Absorption (EM radiation)
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The process by which the energy of a photon is taken up by matter, typically the electrons of an atom. The electromagnetic energy is transformed to another form of energy, e.g. to heat.
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Absorptive Filter
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A filter that absorbes some wavelengths of incident radiation whilst transmitting others.
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Acoustic Absorption
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The process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body.
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Acousto-optic Effect
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A specific case of photoelasticity, where there is a change of a material's permittivity due to a mechanical strain resulting from an acoustic wave which has been excited within a transparent medium giving rise to variation of the refractive index. This creates a diffraction grating moving with the velocity given by the spread on the sound wave in the medium which defracts light forming a prominent diffraction pattern.
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Through Bragg cell deflection into an absorbent region
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Aerosol
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A suspension of fine solid particles or liquid droplets in a gas.
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For example, an aerosol of liquid droplets in air can be used to absorb or attenuate radiation, such as light.
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Angular Momentum Conservation
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In a closed system angular momentum is constant. The conservation of angular momentum explains the angular acceleration of an ice skater as she brings her arms and legs close to the vertical axis of rotation. By bringing part of mass of her body closer to the axis she decreases her body's moment of inertia. Because angular momentum is constant the angular velocity (rotational speed) of the skater has to increase.
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Auxetic Materials
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Materials which, when stretched, become thicker perpendicularly to the applied force, i.e. they have a negative Poisson's ratio. Such materials have interesting mechanical properties such as high energy absorption and fracture resistance. This may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material and sponge mops.
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Though the use of energy to 'power' the auxetic change.
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Biot-Savart Effect
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The generation of a magnetic field by a steady current i.e. an unchanging continual flow of charges (for example through a wire).
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Boiling
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A type of phase transition. The rapid vaporisation of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure.
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The phase change associated with boiling requires the use of energy.
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Brewster's Angle
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Brewster's angle (also known as the polarisation angle) is an angle of incidence at which light with a particular polarisation is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarised light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarised.
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For example, polarised sunglasses use the principle of Brewster's angle to reduce glare from the sun reflecting off horizontal surfaces such as water or road.
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Bridgman Effect
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(or internal Peltier effect) A phenomenon that occurs when an electric current passes through an anisotropic crystal - there is an absorption or liberation of heat because of the non-uniformity in current distribution
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Capacitance
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The property of a device or material medium to store an electric charge as a result of an electric potential. The most common form of charge storage device is a two-plate capacitor.
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Chemical Bonding
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The physical process responsible for the attractive interactions between atoms and molecules, and which confers stability to diatomic and polyatomic chemical compounds. In general, chemical bonding is associated with the sharing of electrons between the participating atoms. Molecules, crystals, and diatomic gases and most of the physical environment around us are held together by chemical bonds.
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The forming of chemical bonds generally requires energy.
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Christiansen Effect
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A narrow bandpass or monochromatic optical filter which consists of an optical cell which is stuffed with a crushed substance (e.g. glass) and a (mostly organic) liquid.
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Compression
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The result of the subjection of a material to compressive stress, resulting in reduction of volume. The opposite of compression is tension. In simple terms, compression is a pushing force.
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Converse Piezoelectric Effect
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Materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.
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Corona Discharge
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An electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient (the strength of the electric field) exceeds a certain value, but conditions are insufficient to cause complete electrical breakdown or arcing.
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Curie Point (ferromagnetic)
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The Curie point of a ferromagnetic material is the temperature above which it loses its characteristic ferromagnetic ability
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Energy is required for the state change at the curie point.
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Curie Point (piezoelectric)
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The Curie point of a piezoelectric material is the temperature above which the material loses its spontaneous polarisation and piezoelectric characteristics
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Energy is required for the state change at the curie point.
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Damping
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Damping is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of an oscillatory system.
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Deformation
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A change in the shape or size of an object due to an applied force. This can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described in terms of strain.
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Energy is required to deform an object.
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Dellinger Effect
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A fadeout of short-wave radios, caused by increased ionization of the D region of the ionosphere due to solar flares.
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Dielectric Permittivity
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The measure of how much resistance is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. Permittivity is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material. Thus, permittivity relates to a material's ability to transmit (or 'permit') an electric field.
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In the case of an electric field.
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Dispersion (of waves)
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In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Media having such a property are termed dispersive media. Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g. a waveguide), such as sound waves.
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Drag
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A drag (sometimes called fluid resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). The most familiar form of drag is made up of friction forces, which act parallel to the object's surface, plus pressure forces, which act in a direction perpendicular to the object's surface.
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Echo
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A reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound.
