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Unformatted text preview: There must be at least one source or generator of electrical energy in an electric circuit . The electromotive force (emf) of a generator, such as a battery, is the maximum potential difference (in volts) that exists between the terminals of the generator. The rate of flow of charge is called the electric current. If the rate is constant, the current I is given by I = ( ∆ q )/( ∆ t ), where ∆ q is the magnitude of the charge crossing a surface in a time ∆ t , the surface being perpendicular to the motion of the charge. The SI unit for current is the coulomb per second (C/s), which is referred to as an ampere (A). When the charges flow only in one direction around a circuit , the current is called direct current (dc). When the direction of charge flow changes from moment to moment, the current is known as alternating current (ac). The definition of electrical resistance is R = V/I , where V is the voltage applied across a piece of material and I is the current through the material. If the ratio V / I is constant for all values of V and I , the relation R = V / I or V = IR is referred to as Ohm's law. Resistance is measured in volts per ampere, a unit called an ohm ( Ω ). The resistance of a piece of material of length L and cross-sectional area A is R = ρ L / A , where ρ is the resistivity of the material. The resistivity of a material depends on the temperature. For many materials and limited temperature....
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