# CBSE Delhi-Set-1-2007

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• Q1

An electron, an alpha-particle and a proton have the same kinetic energy. Which one of these particles have the largest de-broglie wavelength?

Marks:1

The de-broglie wavelength is given by

Since electron has the smallest mass, it will have the longest de-broglie wavelength.

• Q2

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.

• Q3

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.

• Q4

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.

• Q5

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.

• 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

Draw a plot of potential energy of a pair of nucleons as a function of their separation. What is the significance of negative potential energy in the graph drawn?

Marks:2

The plot of potential energy for a pair of nucleons as a function of their separation is given below,

The significance of negative potential energy is that it represents the force of attraction between the pair of nucleons.

• Q8

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

• Q9

Distinguish between the terms ‘average value’ and ‘rms’ value of an alternating current. The instantaneous current from an a.c. source is I = 5 sin (314 t) ampere. What are the average and rms values of the current?

Marks:2

Average value of a.c:- Average value of alternating current may be defined as that value of steady state current which would send the same amount of charge through a circuit in the time of half cycle as is sent by the ac through the same circuit in the same time.
The average value of the a.c is given by the equation,

R.M.S value of a.c:- The root mean square value of alternating current may be defined as that value of direct current that produces the same heating effect in a given resistor as is produced by the given alternating current when passed for the same time.
The r.m.s of a.c is given by the relation,

Irms = Io/2

Numerical:- Given that, I = 5 sin (314 t)  ---------(I)

To find out, Iav = ? and Irms = ?

Comparing equation (I) with the standard equation,

• Q10

Write the relation for the force  acting on a charge carrier q moving with velocity  through a magnetic field  in vector notation.  Using this relation, deduce the conditions under which this force will be (i) maximum (ii) minimum.

Marks:2

The force  acting on a charge carrier q moving with velocity  through a magnetic field  in vector notation is given by relation,

The above equation can also be written as,

(i) The force will be maximum when;

Substituting,

Hence magnitude of force is maximum, when .

(ii) The force will be maximum when;

Subsituting,

Hence magnitude of force will be minimum, when