A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%.

(a) Why are infra-red waves often called heat waves ? Explain.

(b) What do you understand by the statement, ‘‘Electromagnetic waves transport momentum’’ ?

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why ?

Four-point charges Q, q, Q and q are placed at the corners of a square of side ‘a’ as shown in the figure.

Find the

(a) resultant electric force on a charge Q, and

(b) potential energy of this system.

OR

(a) Three-point charges q, – 4q and 2q are placed at the vertices of an equilateral triangle ABC of side ‘l’ as shown in the figure. Obtain the expression for the magnitude of the resultant electric force acting on the charge q.

(b) Find out the amount of the work done to separate the charges at infinite distance.

(a) Define the term ‘conductivity’ of a metallic wire. Write its SI unit.

(b) Using the concept of free electrons in a conductor, derive the expression for the conductivity of a wire in terms of number density and relaxation time. Hence obtain the relation between current density and the applied electric field E.