# UNIVERSITY PHYSICS : Definitions , Derivations , Equations , Concepts

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
• AcceptorA 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.
• ActinometerInstrument 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 processAny 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 :  pvγ = K , here p and v are the pressure and volume of an ideal gas respectively, γ is the ratio of the principal specific heat capacities of the gas and  K is a constant.
• Alpha particlerefers 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 ( I0 ) or its root mean square value ( I0 /√2 ). The instantaneous value of an ac current , I = I0 sin 2π f t , where 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 f t ) – ( ½  2π f t ) , when the resistance , inductance  and the capacitance of a circuit are in series.
• AmmeterAn 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 , μ0 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 = ( μ0 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 .