# CBSE Delhi-Set-2-2005

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• Q1

The power factor of an a.c circuit is 0.5. What will be the phase difference between voltage and current in this circuit?

Marks:1

According to the given condition,

cos= 0.5

= cos-1(0.5)

= 60o

• Q2

What should be the length of the dipole antenna for a carrier wave of frequency 3x108 Hz?

Marks:1

Length of the dipole antenna = /2

= c/2v

= [(3108)/(23108)]

= 0.5 m

• Q3

Why does the electric field inside a dielectric decrease when it is placed in an external electric field?

Marks:1

If we place the dielectric in an external magnetic field then polarization of dielectric takes place which induces a field in the opposite direction. This effect results in decrease of net electric field inside the dielectric.

• Q4

A charged particle enters into a uniform magnetic field and experiences upward force as indicated in the figure. What is the charge sign on the particle?

Marks:1

The given particle will have positive sign.

• Q5

Name the types of communication systems according to the mode of the transmission.

Marks:1

There are two types of communication systems,
(i) Analog communication system.
(ii) Digital communication system.

• Q6

Define ‘electric line of force’ and give its two important properties.

Marks:2

Electric line of force may be defined as a curve which is drawn in such a manner that a tangent to it at any point gives the direction of the electric field at that point.
Its two important properties are given as follows:
1. Two lines of forces can never intersect each other.
2. It is a continuous curve that starts from positive charge and ends on a negative charge.

• Q7

(a) Why does the electric field inside a dielectric decrease when it is placed in an external electric field?
(b) A parallel plate capacitor with air between the plates has a capacitance of 8 pF. What will be the capacitance if the distance between the plates be reduced by half and the space between them is filled with a substance of dielectric constant K = 6?

Marks:2

(a) If we place the dielectric in an external magnetic field then polarization of dielectric takes place which induces a field in the opposite direction. This effect results in decrease of net electric field inside the dielectric.
(b) Let C0 be the capacitance of capacitor in air. C0 is given by,

Co = (oA)/(d) ..........(I)

Now change in the capacitance according to the given condition can be given by the relation,
C = k[(oA)/(d/2)]

C = 2KC0 ------ (from-I)

C = 2 x 6 x 8 = 96 pF.

• Q8

There point charges of +2 C, -3 C and -3 C are kept at the vertices, A, B and C respectively of an equilateral triangle of side 20 cm as shown in the figure. What should be the sign and magnitude of the charge to be placed at the mid-point (M) of side BC so that the charge at A remains in equilibrium?

Marks:2

The force exerted on +2 C charge by the charge at B is given by,

Similarly the force exerted by charge at point C is given by,

The resultant of force F1 and F2 is given by

• Q9

Define the term ‘Magnetic Dip’ and ‘Magnetic Declination’ with the help of relevant diagrams.

Marks:2

(i) Magnetic declination: The angle between the geometric meridian and magnetic meridian at a given place is called magnetic declination at that place. It is denoted by

(ii) Angle of dip: It is the angle made by the earth’s total magnetic field with the horizontal at a given place.

• Q10

The image of a candle is formed by a convex lens on a screen. The lower half of the lens is painted black to make it completely opaque. Draw the ray diagram to show the image formation. How will this image be different from the one obtained when the lens is not painted black?

Marks:2