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 Physics Definitions Alpha Decay (α Decay) : The phenomenon of emission of α-particle from a radioactive nucleus is called α Decay. Atmosphere : The gaseous envelope around the earth is called atmosphere. Atomic Nucleus : The central core of the atom which contains all of the atom's positive charge and most of its mass is known as atomic nucleus. Atomic Number : The number of protons in a nucleus (or nuclide) is called the atomic number and is represented by the symbol Z. In a natural atom, the number of electrons obtain the nucleus is equal to the atomic number Z. Activity : The rate of disintegration (i.e. number of disintegrations per second) in a radioactive substance is known as its activity i.e. Activity of a radioactive substance, A = - dN / dt. Artificial Radioactivity : The phenomenon of making a stable nucleus radioactive by artificial means is known as artificial radioactivity. Amorphous Solids : Solids whose constituent particles are not arranged in an orderly fashion are called amorphous solids. e.g. glass, wax etc. Analog Signal : A continuously varying signal (voltage or current) is called an analog signal. AND Gate : The AND gate is a logic gate that has two or more inputs but only one output. However, the output Y of an AND gates is HIGH when all inputs are HIGH. The output Y of an AND gate is LOW if any or all the inputs are LOW. Attenuation : The loss of strength of the signal while propagating through the medium is known as attenuation. Amplification : The process of raising the strength of a signal is called amplification and is done by an electronic circuit called amplifier. Antenna : An antenna or aerial is a system of elevated conductors which couples the transmitter or the receiver to free space. Amplitude Modulation (AM) : When the amplitude of high frequency carrier wave is changed in accordance with the intensity of the signal, it is called amplitude modulation (AM).