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Eddy Currents
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An electrical phenomenon caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause eddys of circulating current within the conductor which create electromagnets with magnetic fields that opposes the change of the magnetic field.
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Eddy Currents produce a magnetic field opposite to that inducing the eddy current.
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Elasticity
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The physical property of a material when it deforms under stress (e.g. external forces), but returns to its original shape when the stress is removed. The amount of deformation is called the strain. The elastic regime is characterized by a linear relationship between stress and strain, denoted linear elasticity.
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Electrical Resistance
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The degree to which an object opposes an electric current flowing through it (usually measured in Ohms). A function of both its physical geometry and the resistivity of the material the object is made from.
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Electro-Optic Effects
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A change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light. The term encompasses a number of distinct phenomena, which can be subdivided into a) change of the absorption (electroabsorption, Franz-Keldysh effect, Quantum-confined Stark effect, electro-chromatic effect) and b) change of the refractive index(Pockels effect, Kerr Effect, electro-gyration)
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Electrochromism
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A type of electro-optic effect. The creation of an absorption band at some wavelengths in a material in response to an electric field, which gives rise to a change in colour (i.e. the phenomenon displayed by some chemical species of reversibly changing color when a burst of charge is applied).
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Electrohydrodynamics
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Includes the following types of particle and fluid transport mechanisms:Electrophoresis, electrokinesis, dielectrophoresis, electro-osmosis, and electrorotation. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy, and vice versa.
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Electrolysis
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A method of separating chemically bonded elements and compounds by passing an electric current through them. Electrolysis involves the passage of an electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes.
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Electromagnetic Induction
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The generation of electromotive force (EMF) in a current-carrying conductor exposed to a changing magnetic field
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Eddy Currents produce a magnetic field opposite to that inducing the eddy current.
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Electrorheological Effect
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Electrorheological (ER) fluids are suspensions of extremely fine non-conducting particles (up to 50 micrometres diameter) in an electrically insulating fluid. The apparent viscosity of ER fluids changes reversibly by an order of up to 100,000 in response to an electric field. A typical ER fluid can go from the consistency of a liquid to that of a gel, and back, with response times of the order of ms. Sometimes called the Winslow Effect.
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It has been proposed to create bullet-proof vests using an ER fluid because the ability to soak the fluid into cloth creates the potential for a very light vest that can change from a normal cloth into a hard covering almost instantaneously
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Electrostriction
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A property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. All dielectrics exhibit some electrostriction, but certain engineered ceramics, known as relaxor ferroelectrics, have extraordinarily high electrostrictive constants,e.g. Lead magnesium niobate (PMN), Lead magnesium niobate-lead titanate (PMN-PT), Lead lanthanum zirconate titanate (PLZT).
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Electroviscous Effect
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Change in a liquid's viscosity induced by a strong electrostatic field.
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Endothermic Reaction
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A process or reaction that absorbs energy typically (but not always) in the form of heat. The concept is frequently applied in physical sciences to e.g. chemical reactions, where thermal energy (heat) is converted to chemical bond energy.
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By using the energy of the field to enable an endothermic reation.
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Faraday Wave
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(or Faraday Ripples) Nonlinear standing waves that appear on liquids enclosed by a vibrating receptacle. When the vibration frequency exceeds a critical value, the flat hydrostatic surface becomes unstable. The waves can take the form of stripes, close-packed hexagons, or even squares or quasiperiodic patterns.
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Fatigue
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The progressive and localised structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.
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Fluorescence
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Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer (less energetic) wavelength. The energy difference between the absorbed and emitted photons ends up as molecular rotations, vibrations or heat. Sometimes the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range.
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Franz-Keldysh Effect
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A change in optical absorption by a semiconductor when an electric field is applied. Used to create electroabsorption modulators.
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An electroabsorption modulator can be used for modulating the intensity of a laser beam via an electric voltage. Its principle of operation is based on the Franz-Keldysh effect.
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Friction
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The force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid (e.g. air on an aircraft or water in a pipe).
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Harmonic Oscillator
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A system which, when displaced from its equilibrium position, experiences a restoring force proportional to the displacement. Mechanical examples include pendula (with small angles of displacement) and masses connected to springs.
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Heating
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The act of increasing temperature.
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Hooke's Law
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Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit. Materials for which Hooke's law is a useful approximation are known as linear-elastic or 'Hookean' materials.
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Joule Heating
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The process by which the passage of an electric current through a conductor releases heat. Heat generated is a function of current, resistance and time. Also known as the Joule-Lenz effect, Ohmic heating or resitive heating.
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Latent Heat
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Energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process. An example is a state of matter change, meaning a phase transition, such as ice melting or water boiling.
