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Glossary

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Edge Dislocation

One of the simple dislocations that can occur in a metallic grain. In effect it is an extra half plane of atoms slid into the crystalline structure.

Edge Dislocation

Edge dislocation with extra half plane highlighted

(n.b. even though the extra half plane is highlighted the atoms in it are identical to their neighbours)

Elastic Deformation

The temporary deformation of a material when subjected to a stress. When the stress is removed the material returns to its original size. If the elastic limit is exceeded then the deformation becomes permanent.

Deformation Graph

Stress - strain diagram for a ductile material

Elastic Limit

The point beyond which, if a material is stressed, some of the strain becomes permanent. On a stress - strain diagram it can be very hard to determine the precise location of the point. It will however lie somewhere between the limit of proportionality and the yield point for a ductile material. With a purely brittle material (such as glass) the elastic limit and the breaking stress lay at the same point as there can be no plastic deformation.

Electrolyte

A substance that is either molten or a solution and can carry a current. Importantly the current is carried by ions rather than electrons. Increasingly electrolytic gels are being used as these are not prone to spillage if handled roughly.

Electromagnetic Levitation

Frequently abbreviated to EML. A rapidly changing current in a set of coils produces a magnetic field which in turn produces an opposing current and magnetic field in the sample. The result is that the sample hovers inside the coils. The current can also be used to heat and ultimately melt the sample. The main reason for using EML is that the sample is held in either a neutral atmosphere or a vacuum and never comes into contact with a container wall and so is not contaminated. Since the technique relies on a current being induced in the sample it can only be used with conductors, typically metals. A related method, ‘electrostatic levitation’ can be used for insulators which relies on placing an electrical charge on the sample and suspending it in an electric field.

Electromagnetic Levitation Equipment

Electromagnetic levitation coil, supporting a molten metal sample.

(This single coil is for use in a terrestrial laboratory. In microgravity separate coils are used for heating and levitation and the arrangement is symmetrical)

Electronegativity

The tendency of an atom to attract electrons. The higher the electronegativity the more strongly are electrons attracted.

Electrostatic Levitation

See Electromagnetic Levitation

Emissivity

A dimensionless number that defines how far an object deviates from being a perfect blackbody radiator. Emissivity values can in theory vary from zero (never radiates any energy) to one (a perfect blackbody radiator). In practice many rough materials (both natural and manmade) have an emissivity of approximately 0.95 although polished surfaces are much lower.

For many practical situations it is common to assume that the emissivity of an object is the same for all wavelengths of radiation (a so called 'grey body'). In fact the emissivity varies with wavelength, temperature and viewing angle. A very low emissivity at a particular wavelength can be due either to the object being transparent to the radiation or highly reflective. Smoke for example is highly transparent to a wide range of infrared wavelengths and so fire fighters can use IR imagers to rescue victims in a smoke filled room. A multilayer coated filter on the other hand can be highly reflective to a very narrow waveband, determined by the layer thickness.

For a given wavelength, temperature and viewing angle any material or object is always exactly as good an emitter as it is an absorber. Equally an excellent reflector (or transmitter) of a particular wavelength must also be a very poor emitter at that wavelength.

Endothermic

A chemical or physical process that absorbs energy (normally heat energy) from its surroundings.

Enthalpy of Combustion

The energy released when a chemical is burned in excess oxygen under standard conditions. Usually the figure quoted is the ‘standard molar enthalpy change of combustion’ which is for one mole of the chemical.

Eutectic Composition

The precise composition of a solid solution of two (or more) substances that has the lowest possible melting point.

Eutectic Point

The position on the phase diagram for a mixture of two (or more) that corresponds to the melting point of the eutectic mixture.

Phase Diagram for Pb - Sn

Phase diagram for Pb - Sn showing the eutectic point and composition.

Exothermic

A chemical or physical process that releases energy (normally heat energy) into its surroundings.

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