 CBSE Delhi-Set-2-2007

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• Q1

The radioactive isotope D decays according to the sequence, If the mass number and atomic number of D2 are 176 and 71, respectively, what is
(i) the mass number, and

(ii) atomic number of D?

Marks:1

We can represent the above equation as follows:- Therefore,
(i) Mass number of D=172.
(ii)Atomic number of D=70.

• Q2

What will be the values of input A and B for the Boolean expression Marks:1

Substituting the value of A & B = 0 we get,  A=0 and B=0.

• Q3

Why is frequency modulation preferred over amplitude modulation for transmission of music?

Marks:1

We prefer frequency modulation over amplitude modulation, because of its higher fidelity reception.

• Q4

Why should the material used for making permanent magnets have high coercivity?

Marks:1

The magnetisation of highly coercive material is not disturbed by stray magnetic field or temperature variation. Therefore we use this type of material for making permanent magnets.

• Q5

What is the purpose of modulating a signal in transmission?

Marks:1

The purpose of modulating a signal in transmission is given as follows:-
(i) It enables the transmission of low frequency signal over a large distance.
(ii) It avoids the mixing up of signals transmitted from different stations.

• Q6

The output of OR gate is connected to both the inputs of a NAND gate. Draw the logic circuit of this combination of gates and write its truth table.

Marks:2

The logic circuit is given below, The truth table of the above circuit is given as follows:-

 A B Y’=A+B 0 0 0 1 0 1 1 0 1 0 1 0 1 1 1 0

• Q7

A convex lens of refractive index 1.5 has a focal length of 18cm in air. Calculate the change in its focal length when it is immersed in water of refractive index 4/3.

Marks:2 • Q8

Distinguish between the terms ‘effective value’ and ‘peak value’ of an alternating current. An alternating current from a source is represented by
I = 10 sin (314 t)
Write the corresponding values of
(i) its ‘effective value’
(ii)frequency of the source

Marks:2

Effective value of a.c.: It is that value of direct current which produces the same heating effect in a given resistor as is produced by the given alternating current when passed for the same time.
Peak value of a.c: The maximum value attained by an alternating current in either of its half cycle is called its peak value.

Given, I = 10 sin (314 t)

Comparing with equation, I = I0 sin 2pft, we get

I0 = 10 A, 2nf = 314 • Q9

If the nucleons bound in a nucleus are separated apart from each other, the sum of their masses is greater than the mass of the nucleus. Where does this mass different come from? Explain briefly.

Marks:2

In the nucleus, protons and neutrons are bounded by strong attractive nuclear forces. A definite amount of work has to be done in order to separate them from each other. This work done gets converted into mass in accordance with the Einstien’s equation,
E= m x c2.
This is the reason because of which the nucleons bound in a nucleus when separated from each other, the sum of their masses is greater than the mass of the nucleus.

• Q10

The output of 2-input AND gate is fed to a NOT gate. Draw the logic circuit of this combination of gates and write its truth table.

Marks:2 Input A Input B Y’ = A . B 0 0 1 1 0 1 0 1 0 0 0 1 1 1 1 0