14. Current of Electricity

Current of Electricity Electric current, I, is the rate of flow of charged particles, which may be positively or negatively charged, with respect to time, through a cross-sectional area. The electric charge Q passing a point in a circuit is the product of the electric current I in the circuit and the interval of time t during which the electric current is passing that point. t QI Microscopic model of current If n is the number of mobile charge carriers per unit volume (i.e. number density), and q is the charge of each charge carrier, then the total amount of charge for a particular section of a conductor is Q = q(nAx) Suppose that the charge carriers move with an average speed vd , then the displacement in a time interval t will be x = vd t , which implies the average current is t tqnAv t Q dI dqnAvI The drift speed vd is the average speed at which charge carriers move through a conductor when there is an electric current in the conductor. The potential difference (p.d.), V, between two points in a circuit is the amount of energy per unit charg e transferred from electrical energy to other forms of energy when charge passes through those two points. WV Q

Electrical r esistance R, is the ratio of potential difference across a component to the current through it. I VR Resistivity is a relationship between the dimensions of a specimen of a material and its resistance at constant temperature. For a sample of length L, with a uniform cross-sectional area A and resistance R, the resistivity ρ is RA L  I-V Characteristics Resistor at constant temperature (Ohmic resistor) Filament Lamp I-V characteristic of a filament lamp v I I-V characteristic of an ohmic resistor 0

Semiconductor diode I-V characteristic of a semiconductor diode Negative Temperature Coefficient (NTC) Thermistor I-V characteristic of a NTC thermistor Electrical Power Electric power is the rate at which electrical energy is supplied to a circuit or consumed by a load with respect to time. IVP R VRIP 2 2 