LCHS Physics Mark Ewoldsen, Ph.D.

1. LCHS Physics Mark Ewoldsen, Ph.D.

2. Definitions • Voltage: Electric Potential or Potential Difference (Energy added or used) • V (volts) = Joules/Coulomb • One volt = one coulomb of charge gains or loses one joule of energy

3. What is the difference between E and V? • E is the voltage supplied by a battery • V is the voltage measured across a resistor

4. Batteries • Positive and Negative terminals • electrons leave negative terminal • Batteries use a chemical reaction to create voltage • Construction: Two different metals and Acid • e.g. Copper, Zinc, and Citrus Acid • e.g. Lead, Lead Oxide, Sulfuric Acid • e.g. Nickel, Cadmium, Acid Paste

6. Definitions • Current: Flow of electric charge • I (amps) = Q/t = Coulombs/second • Higher current increases heat due to more collisions of ‘free’ electrons with atoms

7. Effect of Currents on the Body • 0.001 A can be felt • 0.005 A is painful • 0.010 A causes involuntary muscle contractions • 0.015 A causes loss of muscle control • 0.070 A can be fatal if the current last for more than 1 second

8. Speed of Electrons in Circuit • Light goes on ‘instantly’ when switch turned on • Electrons do not move at speed of light • c = 3 x 108 • Electrons – 6 x 105 m/s in random directions • Signal (energy) moves at speed of light due to electric energy field • Wire acts as pipe to guide electric field • Electrons in circuit do not come from battery but are from the wire

9. Definitions • Resistance: measure of a material’s ability to resist the flow of of electrons • Ω (ohms) = J-s/C2 • Conductor – low resistance • materials with free electrons • e.g. copper, aluminum, gold, most metals • Insulator – high resistance • materials with no free electrons • e.g. glass, plastics, ceramics, wood

10. Definitions • Resistance • Increases with length – L • Decreases with cross area – A • Resistivity – ρ • material dependent • temperature dependent R = ρL A

11. Definitions • Power is the rate energy is converted into another form • Resistors transform electrical energy into light, mechanical or heat energy • Equation for Power: P = I V (Watts) – Joules/second

12. A Ammeter: measures amp(ere)s Battery: Source of energy Resistor: Removes energy V Voltmeter: measures volt(age) Wires: Definitions

13. Reading a Voltmeter

14. Reading a Ammeter

15. Making Circuits

16. Making Circuits A

17. Making Circuits A V

18. Making Circuits V A V

19. How many batteries? A V

20. How many ammeters? A V

21. How many voltmeters? A V

22. How many light bulbs? A V

23. Ohm’s Law: V = I R V = voltage, I = current, R = resistance For a given voltage, as the resistance goes up, the current will go down OR If the resistance is less, the current is more

24. Ohm’s Law A V

25. Ohm’s Law A V

26. Constant Voltage • For a given pressure on a car’s accelerator • Voltage or energy input • By increasing the steepness of a hill • Changing resistance • The car’s speed will go down • Decrease the current or amps

27. Turned on When light is first turned on • Filament is cold • The resistance is low and • since V = IR, current is high • High current means filament is more likely to burn out

28. Ohm’s Law: V/I = R V = voltage, I = current, R = resistance For a given resistance, as the voltage goes up, the current goes up

29. A A A A V V V V Ohm’s Law – Set Resistance

30. http://www.physics.udel.edu/wwwusers/watson/scen103/98w/note0105.htmlhttp://www.physics.udel.edu/wwwusers/watson/scen103/98w/note0105.html

31. In a simple one resistor circuit, what is the Resistance if the Voltage is 12-V and Current is 3-A?

32. In a simple one resistor circuit, what is the Resistance if the Voltage is 120-V and Current is 5-A?

33. In a simple one resistor circuit, what is the Voltage if the Resistance is 12- and Current is 3-A?

34. In a simple one resistor circuit, what is the Voltage if the Resistance is 3- and Current is 17-A?

35. In a simple one resistor circuit, what is the Current if the Resistance is 3- and Voltage is 17-V?

36. In a simple one resistor circuit, what is the Current if the Resistance is 3- and Voltage is 120-V?

37. Series Circuit

38. Finding V, I & R for a Series Circuit • Determine total E (V from battery) • Find the total Resistance RT = R1 + R2 + R3 … • Determine IT by I = VT/RT • Since I is constant in a series circuit, find V (energy used) by each resistor using V = ITR

39. Finding V, I & R for a Series Circuit • Determine E (V from battery)

40. 12 V 3 Ω 2 Ω 1 Ω

41. Finding V, I & R for a Series Circuit • Determine E (V from battery)- 12V • Find the total Resistance RT = R1 + R2 + R3 …

42. 12 V 3 Ω 2 Ω 1 Ω 6 Ω= 3 Ω + 2 Ω+ 1 Ω

43. Finding V, I & R for a Series Circuit • Determine E (V from battery)- 12V • Find the total Resistance - 6 Ω

44. Finding V, I & R for a Series Circuit • Determine E (V from battery)- 12V • Find the total Resistance - 6 Ω • Determine IT by IT = VT/RT 2 A= 12 V/ 6 Ω

45. Finding V, I & R for a Series Circuit • Determine E (V from battery)- 12V • Find the total Resistance - 6 Ω • Determine that IT = 2 A • Since I is constant in a series circuit, find V (energy used ) for each resistor using V = ITR

46. 12V = 6V + 4V + 2V 12 V 6V = 2A * 3Ω 3 Ω 2 Ω 1 Ω 4V = 2A * 2Ω 2V = 2A * 1Ω

47. Series Circuit • Current is the same at all points I = I1 = I2 = I3 = I4 • Volt is divided among all the resistor E = V1 + V2 + V3 • Resistance accumulates R = R1 + R2 + R3

48. 12 V 3 Ω 4 Ω 1 Ω

49. 12 V 3 Ω 4 Ω 1 Ω What is the total Resistance?

50. 12 V 3 Ω 4 Ω 1 Ω What is the total Resistance? 8 Ω What is the current leaving the Battery?