Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Photoelectric Cell


Photoelectric Cell, also phototube, electron tube in which the electrons initiating an electric current originate by photoelectric emission. In its simplest form the phototube is composed of a cathode, coated with a photosensitive material, and an anode. Light falling upon the cathode causes the liberation of electrons, which are then attracted to the positively charged anode, resulting in a flow of current proportional to the intensity of the irradiation. Phototubes may be highly evacuated or may be filled with an inert gas at low pressure to achieve greater sensitivity. In a modification called the multiplier phototube, or the photomultiplier, a series of metal plates are so shaped and arranged that the photoelectric emission is amplified by secondary electron emission. The multiplier phototube is capable of detecting radiation of extremely low intensity; hence, it is an essential tool for those working in the area of nuclear research.

The photoelectric cell, popularly known as the electric eye, is employed in operating burglar alarms, traffic-light controls, and door openers. A phototube and a beam of light (which may be infrared or invisible to the eye) form an essential part of such an electric circuit. The light produced by a bulb at one end of the circuit falls on the phototube located some distance away. Interrupting the beam of light breaks the circuit. This in turn causes a relay to close, which energizes the burglar-alarm, or other, circuit. Various types of phototubes are used in sound recording, television, and the scintillation counter (see Particle Detectors).