2. 27 Current and Resistance 27-1 Moving Charges and Electric Currents electric currents——that is, charges in motion.

3. a.A loop of copper in electrostatic equilibrium. b. This movement of charges is a current i.

4. 27-2 Electric Current The current I through the conductor has the Same value at planes aa`,bb`,cc`.

5. An electric current I in a conductor is defined by: We can fine the charge that passes through the plane in a time interval extending from 0 to t by Integration:

6. The SI unit for current is the coulomb per Second,also called the ampere(A): 1 ampere=1 A=1 coulomb per second=1C/s

7. The Direction of Currents A current arrow is drawn in the direction in which positive charge carriers would move, Even if the actual charge carriers are negative and move in the opposite direction.

8. CHECKPOINT 1 What are the magnitude and direction of the current I in the lower right-hand wire? What are the magnitude and direction of the current I in the lower right-hand wire?

9. Sample Problem 27-1 charge electrons molecules i= per per per electron molecule second Step one: or

10. Step two: molecules molecules moles mass volume per = per per unit per unit per second mole mass volume second Step three:

11. Step four: Step five:

12. 27-3 Current Density

13. Drift Speed

14. Sample Problem 27-2 Step one: Step two:

15. (b) Step one: Step two:

16. Sample Problem 27-3 atoms atoms moles mass n= per unit = per per unit per unit volume mole mass volume Step one:

17. Step two: Step three:

18. Step four:

19. 27-4 Resistance and Resistivity 1 ohm =1Ω ＝ 1 volt per ampere = 1V/A

20. definition of ρ definition of σ

21. Calculating Resistance from Resistivity Resistance is a property of an object. Resistivity is a property of a material.

23. Variation with Temperature

24. Sample Problem 27-4 Step one: Step two: Step three:

25. 27-5 Ohm ’ s Law Ohm’s law is an assertion that the current through a device is always directly proportional to the potential different applied to the device.

26. A conducting device obeys Ohm’s law when the resistance of the device is independent of the magnitude and polarity of the applied difference. A conducting material obeys Ohm’s law when the resistivity of the material is independent of the magnitude and direction of the applied electric field.

27. 27-6 A Microscopic View of Ohm ’ s Law

29. Sample Problem 27-5 Step one:

31. Step two:

32. 27-7 Power in Electric Circuits

33. or

34. Sample Problem 27-6 Step one:

35. Step two:

36. 27-8 Semiconductors

37. 27-9 Superconductors Superconductors are materials that lose all electrical resistance at low temperatures.