Current and Resistance Chapter 3 Current and Resistance 1. Electric Current 2. Resistance and Ohm’s Law 3. Electrical Energy and electric Power

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3-1 Electric Current DC AC

1- Electric Current Consider a system of electric charges in motion, a current is said to exist. The current is the rate of charge flows certain time t. Where Q=ne

Ex The electric current when an electric charge of 5x10-6 C passes an area each 10-3 sec is: Ex In a particular cathode ray tube, the measured beam current is 30x10-6 A . The number of electrons strikes the tube screen every 40 sec

2- Resistivity and Ohms' law Consider a conductor of cross-sectional area A and length L carrying a current I. A/m2 Ohm’s law Where  is the resistivity المقاومة النوعية

Other Important relation is Ohm`s law Ex :The ratio of an electric potential across a resistor to the passing current is Ex : An aluminum wire having a cross sectional area of A=4 x10-6 m2 carries a current of I= 5 x10-6 A, the current density J is

Calculate the resistance R of an cupper wire that has a length of L=10 m and Cross-section area A= 2x10-4 m2 where the resistivity  = 1.68 x10-8 .m

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EX: An electric heater is constructed by applying a potential difference of 120 V to a Ni-chrome wire that has a total resistance of 8 Ω . Find the current carried by the wire and the power rating of the heater.

Energy E= P t = 330 x (7x24h) = 55440 watt .h = 55.44 k watt .h Power P = IV=220 x1.5 = 330 watt Energy E= P t = 330 x (7x24h) = 55440 watt .h = 55.44 k watt .h Cost = Energy x price = 55.44x 0.05 = 2.772 SR EX Estimate the cost of cooking a turkey for 4 h in an oven that operates continuously at I= 20 A and V= 240 V. Cost=1.54 SR

EX If the cost of electricity in SA is 0.05 SR/kW-h , then the cost of leaving a 60 W light lamp ON for 14 days is Total Power= 60 x14x24 = 20160 W= 2.016 kW The cost = 2.016x0.05= 1.008 SR Ex An electric heater is operated with a potential difference of 110 V to a Tungsten Wire that has a resistance of 10  . The power of the heaters:

Energy dissipation E= P t k watt .h Cost = Energy x price Ex An electric heater is operated with a potential difference of 110 V to a Tungsten Wire that has a resistance of 10  . The power of the heaters: EX If the cost of electricity in SA is 0.05 SR/kW-h , then the cost of leaving a 60 W light lamp ON for 14 days is