ELECTROMAGNETIC WAVES — Physics STD 12 Science — Question

Write an expression for resistance and resistivity of a conductor in terms of number of free electrons per unit volume and relaxation time.

Two metallic wires of the same material have the same length but cross-sectional areas are in the ratio 1 : 2. They are connected (i) in series and (ii) in parallel. Compare the drift velocities of electrons in the two wires in both the cases (i) and (ii).

Explain dielectric strength and dielectric breakdown.

In a Van de Graaff type generator a spherical metal shell is to be a 15 × 10⁶V electrode. The dielectric strength of the gas surrounding the electrode is 5 × 10⁷Vm^–1. What is the minimum radius of the spherical shell required? (You will learn from this exercise why one cannot build an electrostatic generator using a very small shell which requires a small charge to acquire a high potential.)

What is AC current and DC current? Why is AC voltage preferred compared to DC voltage in practice?

Define lens and write its formula.

A nearsighted person cannot clearly see beyond 200cm. Find the power of the lens needed to see objects at large distances.

Derive the equation for the force acting on two parallel current carrying wires.

A copper wire bent in the shape of a semicircle of radius r translates in its plane with a constant velocity v. A uniform magnetic field B exists in the direction perpendicular to the plane of the wire. Find the emf induced between the ends of the wire if:

1. The velocity is perpendicular to the diameter joining free ends.
2. The velocity is parallel to this diameter.