Concept Review Ch.41 Electric Current through Gases

Electric current through gases

Gases are in general poor conductors of electricity. This is because they do not have free charged particles in large numbers which may respond to an electric field.

Electric current may be passed through a gas if we ensure that by some mechanism charged particles are continuously produced in the gas.

This can be done in many ways. One method is to apply a large potential difference across gas at very low pressure. Another method is to heat a metal kept in an evacuated chamber to high temperatures at which electrons are ejected from the metal and these electrons flow through the gas. Another method is to pass X-rays through the gas.

Discharge tube

It is closed tube of length about 30 cm and diameter about 4 cm. It is fitted with two electrodes to function as anode and cathode. There is a pumping arrangement to create low pressure in the tube. The electrodes are connected to a high voltage source, a secondary coil of an induction coil.


Sparking potential

In a gas at low pressure, if the potential difference between the electrodes is gradually increased, sparking occurs at a certain stage.

The minimum potential difference which can cause sparks in a gas is called the sparking potential. It depends on the pressure of gas as well as on the separation between the electrodes.

Paschen's law
Sparking potential of a gas in a discharge tube is a function of the product of the pressure of the gas and the separation between the electrodes.

V = f(pd)

Phenemona in the gas as pressure is decreased

In a typical case, sparking occurs at a pressure of 10 cm of mercury. the sparking is accompanied by crackling noise.

At a pressure of about 10 mm of mercury, the irregular streaks broaden out in a luminous column extending from the anode almost up to the cathode. A buzzing sound comes now. This column is called positive column. The colour depends on the nature of the gas in the tube - reddish for air, bright red for neon, bluish for Carbon dioxide etc.

At a pressure of 3-4 mm of mercury, the length of positive column decreases - it starts from anode, but ends well before the cathode. A bluish glow will be there around the cathode and there will be a dark space between this glow and the positive column. The bluish glow around the cathode is termed cathode glow or negative glow. The dark space between the cathode glow and the positive column is called Faraday dark space.

As the pressure reaches to about 1 mm of mercury, the positive column gets reduced further and the length of Faraday dark space increases. The cathode glow is now some distance away from the cathode and a dark space appears between cathode and the cathode glow. This dark space is called Crookes dark space.

As the pressure is decreased further, the Crookes dark space and the cathode glow expand. At about 0.1 mm of mercury, the positive column gets split into alternate bright and dark bands called striations.

As the pressure is further reduced, the striations of the positive column keep vanishing and finally, positive column disappears totally. At about 0.01 mm of mercury, cathode glow also disappears. Crookes dark space fills the entire tube and the walls of the tube begin to glow. This is fluorescence. If the soda gas is used, the colour of the tube will appear as yellowish green.

If the pressure is further reduced, the tube stops conducting at some stage.

Cathode rays

When Crookes dark space fills the entire tube, the walls of tube begin to glow. This means something is coming out of cathode. This something is a stream of fast moving electrons. Crookes, Thomson and others carried out a series of experiments on this phenomenon and named the stream of electrons as cathode rays.

Properties of cathode rays

1. Cathode rays are emitted normally from the cathode surface. Their direction is independent of the position of the anode.

2. Cathode rays travel in straight lines

3. Cathode rays exert mechanical force on the object they strike.

4. They produce heat when the strike a material surface.

5. They produce fluorescence when they strike a number of crystals, minerals and salts.

6. When cathode rays strike a solid object, specially, a metal, X-rays are emitted from the metal surface.

7. Cathode rays can be deflected by an electric field or a magnetic field. The deflection is similar to the deflection of negatively charged particles. The deflection is independent of the gas present in the discharge tube, material of the cathode, the position of anode etc.

8. They ionize gas through which they are passed.

9. They can penetrate thin foils of metal.

10. They affect photographic plates

Canal rays or positive rays

It is observed that if the cathode of the discharge tube has holes in it and the pressure of the gas is around 1 mm of mercury, streams of faint luminous glow come out from each hole on the backside of the cathode. These rays are called canal rays or positive rays. When gas is ionized, electrons leave the molecules and positive ions move towards the cathode. These rays were observed to deflect in the same manner as a stream of positively charged particles. They cause fluorescence when incident on certain materials.

Discovery of electron

J.J Thomson is credited with the discovery of electron. He studied the properties of electron and suggested that electrons are necessary constituents of all atoms.

Experiment to Determine e/m by Thomson

Cathode rays are subjected to electric and magnetic fields.

If both the electric field and magnetic field are switched on and the values are so chosen that

v = E/B

The magnetic field evB will exactly cancel the electric force eE and the beam will pass undeflected. If the potential difference between the anode and the cathode is V, the speed of the electrons coming out of A is given by

½ mv² = eV

as v = E/B

½ m (e/B) ² = eV
therefore e/m = E²/2B²V

Millikan Oil drop experiment to determine e.

E = 1.6*10^-19 C.

Thermionic emission

When a metal is heated to a high temperature, electrons escape from its surface. This phenomenon is called thermionic emission and the electrons coming out are called thermions.

If n thermions are ejected per unit time by a metal surface, the thermionic current i = ne. This current is given by the Richardson-Dushman equation.

i = ne = AST²e^{- φ/kT}

where
S = surface area
T = the absolute temperature of the surface.
φ = work function of the metal
K = Boltzmann constant
A = constant which depends only the nature of the metal