Aerosol
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A suspension of fine solid particles or liquid droplets in a gas.
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Angular Momentum
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Angular momentum is the rotational counterpart of momentum. A spinning flywheel has angular momentum.
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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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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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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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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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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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For example, Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying).
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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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Electret
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An Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.
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An electret can be considered as a permanent or semi-permanent accumulation of electric charge.
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Electrostatic Induction
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A redistribution of electrical charge in an object, caused by the influence of nearby charges. Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. Electrostatic induction should not be confused with electromagnetic induction; both are often referred to as 'induction'.
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Electrostatics
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The phenomena arising from stationary or slowly moving electric charges
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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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Focusing
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The gathering of wavefronts of a wave (e.g. radiation) into a spherical or cylindical shape. Focusing of light is used in optics, however focusing can be applied to any radiation or wave.
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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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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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Interference
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The addition (superposition) of two or more waves that result in a new wave pattern. As most commonly used, the term interference usually refers to the interaction of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.
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By constructive interference.
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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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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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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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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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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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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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Plasma
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A partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma therefore has properties quite unlike those of solids, liquids or gases and is considered to be a distinct state of matter.
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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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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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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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Surface Acoustic Wave
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(SAW) An acoustic wave traveling along the surface of a material having some elasticity, with an amplitude that typically decays exponentially with the depth of the substrate. This kind of wave is commonly used in devices called SAW devices in electronics circuits.
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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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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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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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Triboelectric Effect
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A type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.
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Viscous Heating
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The work done by a fluid on adjacent layers due to the action of shear forces is transformed into heat.
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