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For example, using ice to absorb the heat from a drink.
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Lever
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A rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object.
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Liquid Crystals
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Substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal.
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Liquid crystals are use to absorb (attenuate) light.
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Lorentz Force
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The force on a point charge due to electromagnetic fields. When a wire carrying an electrical current is placed in a magnetic field, each of the moving charges, which comprise the current, experiences the Lorentz force, and together they can create a macroscopic force on the wire (sometimes called the Laplace force).
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Magnetic Reluctance
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Analogous to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance.
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Magnetic Saturation
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Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetising field cannot increase the magnetisation of the material further, so the total magnetic field levels off. It is a characteristic particularly of ferromagnetic materials, such as iron, nickel, cobalt and their alloys.
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Magnetocaloric Effect
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(or Magnetothermal Effect) A magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to a changing magnetic field. Also known as adiabatic demagnetisation. Can be used to attain extremely low temperatures (well below 1 K), as well as the ranges used in common refrigerators.
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Mechanical Advantage
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Force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force.
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Mechanical Force
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A mechanical influence that causes a free body to undergo an acceleration.
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Melting
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A process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased (typically by the absorbtion of heat) to a specific temperature (called the melting point) at which it changes to the liquid phase.
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Metastability
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A general scientific concept which describes states of delicate equilibrium. A system is in a metastable state when it is in equilibrium (not changing with time) but is susceptible to fall into lower-energy states with only slight interaction. It is analogous to being at the bottom of a small valley when there is a deeper valley close by
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Mirage (photothermal deflection)
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A naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky.
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Moment of Inertia
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(or mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass). The inertia of a rotating body with respect to its rotation. The moment of inertia plays much the same role in rotational dynamics as mass does in linear dynamics,
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Non-Newtonian Fluids
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Fluids whose flow properties are not described by a single constant value of viscosity. In a non-Newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity can not be defined. A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this being more useful for fluids without time-dependent behavior.
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For example, mechanical force, in the case of body armour.
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Peltier Effect
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Heat is evolved or absorbed at the junction of two dissimilar metals carrying a small current, depending upon the direction of the current. One of a number of thermoelectric effects (See Seebeck Effect and Thompson Effect).
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Either by absorbing a thermal field, or by absorbing the electrical field used to power the Peltier effect.
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Phase Change
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The transformation of a thermodynamic system from one phase to another. Most commonly used to describe transitions between solid, liquid and gaseous states of matter, in rare cases including plasma.
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Phononic Crystal
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A phononic crystal is a material which exhibits stop bands for phonons, preventing phonons of selected ranges of frequencies from being transmitted through the material.
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Phosphorescence
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A specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs.
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Photoacoustic Effect
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The formation of sound waves, following light absorption in a material sample. Most generally, electromagnetic radiation of any kind can give rise to a photoacoustic effect. This includes the whole range of electromagnetic frequencies, from gamma radiation and x-rays to microwave and radio.
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Photochromism
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The reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra. Trivially, this can be described as a reversible change of colour upon exposure to light.
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Photoconductivity
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An optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.
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Photodissociation
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A chemical reaction in which a chemical compound is broken down by photons. Any photon with sufficient energy can affect the chemical bonds of a chemical compound. Since a photon's energy is inversely proportional to its wavelength, electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, x-rays and gamma rays are usually involved in such reactions.
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Photoelectric Effect
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The emission of electrons from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light.
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Photoionisation
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The physical process in which an incident photon ejects one or more electrons from an atom, ion or molecule. This is essentially the same process that occurs with the photoelectric effect with metals. In the case of a gas, the term photoionisation is more common.
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Photoluminescence
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A process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. One of many forms of luminescence (light emission) and is distinguished by photoexcitation (excitation by photons).
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Photophoresis
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The phenomenon that small particles suspended in gas (aerosols) or liquids (hydrocolloids) starts to migrate when illuminated by a sufficiently intense beam of light. The existence of this phenomenon is owed to a non-uniform distribution of temperature of an illuminated particle in a fluid medium.
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Photovoltaic Effect
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The creation of a voltage (or a corresponding electric current) in a material upon exposure to light. Though directly related to the photoelectric effect, the two processes are different and should be distinguished. In the photoelectric effect, electrons are ejected from a material's surface upon exposure to radiation of sufficient energy. The photovoltaic effect is different in that the generated electrons are transferred between different bands (i.e. from the valence to conduction bands) within the material, resulting in the buildup of a voltage between two electrodes.
