Modern Physics: Gaseous discharge at various pressure: Discovery of an electron.

Teaching material for 12th grader Physics students:


Q. Describe in brief the discharge of electricity through a gas.


Q. Describe with a neat diagram of the phenomenon of electrical discharge through gases.

Ans: We know that the gases are good insulators of electricity at normal pressure and temperature. But, we can make them conducting by:

  • Applying very high voltage (~30,000 V/cm)
  • Reducing its pressure to a very low value (~ 0.01 mm of Hg)

By imposing the above conditions to the gases, they split into the charged particles, called ions, both positive and negative and hence the electric current flows through them which is known as electrical conduction or discharge in the gas. We use the glass tube to study the electrical conduction through gases at low pressure called the discharge tube. This experiment was conducted initially by Sir William Crookes, Thomson, and many others.

Experimental set-up:

The experimental set-up consists of a strong glass tube of about 0.5 m long and 0.0 m in diameters, closed at both ends and provided with two platinum electrodes A and C, called anode and cathode respectively. These two electrodes are connected to the secondary terminals of a powerful induction coil. The discharge tube is connected with a vacuum pump to reduce the pressure inside the tube and pressure gauge to measure the pressure inside the tube as shown in the figure below.


                                     Fig: Discharge of electricity through gases at low pressure.

As the pressure inside the discharge tube is gradually decreased with the help of pressure gauge, the following phenomena are observed as discussed below:

(i) At a pressure about 10 mm of Hg:

When the pressure in the discharge tube is reduced to about 10 mm of Hg, the discharge occurs in the tube showing luminous streaks between the electrodes A and C, these streaks are called blue streamers which are produced with cracking noise.

(ii) At a pressure about 5 mm of Hg:

At this pressure inside the tube, the blue streaks broaden out in a luminous column which is bright and steady and this column is called Geissler’s discharge. The color of the discharge depends on the nature of the gas.

(iii) At a pressure of 2 mm of Hg:

In this case, a long luminous column appears from A to C inside the tube and this column is called the positive column. Again, the color of the discharge depends on the nature of the gas. For eg: blue for hydrogen and red for air.

(iv) At a pressure of 1 mm of Hg:

When the pressure inside the tube is reduced to 1 mm of Hg, the positive column leaves the cathode and moves to the anode. A blue luminous glow appears at a cathode called a negative glow. A dark space appears between the positive column and negative glow which is called Faraday’s dark space as shown below;

(v) At a pressure of 0.5 mm of Hg:

As the pressure inside the discharge tube is further reduced to about 0.5 mm of Hg, the size of the positive column gets reduced whereas the  Faraday’s dark space extends to a longer size. The negative glow leaves the cathode and another glow appears on the cathode called cathode glow. The negative glow moves towards the anode. Also, another dark space appears between cathode glow and negative glow which is known as Crooke’s dark space.

(vi) At a pressure of 0.05 mm of Hg: 

In this range of pressure, the positive column further shortens and breaks into an alternative bright and dark disc called striations as shown below;

(vii) At a pressure of about 0.01 mm of Hg: 

In this reduced pressure inside the discharge tube, initially, the striations disappear, and then negative and cathode glow vanishes. Then the whole tube is filled with Crooke’s dark space. At this situation, the luminous rays are seen to come out of the cathode which are called cathode rays.

In this way, the cathode rays are produced. If the pressure in the tube is reduced further, the tube stops conducting.

Cathode rays: 

What are the cathode rays? write its properties. [imp: 2 marks]

Ans: Cathode rays are the invisible rays emerging normally from the cathode of a discharge tube, kept at a pressure of 0.01 mm of Hg, and under a very high potential difference of the order of several thousand volts (10-15 KV). These rays are independent of the nature of gas and their propagation is independent of the position of the anode.

Properties of cathode rays:

  1. Cathode rays are emitted normally from the surface of the cathode.
  2. Cathode rays can penetrate the small thickness of matter such as sheets of aluminum foil.
  3. They can travel in a straight line and cast sharp shadows of the targets placed in their path.
  4. They carry negative charge so are deflected by electric and magnetic fields.
  5. They carry momentum and kinetic energy.
  6. They produce heat when they fall upon the matter.

Reference: Principle of physics, Ayam publication.


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