Science (English Medium) Class 12 - CBSE Important Questions for Physics

Explain the term demodulation used in a communication system.

Block diagram of a receiver is shown in the figure:

(a) Identify ‘X’ and ‘Y’.

(b) Write their functions.

In the block diagram of a simple modulator for obtaining an AM signal, shown in the figure, identify the boxes A and B. Write their function.

Distinguish between ‘Analog and Digital signals’.

How does the electric flux due to a point charge enclosed by a spherical Gaussian surface get affected when its radius is increased?

(a) Derive an expression for the electric field E due to a dipole of length '2a' at a point distant r from the centre of the dipole on the axial line.
(b) Draw a graph of E versus r for r >> a.
(c) If this dipole were kept in a uniform external electric field E₀, diagrammatically represent the position of the dipole in stable and unstable equilibrium and write the expressions for the torque acting on the dipole in both the cases.

An infinitely large thin plane sheet has a uniform surface charge density +σ. Obtain the expression for the amount of work done in bringing a point charge q from infinity to a point, distant r, in front of the charged plane sheet. 

1. Define torque acting on a dipole of dipole moment \[\vec{p}\] placed in a uniform electric field \[\vec{E}\] Express it in the vector from and point out the direction along which it acts. Express it in the vector from and point out the direction along which it acts.
2. What happens if the field is non-uniform?
3. What would happen if the external field
   \[\vec{E}\]  is increasing (i) parallel to \[\vec{p}\] and (ii) anti-parallel to \[\vec{p}\]?

A point charge (+Q) is kept in the vicinity of an uncharged conducting plate. Sketch the electric field lines between the charge and the plate?

A hollow cylindrical box of length 1 m and area of cross-section 25 cm² is placed in a three dimensional coordinate system as shown in the figure. The electric field in the region is given by `vecE = 50xhati` where E is NC­⁻¹ and x is in metres. Find

(i) Net flux through the cylinder.

(ii) Charge enclosed by the cylinder.

A charge ‘q’ is placed at the centre of a cube of side l. What is the electric flux passing through each face of the cube?

Define dipole moment of an electric dipole. Is it a scalar or a vector?

An electric dipole of dipole moment `vecP` is placed in a uniform electric field `vecE` with its axis inclined to the field. Write an expression for the torque `vecT` experienced by the dipole in vector form. Show diagrammatically how the dipole should be kept in the electric field so that the torque acting on it is:

1. maximum
2. Zero

Answer the following question.
Derive an expression for the electric field at any point on the equatorial line of an electric dipole. 

The electric flux through a closed Gaussian surface depends upon ______.

Derive the expression for the torque acting on an electric dipole, when it is held in a uniform electric field. identify the orientation of the dipole in the electric field, in which it attains a stable equilibrium.

The magnitude of the electric field due to a point charge object at a distance of 4.0 m is 9 N/C. From the same charged object the electric field of magnitude, 16 N/C will be at a distance of ______.

An electric dipole of length 2 cm is placed at an angle of 30° with an electric field 2 × 10⁵ N/C. If the dipole experiences a torque of 8 × 10^-3 Nm, the magnitude of either charge of the dipole is ______.

A point charge +Q is placed at point O, as shown in the figure. Is the potential difference V_A – V_B positive, negative or zero?

Two parallel-plate capacitors X and Y have the same area of plates and same separation between them. X has air between the plates, while Y contains a dielectric medium of ε_r = 4.

(i) Calculate capacitance of each capacitor if the equivalent capacitance of the combination is 4 μF.

(ii) Calculate the potential difference between the plates X and Y.

(iii) Estimate the ratio of electrostatic energies stored in X and Y.