# CBSE [All India]-Set-1-2006

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• Q1

Give any one difference between FAX and e-mail systems of communication.

Marks:1

The basic difference between FAX and e-mail is that in case of FAX we get a hard copy of the message at the recipient’s terminal, while in case of an e-mail, we get a soft copy of the message at the recipient’s terminal.

• Q2

De Broglie wavelength associated with an electron accelerated through a potential difference V is l. What will be its wavelength when the accelerating potential is increased to 4 V?

Marks:1

The de Broglie wavelength and potential difference is related to each other by the relation,

1/(V)1/2

Now according to given condition,

'/= (V/4V)1/2 = 1/2

' = /2

Hence, the new wavelength is half the original wavelength.

• Q3

Define the term electric dipole moment. Is it a scalar or a vector quantity?

Marks:1

The electric dipole moment of an electric dipole is the product of either charge and separation between the two charges. It is a vector quantity in the direction of the dipole axis from –q to +q. Mathematically it can be represented by equation,
p = q  2a

• Q4

The variation of potential difference V with length l in case of two potentiometers P and Q is as shown. Which one of these two will you prefer for comparing emfs of two primary cells.

Marks:1

Here potentiometer Q is preferred because it has a smaller potential gradient. (k=V/l)

• Q5

Steel is preferred for making permanent magnets whereas soft iron is preferred for making electromagnets. Give one reason.

Marks:1

Steel is preferred for making permanent magnets because of its high coercivity.
Soft iron is used for making electromagnets because of its high permeability and low hysteresis loss.

• Q6

Two capacitors of capacitance of 6 F and 12 F are connected in series with a battery. The voltage across the 6F capacitor is 2 V. Compute the total battery voltage.

Marks:2

Since both the capacitors are connected in series,

Charge on 6F capacitor = charge on 12 F capacitor. Let V be the potential difference across 12F capacitor.

2 = 12  V [ Q = CV]

V = [(62)/12] = 1 volt

Hence the battery voltage = (2 + 1)V = 3V.

• Q7

Sketch a graph between frequency of incident radiations and stopping potential for a given photosensitive material. What information can be obtained from the value of the intercept on the potential axis?
A source of light of frequency greater than the threshold frequency is placed at a distance of 1m from the cathode of a photo-cell. The stopping potential is found to be V. If the distance of the light source from the cathode is reduced, explain giving reasons, what change will you observe in the
(i)photoelectric current,
(ii)stopping potential

Marks:2

Graph between frequency of incident radiation and stopping potential for a given photosensitive surface is given below:

The intercept on the potential axis = - Wo/e

From the above equation it follows that,

Work function, W0 = e x Magnitude of intercept on the potential axis.

If the distance of the light source from the cathode is reduced, then

(i)The photoelectric current increases because the intensity of incident radiation increases.

(ii)The stopping potential V remains the same because it is independent of the intensity.

• Q8

You are given ‘n’ resistors, each of resistance ‘r’. These are first connected to get minimum possible resistance. In the second case, these are again connected differently to get maximum possible resistance. Compute the ratio between the minimum and maximum values of resistance so obtained.

Marks:2

First of all connecting n resistances in parallel combination,

1/(Rmin) = 1/(r) + 1/(r)........(n resistance) = n/r

or 1/(Rmin) = r/n

Now connect the n-resistances in series,

Rmax= r + r ..........(n resistance) = nr

We have to find out,

(Rmin)/(Rmax) = [(r/n)]/[(nr)] = 1/n2 = 1 : n2

• Q9

A parallel plate capacitor with air between the plates has a capacitance of 8 pF. The separation between the plates is now reduced by half and the space between them is filled with a medium of dielectric constant 5. Calculate the value of capacitance of the capacitor in the second case.

Marks:2

Initial capacitance of the capacitor is given by,

CooA/d = 8 pf

Now final capacitance of the capacitor, according to the given condition is given by,

C = [(K oA)/(d/2)] = 2KCo = 258 = 80pf

• Q10

Draw a labeled ray diagram to show the image formation in a refracting type astronomical telescope. Why should the diameter of the objective of a telescope be large?

Marks:2