1. 1 Unit Three: Ohm’s Law John Elberfeld JElberfeld@itt-tech.edu WWW.J-Elberfeld.com ET115 DC Electronics

2. Schedule Unit Topic Chpt Labs 1.Quantities, Units, Safety12 (13) 2.Voltage, Current, Resistance23 + 16 3.Ohm’s Law35 (35) 4.Energy and Power36 (41) 5.Series CircuitsExam I47 (49) 6.Parallel Circuits59 (65) 7.Series-Parallel Circuits610 (75) 8.Thevenin’s, Power Exam 2619 (133) 9.Superposition Theorem 611 (81) 10.Magnetism & Magnetic Devices7Lab Final 11.Course Review and Final Exam 2

3. 3 Unit 3 Objectives - I Describe the relationship among voltage, current, and resistance.Describe the relationship among voltage, current, and resistance. Given two of the three variables in Ohm’s Law, solve for the remaining quantity.Given two of the three variables in Ohm’s Law, solve for the remaining quantity. Solve Ohm’s Law problems using metric prefixes.Solve Ohm’s Law problems using metric prefixes. Construct basic DC circuits on a protoboard.Construct basic DC circuits on a protoboard.

4. 4 Unit 3 Objectives – II Use a digital multimeter (DMM) to measure a predetermined low voltage on a power supply.Use a digital multimeter (DMM) to measure a predetermined low voltage on a power supply. Measure resistances and voltages in a DC circuit using a DMM.Measure resistances and voltages in a DC circuit using a DMM. Explain the Multisim workbench and show how to construct a basic circuit.Explain the Multisim workbench and show how to construct a basic circuit. Test circuits by connecting simulated instruments in MultisimTest circuits by connecting simulated instruments in Multisim

5. Reading Assignment Read and studyRead and study Chapter 3: Ohm’s Law Pages 71-80Chapter 3: Ohm’s Law Pages 71-80 5

6. Lab Assignment Lab Experiment 5:Lab Experiment 5: Ohm’s Law Pages 35-38Ohm’s Law Pages 35-38 Complete all measurements, graphs, and questions and turn in your lab before leaving the roomComplete all measurements, graphs, and questions and turn in your lab before leaving the room 6

7. Written Assignments Answer all questions on the homework handoutAnswer all questions on the homework handout Be prepared for a quiz on questions similar to those on the homework.Be prepared for a quiz on questions similar to those on the homework. If there are any calculations, you must show ALL your work for credit:If there are any calculations, you must show ALL your work for credit: –Write down the formula –Show numbers in the formula –Circle answer with the proper units 7

8. 8 Ohms Law MEMORIZE: V = I RMEMORIZE: V = I R Ohm’s LawOhm’s Law If you increase the voltage, you increase the current proportionallyIf you increase the voltage, you increase the current proportionally –3 times the voltage gives you three times the current –Resistance (ohms) is the proportionality constant and depends on the atomic structure of the material conducting the current

9. 9 Experimental Results Current (x)Voltage (y) 0 A0 V 2 A6 V 4 A12 V 6 A18 V 8 A24 V

10. 10 Graph of Data x I – Current in Amps VVoltageVVoltage x x x x

11. 11Reasoning Ohms Law: V = I ROhms Law: V = I R High voltage produces high current for a given resistanceHigh voltage produces high current for a given resistance Low voltage produces low current for a given resistanceLow voltage produces low current for a given resistance For a given voltage, a high resistance produces a low currentFor a given voltage, a high resistance produces a low current For a given voltage, a low resistance produces a high currentFor a given voltage, a low resistance produces a high current V = I R

12. 12 Electronic Circuit A battery with the voltage V pushes a current I through a resistor RA battery with the voltage V pushes a current I through a resistor R V = I R

13. 13 Ohm’s Law This is the BIG IDEA for the day (year)!This is the BIG IDEA for the day (year)! V = I RV = I R What if we divide both sides by R?What if we divide both sides by R? V = I R R RV = I R R R But R/R = 1, so we don’t need to write it down:But R/R = 1, so we don’t need to write it down: I = VI = V / R RI = VI = V / R R V = I R

14. 14 Ohm’s Law V = I RV = I R What if we divide both sides by I?What if we divide both sides by I? V = I R I IV = I R I I But I / I = 1, so we don’t need to write it down:But I / I = 1, so we don’t need to write it down: R = VR = V / I IR = VR = V / I I V = I R

15. 15 Ohm’s Law Memorize: V = I RMemorize: V = I R Use algebra to find:Use algebra to find: I = V / RI = V / R R = V / IR = V / I If you can, learn all three variations, but you can get by if you memorize:If you can, learn all three variations, but you can get by if you memorize: V = I R

16. 16Practice V = I RV = I R What voltage (V) is needed to push a current of 2 Amperes (I) through a resistance of 18 Ohms (R) ?What voltage (V) is needed to push a current of 2 Amperes (I) through a resistance of 18 Ohms (R) ?

