Having been operated long enough, the ionixer producing `n_(1) = 3.5.10^(9) cm^(-3). S^(-1)` of one pairs per unit volume of air per unit time was switched off. Assuming that the only process tending to reduce the numnber of ions in air is thier recombination with coefficient `r = 1.67.10^(-5)cm^(3)//s`, find how soon after the ioner concentration decreases `eta = 2.0` times.

The wave number of a beam of light in air is 5 xx 10^(6) m^(-1) . If the velocity of light in air is 3 xx 10^(8) m//s , find the frequency of the light.

The radius of an air bubble in increasing at the rate of 0.5cm/s. Find the rate of change of its volume, when the radius is 1.5 cm.

A rain drop of radius 0.5 mm has a terminal velocity in air 2 ms^(-1) . The viscosity of air is 18 xx 10^(-5)" dyne "cm^(-2)s . Find the viscous force on the rain drop.

The air between two closely separated large plates in a scalted glass tube is uniformly ionised by ultraviolet ragdiation. The air volume between the plates is equal 500cm^(3) , the observed saturation current is equl to 0.48muA . Given that at saturation on an average ions take 1 s in travelling from one plate to another. Find: (a) The equilibrium concentration of ions in saturation state. (b) The production rate of iron pairs if the recombination coefficient for air ions is equal to r=1.67xx10^(-6)cm^(3)//s . Consider that the rate at which ion pairs recombine is given as r N^(2) where N is the ion concentration.

Strain energy of a wire is 1.8 xx 10^(-3) J and strain energy per unit volume of the same wire under the same conditions is 6 xx 10^(-3) J//m^(3) . The volume of the wire will be

An air cored solenoid with length 30 cm , area of cross section 25 cm ^2 and number of turns 500 carries a current of 2.5 A. The current is suddenly switched off in a brief time of 10 ^(-3)s . What is the average back emf induced across the ends of the open switch in the circuit ?

Calculate the mean free path and the collsion frequency of air molecules, if the number of molecules per cm^(3) is 3 xx 10^(19) , the diameter of the molecule is 2 xx 10^(-8) cm and average molecule speed is 1 km s^(-1) .

A semiconductor is known to have an electron concentration of 5 xx 10^(13)//cm^(3) and hole concentration of 8 xx 10^(12)//cm^(3) . The semiconductor is