1. CH. 23 ELECTRIC CURRENT

2. Flow of Charge  When the ends of an electrical conductor are at different electric potentials—potential difference or VOLTAGE—charge flows from one end to the other.  Analogous to water flowing from higher pressure to lower pressure  Or think of heat flowing from Hot to Cold!

3. Flow of Charge  In time, the potential will even out and it will reach a common potential (think of equilibrium)  In order to keep the flow, a potential difference must be maintained!  Think of a water pump:

4. Electric Current Electric current  Flow of charged particles (think of H 2 0 molecules moving with a flow of water)  In metal wires Conduction electrons are charge carriers that are free to move. Protons are bound within the nuclei of atoms.  In fluids Positive ions and electrons can flow.

5. Electric Current  Rate of electric flow: - Measured in amperes (usually amps) - One amp = 1 coulomb/second  Speed of electrons (drift speed) is SLOW! (They are constantly bumping into each other)  Remember: Charge flows through a circuit; Voltage is established across a circuit.

6. Which of these statements is true? A. Electric current is a flow of electric charge. B. Electric current is stored in batteries. C. Both A and B are true. D. Neither A nor B are true. Electric Current CHECK YOUR NEIGHBOR

7. Which of these statements is true? A.Electric current is a flow of electric charge. B. Electric current is stored in batteries. C. Both A and B are true. D. Neither A nor B are true. Explanation: Voltage, not current, is stored in batteries. The voltage will produce a current in a connecting circuit. The battery moves electrons already in the wire, but not necessarily those in the battery. Electric Current CHECK YOUR ANSWER

8. Voltage Sources  Requires some sort of conductor (wires, etc.) Electric potential difference:  Difference in potential between two points  Charges in conductor flow from higher potential to lower potential.  Flow of charge persists until both ends of conductor reach the same potential.  Maintained for continuous flow by pumping device.  No charge will flow if potential difference is Zero!  What happens when you discharge a Van De Graff generator?

9. Voltage Sources Electric potential difference (continued) Example: Water from a higher reservoir to a lower one—flow continues until no difference

10. Voltage Sources Electric potential difference (continued)  A battery or generator can maintain a steady flow of charge.  Work is done in pulling negative charges apart from positive ones.  Electromagnetic induction at the generator terminals provides the electrical pressure to move electrons through the circuit.

11. Voltage Sources Electric potential difference  In chemical batteries  Work by chemical disintegration of zinc or lead in acid.  Energy stored in chemical bonds is converted to electric potential energy.

12. Voltage Sources  Electrical plug - 120 Volts are delivered to home outlets - Potential difference between the two holes averages 120 volts. - When you plug something in, you allow the “pressure” of 120 volts to pass between the prongs. - 120 J for each coulomb that is moved.

13. Remember!  Charge flows Through a circuit  Voltage is placed across a circuit.  Think: water flows through a pipe if there is a difference in pressure between the ends of the pipe!  Voltage causes current!

14. Electric Resistance  Current in a circuit depends on: - voltage - electrical resistance (measured in Ohms ~ Ω )  Electrical Resistance: resistance of the conductor to the flow of charge.  Resistors: circuit elements that regulate current flow. Resistors. The symbol of resistance in an electric circuit is.

15. Electric Resistance  What factors influence resistance? - conductivity of the material (rubber has more resistance than copper) - Length of the material (longer material, higher resistance) - Thickness, or area, of the material (Thick wires have less resistance than thinner ones.)

16. Electric Resistance Factors affecting electrical resistance (continued) Temperature – the higher the temperature, the more the resistance

17. Electric Resistance Semiconductors  Refers to materials that can alternate between being conductors and insulators Example: germanium silicon

18. Electric Resistance Superconductors  Materials with zero electrical resistance to the flow of charge.  Flow of charge is without generation of heat. High-temperature superconductors  Refers to ceramic materials that can carry much current at a low voltage.

19. In Review:  Charge will flow when there is a conductor that has a potential difference!  Charge flows (Current) through a circuit, voltage is placed across it.  Resistance and Voltage determine the current in a circuit.

20. The flow of charge in an electric circuit is much  like the flow of water in a system of pipes.  different than water flow in pipes.  like an electric valve.  like an electric pump.

21. The flow of charge in an electric circuit is much  like the flow of water in a system of pipes.  different than water flow in pipes.  like an electric valve.  like an electric pump.

22. Electric charge will flow in an electric circuit when  electrical resistance is low enough.  a potential difference exists.  the circuit is grounded.  electrical devices in the circuit are not defective.

23. Electric charge will flow in an electric circuit when  electrical resistance is low enough.  a potential difference exists.  the circuit is grounded.  electrical devices in the circuit are not defective.

24. The electric current in a copper wire is normally composed of  electrons.  protons.  ions.  All or any of these.

25. The electric current in a copper wire is normally composed of  electrons.  protons.  ions.  All or any of these. Explanation: Although current can consist of protons and ions, in a copper wire current consists of electrons appropriately called conduction electrons.

26. Which statement is correct?  Voltage flows in a circuit.  Charge flows in a circuit.  A battery is the source of electrons in a circuit.  All are correct.

