دانلود کتاب فیزیک مدرن برای JEE پایه و پیشرفته

At normal pressure, gases are poor conductor of electricity because they have very few charged particles for conduction. These charged particles are produced due to cosmic rays. When a potential difference is applied across the discharge tube, the charged particles start accelerating. At low pressure these charged particles can move a considerable distance and aquire enough energy to ionise the other molecules after collisions. The free electrons and positive ions are produced inside the discharge tube, and conduction starts between electrodes fitted in the tube. When pressure inside the discharge tube decreases gradually from 10 mm of mercury to 0.1 mm of mercury, Crookes dark space, Cathode glow, Faraday dark space and then alterate dark and bright bands are observed. At the pressure of 10–3 mm of mercury, the molecules of the gas ionise and the free electrons move towards anode. The positive ions hit the cathode and cause emission of electrons from cathode. These emitted electrons also move towards anode. Thus cathode rays in the discharge tube are the stream of electrons produced due to collisions. Cathode rays were discovered by Sir William Crookes.