Physics Kenya
Cathode Rays and Cathode Ray Tube
Cathode Rays and Cathode Ray Tube
Cathode Rays and Cathode Ray Tube
Production of cathode rays, cathode ray tube
Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they first must be detached from the atoms of the cathode.
Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament which is heated by a separate electric current passing through it. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament, into the evacuated space of the tube.
Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence
Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be travelling in straight lines from the cathode.
After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube. The current in a beam of cathode rays through a tube can be controlled by passing it through a metal screen of wires (a grid) to which a small voltage is applied.
The electric field of the wires deflects some of the electrons, preventing them from reaching the anode. Thus a small voltage on the grid can be made to control a much larger voltage on the anode. This is the principle used in vacuum tubes to amplify electrical signals.
High speed beams of cathode rays can also be steered and manipulated by electric fields created by additional metal plates in the tube to which voltage is applied, or magnetic fields created by coils of wire (electromagnets). These are used in cathode ray tubes, found in televisions and computer monitors, and in electron microscopes.
Properties of cathode rays
Properties of Cathode Rays Include:
- Cathode rays travel in straight lines. That is why, cathode rays cast shadow of any solid object placed in their path. The path cathode rays travel is not affected by the position of the anode.
- Cathode rays consist of matter particles, and posses energy by the virtue of its mass and velocity. Cathode rays set a paddle wheel into motion when it is placed in the path of these rays one the bladder of the paddle wheel.
- Cathode rays consist of negatively charged particles. When cathode rays are subjected to an electrical field, these get deflected towards the positively charge plate (Anode).We know that a positively charged body would attract only a negatively charged body, therefore the particles of cathode rays carry negative charge.Cathode rays also get deflected when these are subjected to a strong magnetic field.
- Cathode rays heat the object only which they fall. The cathode ray particles possess kinetic energy. When these particles strike an object, a part of the kinetic energy is transferred to the object. The causes a rise in the temperature of the object.
- Cathode rays cause green fluorescence on glass surface, i.e., the glass surface only which the cathode rays strike show a colored shine.
- Cathode rays can penetrate through thin metallic sheets.
- Cathode rays ionize the gases through which they travel.
- Cathode rays when fall only certain metals such as copper, but rays produced. The X-rays are not deflected by electrical or magnetic fields. X-rays pass through opaque materials such as black paper, but stopped by solid objects such as bones.
- Cathode rays travel with speed nearly equal to that of light.
C.R.O and T.V tubes
Uses of C.R.O
Problems on C.R.O
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