Electric Spark
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A type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air. This causes an increase in the number of free electrons and ions in the air, temporarily causing the air to become an electrical conductor through dielectric breakdown.
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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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Gravitational Lensing
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A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is 'bent' around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein's general theory of relativity.
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Magnetic Field
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A vector field which surrounds magnets and electric currents, and is detected by the force it exerts on moving electric charges and on magnetic materials. When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field. Magnetic fields also have their own energy with an energy density proportional to the square of the field intensity.
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Magnetism
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One of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.
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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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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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Reflection
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The change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves.
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For example, by the use of a curved mirror.
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Refraction
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The change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth.
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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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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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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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