What goes around, comes around

1. 22 Electric Current What goes around, comes around

2. 22.1 Electric Current Current and Circuits

3. 22.1 Objectives • Students define an electric current and the ampere. • Students describe conditions that create current in an electric circuit. • Students draw circuits and recognize they are closed loops. • Students define power in electric circuits. • Students define resistance and describe Ohm’s law.

4. Bill Nye Electric Current • 0:00 to 3:40

5. What is Electric Current? • Definition: Electricity in flow (moving; running) • Current comes from the Latin currereto run and is used in Electricity to mean the flow of electrons. • Current clothes, current events, ocean current.

6. Electric Current • Electric current is simply the flow of charge • Electrons flowing in a wire constitute a current • Measured in Coulombs per second, or Amperes • Colloquially, Amp (A) • refers to amount of charge crossing through cross-sectional area per unit time • Electrons have a charge of –1.610-19 Coulombs • so (negative) one Coulomb is 61018 electrons • one amp is 61018 electrons per second • subtle gotcha: electrons flow in direction opposite to current, since current is implicitly positive charge flow, but electrons are negative

7. Electric Current

8. What is an Electric Circuit? • Definition: An electric circuit is a closed loop through which charges can flow. • Circuit comes from the Latin circumire circum (around) and ire (to go). • Circuit court judge, circuit drill, Circuit City

9. Would This Work?

10. Would This Work?

11. Would This Work?

12. The Central Concept: Closed Circuit

13. battery + bulb Circuit in Diagram Form _ current In a closed circuit, current flows around the loop electrons flow opposite the indicated current direction! (repelled by negative terminal) Current flowing through the filament makes it glow. No Loop  No Current  No Light

14. Current is the Flowrate It sometimes helps to think of current as flow of water, which is more familiar to us. High current means lots of water flow per unit time. Low current is more like a trickle. In electronics, it is the flow of charge, not water, that is described by the word current. And it’s always electrons doing the flowing (thus electronics)

15. Voltage is the Pressure It sometimes helps to think of voltage as water pressure. High voltage means lots of water pressure. If the water faucet valve is closed, no water flows no matte how high a pressure there is. No current is present if no electrons flow. In electronics, it is the flow of charge, not water, that is described by the word current. And it’s always electrons doing the flowing (thus electronics)

16. Resistance is the Valve It helps to think of resistance as the valve. High resistance means the valve is closed and little water flows through. Low resistance means the valve is open and a lot of water flows through.

17. Currents Divide and Merge at Junctions +  How much would the currentthrough the batterychange if I unscrewed one of the 2 bulbs? How would thebrightness of “A”change if I unscrewed “B”? A B

18. Answer • The battery is supplying an equal amount of current to each of the two bulbs. If one of the bulbs is disconnected, the current through the battery will be halved. • Unscrewing “B” would not affect the current through “A” so it will stay the same brightness. • Why wouldn’t more current flow through A? • The battery does not supply constant current (is there current even when the battery is disconnected? NO. But there is voltage!)

19. Paul Hewitt Video #98 • Caution on Handling Electrical Wires

20. Bill Nye Electric Current • 3:40 to 6:54

21. Electric Circuit Symbols

22. Series Circuits • A connection or arrangement of devices that provides only one path for a current. • The same current flows through each device (resistance).

24. Parallel Circuits • A connection or arrangement of two or more devices that provides more than one path for a current. • The current is divided through each path and merges again after each device. • Each device gets that same voltage.

25. Series and Parallel Circuits Parallel Series

26. Series and Parallel Combo Series Parallel

27. Meters in a Circuit – Ammeter • The ammeter measures current (I) in amperes or amps or Coulombs/second • It must be connected in series. Why? • The voltmeter measures potential difference (ΔV) in volts or Joules/Coulomb • It must be connected in parallel. So what is I(ΔV)?

28. Circuit Diagram This is useful for measuring resistance, flow of charge is impeded by fixed atoms in conductors R = ΔV/I measured as volts/amp or ohms (Ω) An iron has a 6.9 A current when connected to a 120 V outlet, what is its resistance? How is this a circuit?

29. Resistance • Increasing ΔV is like increasing the slope of a water run, it increases current (I), they are directly related • The constant of proportionality is the resistance of the conductor • Obstructions such as rocks act as the resistance, decreasing the current • Resistance in a circuit is due to collisions between the electrons carrying the current with the fixed atoms inside the conductor

30. Georg Simon Ohm • 1787 – 1854 • Formulated the concept of resistance • Discovered the proportionality between current and voltages • For many materials, including most metals, resistance remains constant over a wide range of applied voltages or currents • This statement has become known as Ohm’s Law, ΔV = IR

31. Paul Hewitt Video #100 • Ohm’s Law

32. Ohmic Devices • Ohm’s Law is an empirical relationship that is valid only for certain materials • Materials that obey Ohm’s Law are said to be ohmic • The resistance is constant over a wide range of voltages • The relationship between current and voltage is linear • How do light dimmers work?

33. Non-ohmic Materials • The resistance increases as the voltage increases • Notice that current is small and resistance is high when reversed • It therefore acts as a one way valve for current • A diode is a common example of a non-ohmic device

34. Paul Hewitt Video #102 • Electric Circuits

35. Paul Hewitt Video #99 • Birds on Wires

36. 22.2 Electric Current Using Electric Energy

37. 22.2 Objectives • Students explain how electric energy is converted into thermal energy. • Students determine why high-voltage transmission lines are used to carry electric energy over long distances. • Students define kilowatt-hour in electric energy delivery.

38. Power • The POWER dissipated in a resistor (or resisting appliance) is proportional to the square of the current that passes through it and to the resistance.

39. Energy • POWER is Electric ENERGY per unit time.

40. Resistance Becomes Heat Electric energy becomes thermal energy when the friction of electrons moving in a wire dissipates heat. Resistors are coils of wire that can dissipate heat.

41. Alternating Current • Current comes to your house as a sine wave that alternates between positive and negative current. The is called alternating current (AC). • This oscillating current has to be converted to direct current (DC = one direction current like a battery). • Wonder how we get direct current?

42. Rectification of AC Current • We must get the down humps to flip to up-humps by rectification!

43. Let’s see if Mr. Mosher can Draw! • Mr. Mosher will now explain how diodes can be used to separated the up-humps and down-humps of a sine wave. • A capacitor is used to flip (rectify) the down-humps to up-humps. • The combination is DC! • P.S. - Please be patient with Mr. Mosher!

44. AC/DC Converter • A diode is used to capture current in ONE DIRECTION ONLY. Use another diode to get the OTHER DIRECTION. • Put them together and what have you got? Direct Current! • Now go and play!!!!!!!!

45. Power Plants and You

46. Power Plants and You Electric energy becomes thermal energy due to the friction of electrons. By keeping the current very low and wire resistance minimized. Some long distance lines use 500,000 volts with low amperes. Current can be increase in your home with transformers.

47. Power Costs and You Power at your house is measured in Kilowatt (1,000 Watts) - hours.