17. 17Practice V = I RV = I R What voltage (V) is needed to push a current of 2 Amperes (I) through a resistance of 18 Ohms (R) ?What voltage (V) is needed to push a current of 2 Amperes (I) through a resistance of 18 Ohms (R) ? V = I RV = I R V = 2 A x 18 ΩV = 2 A x 18 Ω V = 36 VV = 36 V

18. 18Examples Ohms Law: V = I R k = 10 3 μ = 10 -6Ohms Law: V = I R k = 10 3 μ = 10 -6 How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow?How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow? V = I RV = I R ? V 1.2k Ω 575 μA

19. 19Examples Ohms Law: V = I R k = 10 3 μ = 10 -6Ohms Law: V = I R k = 10 3 μ = 10 -6 How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow?How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow? V = I RV = I R V = 575 μA 1.2 k ΩV = 575 μA 1.2 k Ω V =.69V = 690 x 10 -3 V = 690 mVV =.69V = 690 x 10 -3 V = 690 mV ? V 1.2k Ω 575 μA

20. 20Examples Ohms Law: V = I ROhms Law: V = I R How much current flow through a 25 Ω resistor with 10 V across it?How much current flow through a 25 Ω resistor with 10 V across it? V = I RI = V / RV = I RI = V / R 10 V 25 Ω 10 3 =k 10 -3 = m 10 -6 = μ

21. 21Examples Ohms Law: V = I ROhms Law: V = I R How much current flow through a 25 Ω resistor with 10 V across it?How much current flow through a 25 Ω resistor with 10 V across it? V = I RI = V / RV = I RI = V / R 10 V = I 25 Ω10 V = I 25 Ω I = 10 V / 25 ΩI = 10 V / 25 Ω I =.4 A or 400 x 10 -3 A = 400 mAI =.4 A or 400 x 10 -3 A = 400 mA 10 V 25 Ω 10 3 =k 10 -3 = m 10 -6 = μ

22. 22Examples Ohms Law: V = I ROhms Law: V = I R If a certain resistor allows 250 mA to flow when 35 V are across it, what is the resistance?If a certain resistor allows 250 mA to flow when 35 V are across it, what is the resistance? V = I RR = V / IV = I RR = V / I 35 V 250 mA 10 3 =k 10 -3 = m 10 -6 = μ

23. 23Examples Ohms Law: V = I ROhms Law: V = I R If a certain resistor allows 250 mA to flow when 35 V are across it, what is the resistance?If a certain resistor allows 250 mA to flow when 35 V are across it, what is the resistance? V = I RR = V / IV = I RR = V / I 35 V = 250 mA R35 V = 250 mA R R = 35 V / 250 maR = 35 V / 250 ma R = 140 ΩR = 140 Ω 35 V 250 mA 10 3 =k 10 -3 = m 10 -6 = μ

24. 24Examples Ohms Law: V = I ROhms Law: V = I R How much current flow through a 3.3k Ω resistor with 4.5 mV across it?How much current flow through a 3.3k Ω resistor with 4.5 mV across it? V = I RI = V / RV = I RI = V / R 4.5 mV 3.3k Ω 10 3 =k 10 -3 = m 10 -6 = μ

25. 25Examples Ohms Law: V = I ROhms Law: V = I R How much current flow through a 3.3k Ω resistor with 4.5 mV across it?How much current flow through a 3.3k Ω resistor with 4.5 mV across it? V = I RI = V / RV = I RI = V / R 4.5 mV = I 3.3k Ω4.5 mV = I 3.3k Ω I = 4.5 mV / 3.3k ΩI = 4.5 mV / 3.3k Ω I = 1.36 μ AI = 1.36 μ A 4.5 mV 3.3k Ω 10 3 =k 10 -3 = m 10 -6 = μ

26. 26 DIRECT AND INVERSE RELATIONSHIPS I = V RI = V R

27. 27 DIRECT AND INVERSE RELATIONSHIPS I = V RI = V R

28. And Still More Practice V = I RI = V / RR = V / I 1.6 mA 2.2 kΩ 250 μA1.0 kΩ 500 mA1.5 MΩ 850 μA10 MΩ 75 μA47 Ω 3 mA27 kΩ 5 μA100 MΩ 2.5 A47 Ω 28

29. 29Practice V = I RV = I R What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery?What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery?

