what are cathode rays
what are cathode rays ? How are they а produced ?What are important properties of cathode rays ?
Ans- When the pressure in the discharge tube (Fig. 4.1) falls below 0.001 mm Hg and a potential difference of about 10,000 volts is applied across the electrodes, the walls of the discharge tube opposite to the cathode starts glowing with a faint greenish light. This is due to the bombardment of walls by some rays emerging from the cathode or negative electrode.
These rays are known as cathode rays. These observations lead to the conclusion that cathode rays consist of rapidly moving negatively charged particles. These are called electrons and are shot out from the cathode of a discharge tube when an electric current is passed at high voltage through a gas at very low pressure.
What are important properties of cathode rays ?
Give four characteristics of cathode rays.
Ans- (i) Cathode rays travel in straight lines and thus cast shadows of objects placed in their path.
(ii) Cathode rays possess material particles because they can rotate a light paddle wheel placed in their path.
(iii) They are deflected towards positive plate thus showing that these are negatively charged particles known as electrons.
(iv) They ionise gas through which they pass.
(v) They are deflected by magnetic fields.
(vi) The nature of cathode rays is independent of the material of cathode. Hence they are common constituents of all matter.
(vii) They can penetrate through thin metallic sheet.
(viii) They can produce X-rays.
(ix) The mass of a cathode ray particle is very-very small as compared to the mass of the atom from which it is formed.
What is a discharge tube ?
Ans- A discharge tube is a glass tube about 70 cm long and having a diameter of 5 cm. Two metal electrodes are sealed at the two ends, one of which is connected to the negative terminal of a battery and the other to the positive terminal. A side tube is fused at the centre of the glass tube which serves to pump out air from using it, a suction pump.
State the observations made, while conducting a discharge experiment,
which showed that
(i) Cathode rays travel in straight lines.
(ii) Cathode rays are made up of material particles having mass and kinetic energy.
(iii) Cathode rays are negatively charged.
Ans- (i) Cathode rays cast shadow of the objects placed in their path. This observation shows that cathode rays travel in straight lines.
(ii) It was observed that cathode rays could rotate the paddle of light wheel. This suggested that cathode rays possess mass and hence kinetic energy.
(iii) The cathode rays were deflected towards positive plate of the electric field applied in their path. This shows that cathode rays are negatively charged particles because only opposite charge attract each other.
How is the flow of current in a cathode ray experiment explained knowing that gases are bad conductors of electricity ?
Ans- The gas in the discharge tube experiment (at low pressure and high voltage) get decomposed. Negatively charged particles are produced which travel from cathode to anode, thus facilitating the flow of current. Under other conditions, e.g., 1 atmosphere pressure, no dissociation of gas occurs and no current flows.
Why does a part of the glass glow in the discharge tube experiment ? Name the scientist who first performed the discharge tube experiment.
Ans- The part of the glass tube opposite the cathode glow in the discharge tube because the invisible rays called cathode rays emit from the cathode and strike the positive electrode (anode) and produce light radiations. William Crooks in 1879 first performed this experiment.
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