kirchhoff’s rules to obtain conditions for the balance condition in a Wheatstone bridge.

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Name the parts of the electromagnetic spectrum which is

(a) suitable for radar systems used in aircraft navigation.

(b) used to treat muscular strain.

Write in brief, how these waves can be produced.

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A cell of emf ‘E’ and internal resistance ‘r’ is connected across a variable load resistor R. Draw the plots of the terminal voltage V versus
(i) R and
(ii) the current I.

It is found that when R = 4 Ω , the current is 1 A and when R is increased to 9 Ω , the current reduces to 0.5 A. Find the values of the emf E and internal resistance r.

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Two capacitors of unknown capacitances C₁ and C₂ are connected first in series and then in parallel across a battery of 100 V. If the energy stored in the two combinations is 0.045 J and 0.25 J respectively, determine the value of C₁ and C₂. Also, calculate the charge on each capacitor in parallel combination.

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State the principle of working of a galvanometer.

A galvanometer of resistance G is converted into a voltmeter to measure upto V volts by connecting a resistance R₁ in series with the coil. If a resistance R₂ is connected in series with it, then it can measure upto V/2 volts. Find the resistance, in terms of R₁ and R₂, required to be connected to convert it into a voltmeter that can read upto 2 V. Also find the resistance G of the galvanometer in terms of R₁ and R₂.

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9.
An inductor L of inductance X_Lis connected in series with a bulb B and an ac source.
How would brightness of the bulb change when (i) number of turn in the inductor is reduced, (ii) an iron rod is inserted in the inductor and (iii) a capacitor of reactance X_C = X_L is inserted in series in the circuit. Justify your answer in each case.

i) Net resistance in the circuit is given by,

Inductance is given by,

As the number of turns decreases, L decreases.

Inductance is given by,

Therefore, will also decrease thereby, reducing the net resistance in the circuit. Thus, current increases and the brightness of the bulb is increased.

ii) When a soft iron rod is inserted into the circuit, L increases. Therefore, inductive reactance also increases. Net resistance increases and the flow of current in the circuit decreases. Thus, the brightness of the bulb will decrease.

iii) When a capacitor of reactance X_L = X_C is connected in series with the circuit net resistance becomes,

; where R is the resistance of the bulb.

Here, we will have Z= R which is the condition of resonance.

At resonance, maximum current will flow through the circuit. Therefore, the brightness of the bulb will increase.