Cathode Rays & Positive Rays

Cathode rays are streams of fast moving electrons.

Properties : (i) Cathode rays are emitted normally from the cathode surface. Their direction is independent of the position of the anode.

(ii) Cathode rays travel in straight lines. (Cast shadows of objects placed in their path).

(iii) Cathode rays exert mechanical force on the objects they strike.

(iv) Cathode rays produce heat when they strike a material surface.

(v) Cathode rays produce fluorescence when they strike a number of crystals, minerals and slats.

(vi) When cathode ray strike a solid object, specially a metal, X-rays are emitted from the object. (It is not safe to use Geissler tube at potential differences above about 5–6 kV because of this X-ray emission, generated by striking of cathode rays of metal anode).

(vii) Cathode rays are deflected by an electric field and also by a magnetic field. The direction of deflection is the same as that of a stream of negatively charged particles.

(viii) Cathode rays ionize the gas through which they are passed.

(ix) Cathode rays can penetrate thin foils of metal.

(x) Cathode rays affect photographic plates.

(xi) Cathode rays are found to have velocity upto one tenth of the velocity of light. The e/m of electrons was measured by J.J. Thomson (so credit of discovering electron is given to Thomson).

The e of electron was measured by R.A./Millikan e/m = 1.759 x 1011 c/kg e = 1.602 x 10–19 C m = 9.09 x 10–31 kg

The e/m of an electron is called the specific charge of an electron.

If one includes the relativistic variation of mass with speed then specific charge of an electron decreases with the increase in the velocity of the electron.

Positive Rays :

Positive rays are sometimes known as the canal rays. These were discovered in 1896 by Goldstein. If the cathode of a discharge tube has holes in it and the pressure of the gas is around 1 mm of mercury than faint luminous glow come out from each hole on the backside of the cathode.

This shows that something is coming out of the holes. These are called canal rays or positive rays.

Positive - RAY

Origin of positive rays :

When potential difference is applied across the electrodes, electrons (cathode rays) are emitted from the cathode. As they move towards anode, they gain energy.

These energetic electrons which collide with the atoms of the gas in the discharge tube, they ionize the atoms. The positive ions formed at various places between cathode and anode, travel towards the cathode. Since during their motion, the positive ions when reach the cathode, some pass through the holes in the cathode. These stream of positive ions are the positive rays or canal rays.

If the discharge tube is totally evacuated then no positive rays are produced. Thus positive rays are positive ions of the gas in the tube. Properties of positive rays were studied by Thomson. The q/m of the hydrogen was found to be ~108 C/kg much less then the e/m of electron ~1011 C/kg indicating that mass of positive rays is much greater than that of electrons.

Properties of Positive Rays :

(i) These are positive ions having same mass if the experimental gas does not have isotopes. However if the gas has isotopes then positive rays are group of positive ions having different masses.

(ii) They travel in straight lines and cast shadows of objects placed in their path. But the speed of the positive rays is much smaller than that of cathode rays.

(iii) They are deflected by electric and magnetic fields but the deflections are small as compared to that of cathode rays.

(iv) They show a spectrum of velocities. Different positive ions move with different velocities.

(v) q/m ratio of these rays depends on the nature of the gas in the tube (while in case of the cathode rays q/m is constant and does not depend on the gas in the tube).

(vi) They carry energy and momentum.

(vii) They cause ionization (which is much more than that produced by cathode rays).

(viii) They cause fluorescence (in ZnS or CdS screen) and affect photographic plates.

(ix) They have a little penetration power (but much less in comparison to cathode rays)

Author: Rajesh Jha

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