Electricity and Magnetism Answers

Explain the lorentz force.

A piece of iron was found along the street of london by a school boy. At what angle should the iron be hammered into the ground inoder to make it a magnet?

Q.1 Consider three charge distributions
A. ρ (x, y, z) = kx

B. ρ (x, y, z) = k √ (x2 + y2)

C. ρ (x, y, z) = k √ (x2 + y2 + z2)

Consider a hydrogen atom undergoing a 2P 1S transition. The lifetime tsp of the 2P
state for spontaneous emission is 1.6 ns and the energy difference between the levels is
10.2 eV. Assuming that the refractive index of the medium n0 = 1, the ratio of Einstein
coefficients for stimulated and spontaneous emission   B 21/ A21 is given by

A policeman directing traffic is standing upright with his arms down at his side. He motions traffic by raising his arms to horizontal and pointing to the left with both arms perfectly straight at shoulder height. His overall center of gravity has been:

1. Raised and shifted right

2. Lowered and shifted right

3. Raised and shifted left

4. Lowered and shifted left

Point charges are placed at the vertices of a square of side L as shown in figure. what should be sign of charge q and magnitude of the ratio |q/Q| so that

(a) net force on each Q is zero?

(b) net force on each q is zero?

(c) Is it possible that the entire system could be in electrostatic equilibrium?

The electric field in a region is given by Ē = 20î + 30ĵ in certain units. As one goes a small distance dl in the direction of ñ = 30î - 20ĵ , the potential will :-

(a) increase

(b) decrease

(c) remain constant

(d) change sign

(Note:- This question have one or more than one correct choice(s) out of the four given choices. Any number of options may be correct.)

Consider three charge distributions
A. ρ (x, y, z) = kx

B. ρ (x, y, z) = k √ (x2 + y2)

C. ρ (x, y, z) = k √ (x2 + y2 + z2)

Here k is a constant. A, B, C are

(a) Planar Symmetric

(b) Planar, Spherically, Cylindrically Symmetric respectively

(c) Spherically, Planar and cylindrically symmetric respectively

(d) Planar, Cylindrically, Spherically symmetric respectively

If the z-axis is vertical and r̂, θ̂^ and φ̂ are unit vectors in spherical coordinates (standard symbols)


(a) r̂ is always horizontal

(b) θ̂^ is always horizontal

(c) φ̂ is always horizontal

(d) θ̂^ can be vertical

[Note:- This question have one or more than one correct choice(s) out of the four given choices. Any number of options may be correct.]

If the z-axis is vertical and ŝ, φ̂, ẑ are unit vectors in cylindrical coordinates (standard symbols),

(a) ŝ is horizontal

(b) φ̂ is horizontal

(c) φ̂ is vertical

(d) ŝ is vertical

(Note:- This question have one or more than one correct choice(s) out of the four given choices. Any number of options may be correct.)