30. 30Practice What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery?What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery? V = I RI = V / RV = I RI = V / R 24 V = I x 8 ΩI = 24 V / 8 Ω24 V = I x 8 ΩI = 24 V / 8 Ω I = 24 V / 8 ΩI = 3 AI = 24 V / 8 ΩI = 3 A I = 3 AI = 3 A V = I R

31. And Still More Practice V = I RI = V / RR = V / I 40 V68 kΩ 1 kV2 kΩ 66 kV10 MΩ 12 V10 Ω 25 V10 kΩ 5 V2.2 MΩ 15 V1.5 kΩ 31

32. 32Practice What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply?What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply? V = I R

33. 33Practice What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply?What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply? V = I RR = V / IV = I RR = V / I 10 V = 2 A x RR = 10 V / 2 A10 V = 2 A x RR = 10 V / 2 A R = 10 V / 2 A R = 5 ΩR = 10 V / 2 A R = 5 Ω R = 5 ΩR = 5 Ω V = I R

34. And Still More Practice V = I RI = V / RR = V / I 500 V250 mA 50 V500 μA 1 kV1 mA 6 V2 mA 8 V2 A 12 V4 mA 39 V150 μA 34

35. Lab 5 - Ohm’s Law Ohm’s Law describes the relationship among voltage, current, and resistance – it does not control it!Ohm’s Law describes the relationship among voltage, current, and resistance – it does not control it! In lab, you will prove to yourself that Ohm’s Law applies to circuitsIn lab, you will prove to yourself that Ohm’s Law applies to circuits Use the special handout to organize your informationUse the special handout to organize your information 35

36. Select and Measure Resistors Your resistors can off by +/- 5% from the marked valueYour resistors can off by +/- 5% from the marked value You must measure as accurately as possible the real resistance used in your experimentYou must measure as accurately as possible the real resistance used in your experiment 36

37. Use TWO meters Use TWO DMMs in your experimentUse TWO DMMs in your experiment Record as many digits as possible for both voltage and currentRecord as many digits as possible for both voltage and current You must BREAK the circuit to measure currentYou must BREAK the circuit to measure current 37 A V

38. Plot Your Points Your lab handout says to plot I along the x axis and V along the y axisYour lab handout says to plot I along the x axis and V along the y axis The slope is Δy / Δx = ΔV/ ΔIThe slope is Δy / Δx = ΔV/ ΔI Based on Ohm’s Law, R = V / I, just like the slopeBased on Ohm’s Law, R = V / I, just like the slope 38

39. 39 Lab 4 – Voltage Measurement 1. Select the correct voltage mode (ac or dc). 2. Select range higher than expected voltage. 3. Connect the meter across the points. Red, positive (+), Black, common (–)

40. 40 Next Steps 4. Reduce the range setting until the reading fails4. Reduce the range setting until the reading fails 5. Increase the range setting one step and record all the numbers, with the proper units, shown on the meter5. Increase the range setting one step and record all the numbers, with the proper units, shown on the meter 34.67 mV, for example34.67 mV, for example

41. Voltage Notation Voltage is always the difference between TWO points.Voltage is always the difference between TWO points. Measure V BC by attaching the RED lead to B and the BLACK lead to CMeasure V BC by attaching the RED lead to B and the BLACK lead to C 41 AB CD V

42. Voltage If only one letter is given, attach the RED lead to that letter, and the BLACK lead to the reference point or ground.If only one letter is given, attach the RED lead to that letter, and the BLACK lead to the reference point or ground. If D is your reference point, V B is:If D is your reference point, V B is: 42 AB CD V

43. Voltage Differences If D is your reference point, thenIf D is your reference point, then V B is really V BDV B is really V BD V C is really V CDV C is really V CD Electrically, thenElectrically, then V BC = V BD - V CDV BC = V BD - V CD Voltage is the difference between two pointsVoltage is the difference between two points Choosing a different reference point does NOT change the real voltageChoosing a different reference point does NOT change the real voltage 43

44. 44 Unit 3 Summary 1. Ohm’s Law 2. Solving for voltage, current, or resistance in a one-load circuit 3. Ohm’s Law using metric prefixes