27. Which statement is correct?  Voltage flows in a circuit.  Charge flows in a circuit.  A battery is the source of electrons in a circuit.  All are correct. Explanation: Voltage causes the flow of electrons, and doesn’t flow. Charges flow. A battery is a source of energy, not electrons.

28. AC vs. DC The War of the Currents  Tesla Vs. Edison Tesla Vs. Edison

29. Direct and Alternating Current  Direct current (dc)  Flows in one direction only.  Electrons always move from the negative terminal toward the positive terminal.  Alternating Current (ac)  Electrons move first in one direction and then in the opposite direction about a fixed position  How? Alternate the polarity of voltage at the voltage source.

30. Direct and Alternating Current  Commercial electricity in North America  Alternating current (ac)  60 cycles per second  Voltage is 120 V  Power transmission is more efficient at higher voltages.  Europe adopted 220 V as its standard.  U.S. continued with 120 V because so much equipment was already installed.

31. Direct and Alternating Current  Converting from ac to dc  Household current is ac, but current in laptop is dc.  The converter uses a diode, a tiny electronic device that acts as a one-way valve to allow electron flow in one direction only.

32. Direct and Alternating Current  Converting from ac to dc  When input to a diode is ac, output is pulsating dc.  Slow charging and discharging of a capacitor provides continuous and smoother current.  A pair of diodes is used, so there are no gaps in current output.

33. Speed and Source of Electrons in a Circuit When we flip the light switch on a wall and the circuit, an electric field is established inside the conductor. The electrons get nudged by the E field. Current is established through the wires at nearly the speed of light. It is not the electrons that move at this speed. It is the electric field that can travel through a circuit at nearly the speed of light. Electrons move at about 0.01 cm/s!!! SLOWWWWW!!!!!!

34.  DC source (Battery): electric field lines are in one direction.  Conduction electrons are accelerated by the field in a direction parallel to the field lines.  Can’t get much speed because they “bump” into the wire and give off some Kinetic Energy. Speed and Source of Electrons in a Circuit

35. Ideas about current… “Current is propagated through the conducting wires by electrons bumping into one another, passing energy.” True or False?  False! Electrons that are free to move in a conductor are accelerated by the electric field impressed upon them.  They do bump into one another and other atoms, but this slows them down and offers resistance to their motion.  Electrons react simultaneously to the electric field and accelerate.  Think of “Forward MARCH!”

36. Speed and Source of Electrons in a Circuit Another idea: “Electrical outlets in the walls of the homes are a source of electrons.” True or False?  False! The outlets in homes are ac. Electrons move back and forth in ac, not through the circuit.  When you plug a lamp into an outlet, energy flows from the outlet into the lamp, not electrons. Energy is carried by the pulsating electric field and causes vibratory motion.  Electrical utility companies sell energy. You provide the electrons.

37. 1. Apply heat to a copper wire and the resistance of the wire  decreases.  remains unchanged.  increases.  vanishes with enough heat.

38. 2. The amount of current in a circuit depends on the  voltage across the circuit.  electrical resistance of the circuit.  Both of these.  None of these.

39. 3. When you double the voltage in a simple electric circuit, you double the  current.  resistance.  Both of these.  None of these.

40. 4. When 110 volts are impressed across a 22- ohm resistor, the current in the resistor is  5 A.  10 A.  132 A.  2420 A.

41. 5. To receive an electric shock there must be  current in one direction.  moisture in the electrical device being used.  a difference in potential across part or all of the body.  high voltage and low body resistance.

42. 6. The difference between dc and ac in electrical circuits is that in dc the electrons  flow steadily in one direction only.  flow in one direction only.  steadily flow to and fro.  flow to and fro.

43. 7. What travels at about the speed of light in an electric circuit?  Electric charges  Electric current  Electric field  All of these.

44. 8. The drift speed of electrons that make up current in a circuit is  near the speed of light.  near the speed of sound.  about a snail’s pace.  imaginary.

45. 1. Apply heat to a copper wire and the resistance of the wire  decreases.  remains unchanged.  increases.  vanishes with enough heat.

46. 2. The amount of current in a circuit depends on the  voltage across the circuit.  electrical resistance of the circuit.  Both of these.  None of these.

47. 3. When you double the voltage in a simple electric circuit, you double the  current.  resistance.  Both of these.  None of these. Explanation: This is straight-forward Ohm’s law. Current = voltage/resistance.

48. 4. When 110 volts are impressed across a 22- ohm resistor, the current in the resistor is  5 A.  10 A.  132 A.  2420 A.

49. 5. To receive an electric shock there must be  current in one direction.  moisture in the electrical device being used.  a difference in potential across part or all of the body.  high voltage and low body resistance.

50. 6. The difference between dc and ac in electrical circuits is that in dc the electrons  flow steadily in one direction only.  flow in one direction only.  steadily flow to and fro.  flow to and fro.

51. 7. What travels at about the speed of light in an electric circuit?  Electric charges  Electric current  Electric field  All of these. Explanation: The electric field in a circuit travels at nearly the speed of light, not the electrons nor the current.

52. 8. The drift speed of electrons that make up current in a circuit is  near the speed of light.  near the speed of sound.  about a snail’s pace.  imaginary.

53. Answers in order are: Mobile Ohm; Ohm Run; Ohm Stretch; Ohm Sick; Ohmwork; Ohmless; Ohm on the Range; Broken Ohm.