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

3. V V V  R

4. 3-1 Electric Current DC AC

5. 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

6. 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

8. 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

9. 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

10. 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  = x10-8 .m

11. 3-

12. 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.

13. 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) = watt .h = k watt .h
Cost = Energy x price = 55.44x 0.05 = 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

14. EX
If the cost of electricity in SA is SR/kW-h , then the cost of leaving a 60 W light lamp ON for 14 days is
Total Power= 60 x14x24 = W= kW
The cost = 2.016x0.05= 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:

15. Energy dissipation E= P t k watt .h Cost = Energy x priceEx
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 SR/kW-h , then the cost of leaving a 60 W light lamp ON for 14 days is