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# CBSE Physics 2011 Exam Questions

11.

What happens to the width of depletion layer of a p-n junction when it is (i) forward biased, (ii) reverse biased?

(i) When forward biased, the width of depletion layer decreases.

(ii) When reverse biased, the width of depletion layer increases.

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12.

Using the curve for the binding energy per nucleon as a function of mass number A, state clearly how the release in energy in the processes of nuclear fission and nuclear fusion can be explained.

The binding energy per nucleon increases when a heavy nucleus breaks into two lighter nuclei. That is, energy would be released during the process of nuclear fusion.

When light nuclei join to form a heavy nucleus, the binding energy per nucleon of fused heavier nucleus is more than the binding energy per nucleon of lighter nuclei. Again, there is release of energy in this process
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13.

What is sky wave communication? Why is this mode of propagation restricted to the frequencies only up to few MHz?

Sky wave propagation is a mode of propagation in which communication of radio waves in frequency range 30 MHz–40 MHz takes place due to reflection from the ionosphere.

For frequencies higher than few MHz, the sky waves penetrate the ionosphere and are not reflected back.

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14.

Draw a labelled diagram of a full wave rectifier circuit. State its working principle. Show the input-output waveforms.

The circuit of Full wave rectifier is as shown below:

Working principle: The underlying working principle of full wave rectifier is that the p-n junction conducts when it is forward biased and does not conduct when it is reverse biased.

The input and output waveform is shown below:

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15.

In the meter bridge experiment, balance point was observed at J with AJ = l.

(i) The values of R and X were doubled and then interchanged. What would be the new position of balance point?

(ii) If the galvanometer and battery are interchanged at the balance position, how will the balance point get affected?

i) Balance point is obtained by,

When both R and X are doubled and then interchanged, new balance length becomes l’ which is given by,

Now, from the above two equations, we have

ii) The balance point will not get affected if galvanometer and balance point are interchanged at this position.

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16.

Write any two characteristic properties of nuclear force.

(i) Nuclear forces are short range attractive forces.

(ii) Nuclear forces are charge – independent.

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17.

Two heating elements of resistance R1 and R2 when operated at a constant supply of voltage, V, consume powers P1 and P2 respectively. Deduce the expressions for the power of their combination when they are, in turn, connected in (i) series and (ii) parallel across the same voltage supply.

i) When they are in series combination,

Net resistance, R = R1 + R2                                                 … (1)

Heating elements are operated at same voltage V, we have

So, from equation (1), we have

ii) When the resistors are connected in parallel combination,

Net resistance is given by,

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18.

Define the term ‘stopping potential’ in relation to photoelectric effect.

Stopping potential is the minimum retarding or negative potential of anode of a photoelectric tube for which photoelectric current stops or becomes zero.

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19.

A parallel plate capacitor is being charged by a time varying current. Explain briefly how Ampere’s circuital law is generalized to incorporate the effect due to the displacement current.

When AC current is passed through capacitor, current moves from one plate to another. According to Ampere’s circuital law, a magnetic field has to exist between the plates of the capacitor for the current transfer to happen. So, Maxwell modified Ampere’s circuital law with the help of continuity equation.

The modified Ampere’s law by Maxwell is given by,

where, iD, is called the Displacement current.

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20.

Net capacitance of three identical capacitors in series is 1 F. What will be their net capacitance if connected in parallel?

Find the ratio of energy stored in the two configurations if they are both connected to the same source.

Let,C be the capacitance of each capacitor,

In a series combination,

When these capacitors are connected in parallel, net capacitance, CP = 3 C = 33 = 9F

Ration of energy stored is given by,

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