CHAPTER 22 PART A

1. CHAPTER 22PART A CURRENT AND RESISTANCE

2. The flow of electrons in a circuit per unit time is called current. I = Q T I = Current- amperes (A) Q = Charge- Coulombs (C) T = time – second (s)

3. The SI unit for current is amperes (A) 1 ampere = 1 Coulomb/second One ampere of current is said to flow if 1 coulomb of charge passes a certain point in one second.

4. electron = 1.6 x 10-19 C • Thus one coulomb of charge has 6.25 x 1018 electrons. • One Ampere of current means the flow of 1 C or 6.25 x 1018 electronsper second.

5. Example 1 The amount of charge that passes through the filament of a certain light bulb in 2 seconds is 1.67 C. A) What is the current in the light bulb? B) What is the number of electrons that pass through the filament in one second?

7. Example 2 1020 electrons flow through a conductor in 3 s. What is the charge flowing through the conductor? What is the current in the circuit?

9. #1 How many electrons pass a point in 5 s if a constant current of 8 A is maintained in a conductor?

11. #2 The amount of charge that passes through the filament of a certain light bulb in 2 seconds is 12C. a) What is the current in the light bulb? b) What is the number of electrons that pass through the filament in one second?

13. #3 How many electrons pass a point in 10 s if a constant current of 20 A is maintained in a conductor?

15. #4 How many amperes of current flow in a wire through which 1018 electrons pass per second?

17. Electric Current A negatively charged object has electrons with potential energy. The difference in potential energy causes the electrons to flow places of higher potential energy to those of lower potential energy.

18. Conventional Current is defined as the movement of positive charges. The direction of conventional current is opposite to the direction in which the electrons move.

19. The difference in potential between two different places is the potential difference.

20. Potential Difference is measured in volts (V). Potential difference is often called voltage. It is measured with a voltmeter.

21. Circuit: is a closed path through which electrons can flow. Electrons in a circuit will continue to flow as long as there is a potential difference.

22. For current to flow we need a voltage source to provide a potential difference. Examples of voltage sources – *dry cells, wet cells, and electric generators

23. In dry cells and wet cells, energy released from a chemical reaction is converted into electrical energy. In electrical generators mechanical energy is converted into electrical energy.

24. In the US the potential difference across the two holes in a wall socket is 120 V. Electrical energy is provided by the electric generator at the power point.

25. A 12 V battery performs 12 J of work on each coulomb of charge transferred from the high potential side (negative) to a low potential side (positive).

26. Ohm’s Law States that the amount of current in the circuit is directly proportional to the voltage impressed across the circuit, and inversely proportional the resistance of the circuit.

27. Ohm’s Law R= V I I = Current (amperes) (A) V= Voltage (volts) (V) R = Resistance (ohms) ()

28. Other Forms Ohm’s Law V = IR and I = V R

29. #1 The headlights of a typical car are powered by a 12 V battery. What is the resistance of the headlights if they draw 3 A of current when turned on?

31. #2 Find the resistance of a portable lantern that uses a 24 V power supply and draws a current of 0.80 A.

33. #3 The current in a resistor of is 0.50 A when connected across a voltage of 120V. What is its resistance?

35. #1 An automobile headlight with a resistance of 30 ohms is placed across a 12 V battery. What is the current through the circuit?

36. #2 A motor with an operating resistance of 32 ohms is connected to a voltage source. The current in the circuit is 3.8 A. What is the voltage of the voltage source?

37. #3 A transistor radio uses .0002 A of current, when a 3V battery operates it. What is the resistance of the radio circuit?

38. #4 What is the resistance of an electric frying pan that draws 12 amperes of current when connected to a 120 V circuit?

39. #5 How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

40. #6 The plate on a certain steam iron states that the iron carries a current of 6.4A when connected to a 120V source. What is the resistance of the steam iron?

41. #7 The resistance of the hot plate is 48 ohms. How much current does the plate carry when connected to a 120 V source?

42. Resistance: is the tendency of a conductor to oppose the flow of current through it. This will result in changing electrical energy into thermal energy and light. Example-Light Bulb

43. Resistance Depends upon: 1. The type of material 2. The thickness of the material 3. The length of the material

44. The greater the thickness of the wire the smaller the resistance. The shorter the wire the smaller the resistance.

45. Copper is an excellent conductor, it has low resistance to the flow of electrons. Copper is used in household wiring because little electrical energy is converted into thermal energy when current passes through it.

46. The resistance of a conductor at a given temperature is directly proportional to its length, and inversely proportional to its cross-sectional area, and dependent on the material from which it is made.

47. Every material has a characteristic resistivity that depends on its electronic structure and temperature. For most materials resistance increases with temperature.

49. #7 What is the resistance of a 20m length of copper wire with a diameter of .8mm? Resistivity = 1.72 x 10-8 m.