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Piezoelectric Effect
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Generation of electrical charge in certain solid materials (notably crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical strain. The piezoelectric effect is a reversible process in that materials also exhibit the reverse piezoelectric effect (the internal generation of a mechanical force resulting from an applied electrical field).
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Pockels Effect
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Produces birefringence in an optical medium induced by a constant or varying electric field. Used to make Pockels cells, which are voltage-controlled wave plates.
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Poisson's Effect
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When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression.
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Pyroelectric Effect
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The ability of certain materials to generate an electrical potential when they are heated or cooled. As a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence, an electrical potential is established.
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Rayleigh Scattering
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The elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light.
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Reaction (physics)
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In classical mechanics, Newton's third law states that forces occur in pairs: the Action and the Reaction. Both forces are equal in magnitude and opposite in direction. The distinction between action and reaction is purely arbitrary: any one of the two forces can be considered an action, in which case the other (corresponding) force automatically becomes its associated reaction.
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Resonance
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The tendency of a system to oscillate with larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.
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Reverberation
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The persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air.This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they can no longer be heard.
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Rheopecty
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(or rheopexy) The rare property of some non-Newtonian fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shearing force, the higher its viscosity. Rheopectic fluids, such as some lubricants, thicken or solidify when shaken (the opposite type of behaviour, in which fluids become less viscous the longer they undergo shear, is called thixotropy and is much more common).
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For example, mechanical force, in the case of body armour.
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Scattering
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A general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass (includes deviation of reflected radiation from the angle predicted by the law of reflection).
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Shadow
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An area where direct light (or other radiation) from a source cannot reach due to obstruction by an object. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the light (or radiation).
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Shear Thickening
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(or Dilitant) The property of a material for which viscosity increases with the rate of shear. Such a shear thickening fluid is an example of a non-Newtonian fluid.
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For example, mechanical force, in the case of body armour.
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Skin Effect
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The tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the 'skin' of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.
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Static Friction
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(or Stiction) Two solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Static Friction is a threshold, not a continuous force.
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Stick-slip Phenomenon
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The spontaneous jerking motion that can occur while two objects are sliding over each other.
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Stress Relaxation
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The decrease in stress with time in a viscoelatic material held at constant strain or deformation.
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Sublimation
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The change of the physical state of aggregation (or simply state) of matter directly from solid phase to gaseous phase without passing through an intermediate liquid phase.
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Superconductivity
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An electrical resistance of exactly zero which occurs in certain materials below a characteristic temperature. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is also characterized by a phenomenon called the Meissner effect, the ejection of any sufficiently weak magnetic field from the interior of the superconductor as it transitions into the superconducting state.
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Tension
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The pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression.
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Terminal Velocity
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A free falling object achieves its terminal velocity when the downward force of gravity equals the upward force of drag. This causes the net force on the object to be zero, resulting in an acceleration of zero. Mathematically an object asymptotically approaches and can never reach its terminal velocity.
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Thermal Expansion
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The tendency of matter to change in volume in response to a change in temperature or when a substance is heated.
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Thermoacoustic Effect
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Acoustic waves can be used for heat transfer. The pressure variations in the acoustic wave are accompanied by temperature variations due to compressions and expansions of the gas which, being inherantly associated with a displacement, results in a net transfer of heat. Direction of heat flow can be fixed by, for example, a standing wave pattern generated by an acoustic resonator.
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Thermochromism
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The ability of substance to change color due to a change in temperature. Thermochromism is one of several types of chromism. The two basic approaches are based on liquid crystals and leuco dyes.
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Thermoluminescence
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A form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon heating of the material. This phenomenon is distinct from that of black body radiation.
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Thermolysis
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(or Thermal decomposition) A chemical decomposition caused by heat. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion.
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Thermophoresis
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(also called thermodiffusion or Soret Effect) The effect of temperature gradient on multicomponent (or isotopic) mixtures of particles (i.e. particle movement from hotter to colder regions or vice versa). Regarded as 'positive' when molecules move from hot to cold and 'negative' when the reverse is true. Typically the heavier/larger species in a mixture exhibits positive behavior while the lighter/smaller species exhibit negative.
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Thompson Effect
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The heating or cooling of a current-carrying conductor with a temperature gradient Depending on the material it may cause heating or cooling. One of a number of thermoelectirc effects (see Seebeck effect and Peltier Effect).
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Total Internal Reflection
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An optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.
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Velocity Ratio
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In a machine, the ratio between the distance moved by the point of effort to that of the point of load.
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Viscoelasticity
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The property of exhibiting both viscous and elastic characteristics when undergoing deformation.
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Wear
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The erosion of material from a solid surface by the action of another surface.
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