## A double slit apparatus is immersed in a liquid of refractive index 1.33. It has slit separation of 1mm and distance …..

Q. A double slit apparatus is immersed in a liquid of refractive index 1.33. It has slit separation of 1mm and distance between the plane of slits and screen 1.33m. The slits are illuminated by a parallel beam of light whose wavelength in air is 6300Å. The fringe width is

(a)(1.33×0.63)mm

(b) 0.63/1.33 mm

(c)0.63/(1.33)² mm

(d) 0.63 mm

Ans: (d)

Fringe Width $\large \beta = \frac{\lambda D}{\mu d}$

$\large \beta = \frac{6300 \times 10^{-10}\times 1.33}{1.33 \times 1 \times 10^{-3}}$

= 0.63 mm

## Why transverse mechanical waves can not be propagated in liquids and gases ?

Q. Why transverse mechanical waves can not be propagated in liquids and gases ?

Ans: To transmit a transverse mechanical wave the medium must be elastic so as to provide a restoring force when acted on by shearing stress. But liquids and gases can not sustain shear stress to provide restoring force, but rather start flowing when acted on by shearing stress.

#### Note :

(a) All non-mechanical waves are transverse.

(b) In gases and liquids mechanical waves are always longitudinal e.g. sound waves in air and water.

(c) In solids mechanical waves can be either transverse or longitudinal depending on the mode of excitation. The speeds of the two waves in the same solid are also different.

## Suppose you have three resistors of 20ohm, 50 ohm and 100 ohm. What minimum and maximum resistances…..

Q. Suppose you have three resistors of 20 ohm , 50 ohm and 100 ohm . What minimum and maximum resistances can you obtain from these resistors?

(a) 15Ω , 170Ω

(b) 12.5Ω , 170Ω

(c) 12.5Ω , 150Ω

(d) 15Ω, 150Ω

Ans: (b)

For minimum resistance , all the resistors are connected in parallel.

$\displaystyle \frac{1}{R_{min}} = \frac{1}{20} + \frac{1}{50} + \frac{1}{100}$

For maximum Resistance , all the resistors are connected in series

Rmax = 20 + 50 + 100 = 170 ohm

## The potential difference between the terminals of a 6 V battery is 7.2V when it is being charged by a current of 2 A…..

Q. The potential difference between the terminals of a 6 V battery is 7.2V when it is being charged by a current of 2 A. What is the internal resistance of the battery ?

(a) 0.4Ω

(b) 0.6Ω

(c) 1 Ω

(d) 0.5Ω

Ans: B

Sol.

V = E + Ir

7.2 = 6 + 2 r

2 r = 1.2

r = 0.6 ohm

## Calculate the electric field in a copper wire of cross-sectional area 2 mm² carrying a current of 1A. The resistivity of copper = 1.7 x 10^-8 ohm-m

Q. Calculate the electric field in a copper wire of cross-sectional area 2 mm2 carrying a current of 1A . The resistivity of copper ρ = 1.7 x 10-8 ohm-m

(a) 8.5 mvm-1

(b) 8 mvm-1

(c) 9 mvm-1

(d) 7 mv/m

Ans: (a)

Resistance $\displaystyle R = \rho \frac{L}{A}$

$\displaystyle L = \frac{R A}{\rho}$

Since V = IR and V = EL

$\displaystyle E = \frac{V}{L} = \frac{I R}{L}$

$\displaystyle E = \frac{I R}{\frac{R A}{\rho}}$

$\displaystyle E = \frac{I \rho}{A}$