# Physics Glossary – T

Terminal Velocity

When an object which is falling or acted on by a constant driving force it is subjected to a resistance or drag force which increases with velocity, at some point it will reach a maximum velocity where the drag force equals the driving force or weight in the case of falling. After the object has reached this velocity it will fall at that velocity, this is called the terminal velocity.

Thermistor

Is a semiconducting material used to measure temperature changes, relying on the principle that the resistance of a semiconductor decreases as the temperature increases. Thermistors are usually a thin coil of a semiconducting material. They work because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge – it promotes them into the conducting band. The more charge carriers available, the more current a material can carry.

Thermocouple

Is a temperature measurement device that works by measuring the voltage produced between conductors placed in hot and cold bodies. Thermocouples rely on the thermoelectric effect.

Thermoelectric Effect

A an electromotive force (EMF) is produced between conductors placed in the two bodies. By measuring the voltage between them, the change in temperature between the bodies can be found. Then if the absolute temperature of one is known the other can be worked out.

Thermopile

A device for measuring the heat radiation. It is made from rods of antimony and bismuth, connected in series. When the device is placed near a hot object, thermoelectric currents flow due to the thermoelectric effect, this current is then measured.

Transistor

The transistor is an amplifying or switching semiconductor device. The transistor is the key component in all modern electronics. In digital circuits, transistors are used as electrical switches, and arrangements of transistors can function as logic gates, RAM-type memory and other devices. In analog circuits, transistors are essentially used as amplifiers.

Transverse Wave

Means the electric and magnetic field vectors are perpendicular to the direction that the wave travels

Tyndall Effect

If a beam of converging rays, say, from a projection lantern, is passed through a liquid containing minute particles in suspension, each of these particles scatters the light rays that fall on it, becoming, in a sense, a luminous point. Thus, the entire path of the rays through the liquid becomes visible, having the appearance of a bright cone, if viewed in a darkened room.

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