Chapter 3 Current and Resistance 1 Electric Current

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, current is said to exist. The current is the rate of charge flows certain time t. Where Q=ne a

Ex The electric current when an electric charge of 5 x 10 -6 C passes an area each 10 -3 sec is: Ex In a particular cathode ray tube, the measured beam current is 30 x 10 -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/m 2 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 x 10 -6 m 2 carries a current of I= 5 x 10 -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= 2 x 10 -4 m 2 where the resistivity = 1. 68 x 10 -8 . m

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 1. the current carried by the wire and 2. the power rating of the heater.

Power P = IV=220 x 1. 5 = 330 watt Energy E= P t = 330 x (7 x 24 h) = 55440 watt. h = 55. 44 k watt. h Cost = Energy x price = 55. 44 x 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/k. W-h , then the cost of leaving a 60 W light lamp ON for 14 days is Total Power= 60 x 14 x 24 = 20160 W= 2. 016 k. W The cost = 2. 016 x 0. 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/k. W-h , then the cost of leaving a 60 W light lamp ON for 14 days is