D
Damping
The dissipation of a vibration's energy into heat energy, or the frictional force that causes the loss of energy.
Deceleration
Deceleration basically means slowing down - reducing speed. A negative answer for an acceleration would actually be a deceleration. Like acceleration, deceleration is also measured in ms-2.
Decibel (dB) Scale
The decibel scale is used to measure the volume (intensity or loudness) of sound. The abbreviation for this unit is dB.
Decoder
The decoder section of a radio essentially removes the carrier wave from the modulated signal, leaving only the audio wave which is then amplified. In TVs there are two separate decoders - one for the picture and one for the sound.
Deflection Plates
Deflection plates are used to direct the electron beam to all parts of the cathode ray tube of an oscilloscope. There are two sets of plates, one set for controlling horizontal movement of the beam and the other set for vertical movement. Each set of plates controls the beam by having a certain voltage applied across the plates.
Destructive interference
The condition in which two waves arriving at the same point at the same time out of phase add amplitudes to create zero total disturbance.
Dew point temperature
The temperature at which condensation begins.
Dew
Condensation of water vapor into droplets of liquid on surfaces.
Diffraction
Diffraction is a wave phenomenon - i.e. all types of wave can diffract. Diffraction means the bending of wave (change in direction) and it occurs whenever a wave passes near to an obstacle. Longer wavelength waves diffract through a greater angle than shorter wavelength waves.
Diffuse reflection
Light rays reflected in many random directions, as opposed to the parallel rays reflected from a perfectly smooth surface such as a mirror.
Digital
Digital can refer to electronic signals or to components of an electronic system. Digital signals have only two voltage levels, described as 'HI' and 'LO', or 'OFF' and 'ON'. In a real component, 'HI' might be represented by +5V d.c. and 'LO' by 0V d.c. A digital component is one which works with only digital signals. Here are some examples of digital components: LED (output), photodiode (input), switch (input).
Diode
A diode is a semi-conductor device. Diodes will allow current to pass through them in only one direction.
Direct current
An electrical current that always moves in one direction.
Direct proportion
When two variables increase or decrease together in the same ratio (at the same rate).
Dispersion
The effect of spreading colors of light into a spectrum with a material that has an index of refraction that varies with wavelength.
Displacement
The change in the position of an object in a particular direction is called displacement. Displacement may also be defined as the shortest distance between the initial and final position of a moving body. It is a vector quantity.
Distance
The actual length of the path traveled by a body irrespective of the direction is called the distance traveled. It is a scalar quantity.
Doppler effect
An apparent shift in the frequency of sound or light due to relative motion between the source of the sound or light and the observer.
Dose Equivalent
This is the biological risk due to exposure to radiation. The SI unit of dose equivalent is the sievert (Sv).
Double Insulation
An electrical appliance which is double insulated does not have an earth wire fitted. The appliance is designed in such a way that the electrical parts can never come into contact with the outer casing of the device. Common double insulated appliances are hair dryers, radios and cassette players.
A wet double insulated appliance is exceptionally dangerous - water is a good conductor of electricity and will easily reach the live electrical components within the case. Any human user touching the casing will then receive an electric shock. It is important to note that the human conductivity is increased when hands are wet, allowing a greater current to flow through the casualty.
Drag
Drag is a type of friction force usually associated with movement through a fluid like air or water. Drag forces generally increase at high speeds.
Driving force
An external force that pumps energy into a vibrating system.
Dynamo
A device (a traducer) which converts kinetic energy into electrical energy. Dynamos are often used to produce a.c. electricity on a push-bike. Unlike a full-blown generator, the rotor coil is replaced with rotating permanent magnets. These magnets rotate near to the stator coil, (wrapped around a soft iron core) and cause a small current to flow (via electromagnetic induction).