** A**

**Acceleration**[ a ] – The rate of increase of speed or velocity. For a body moving linearly with constant acceleration a with initial velocity u and final velocity**v , a = ( v – u ) /t = ( v² – u²) / 2s**. For non -constant acceleration**dv/dt = d² s/d t²**. If the movement is not linear , the vector changes of displacement , velocity and acceleration are to be taken into account. Measured in m/s²**Acceleration of free fall**[ g ] – Also called the acceleration due to gravity . The acceleration experienced by a body falling freely in the gravitational feild of earth. It’s not related to the nature of the body , provided air – resistance is not considered. Due to the earth’s rotation and non -spherical nature , g varies with latitude . Value of g is ≈ 9.8 m/s²**Acceptor**– A substance used as an impurity in a semi-conductor for it’s ability as an electron acceptor . It causes p type conduction by means of mobile positive holes.**Actinometer**– Instrument used to measure the intensity of Electromagnetic radiation . For this it uses the photoelectric effect .**Action spectrum**– A graphical plot denoting the effeciency of electromagnetic radiation in producing the photochemical reaction against the wavelength of the radiation used .**Adiabatic process**– Any process in a system , where heat doesn’t enter or leave it . Such a process is usually characterized by the rise or fall of temperature . Adiabatic equation :*pv*^{γ}= K**Alpha particle**– refers to the helium-4 nucleus emmited by a heavier nucleus during alpha decay. As it consists of two protons and two neutrons , alpha particle emission reduces nucleon no by 4 and atomic no by 2.**Alternating current ( a.c )**– refers to an electric current which reverses its direction with a constant frequency . If the related current / time graph has a sine wave , it’s a sinusoidal current. In such a case , it’s magnitude is either given as its peak value ( I_{0}) or its root mean square value ( I_{0}/√2 ). The instantaneous value of an ac current ,**I = I**, where_{0}sin 2π*f*t*f*is the current frequency. In a circuit an ac current is opposed both by the circuits resistance as well as its reactance. Resistance ,**( X ) = ( 2π**, when the resistance , inductance and the capacitance of a circuit are in series.*f*t ) – ( ½ 2π*f*t )**Ammeter**– An instrument to measure electric current . 3 types – moving-iron , moving- coil and thermoammeter. Of these , moving – coil ammeter can be used only for a dc current .**Ampere**[ A ] – S.I unit of electric current . Defined as the constant current , which when maintained in two straight parallel infinite conductors of negligible cross section placed one metre apart in a vacuum, would produce a force between the conductors of 2x 10^{-7}N/m .**Ampere-Laplace [ Ampere’s ] Law**– The law which defines the magnitude of Magnetic flux caused by a charge carrying conductor. If dB – infinitesimal element of the magnitude of the magnetic flux density at a distance r at a point P from the element dl of a conductor , μ_{0}is the magnetic permeability of free space , I is the current carried by the conductor, and θ is the angle between the direction of the electric current and the line joining the element of the conductor and P , then**dB = ( μ**._{0}I sinθ dl ) / 4 π r²**Ampere’s rule**– The rule which relates the direction of the electric current passing through a conductor and the resulting magnetic feild. It states that , when an electric current is moving away from an observer , the direction of the lines of force of the magnetic feild surrounding the conductor is clockwise and if the electric current is moving towards the observer , the direction of the lines of force are counter-clockwise . Also referred to as the corkscrew rule .**Amplitude**– One of the important characters of a wave motion , defined as the maximum difference of the disturbed quantity from its mean value. Its usually half the peak to peak value .