Unit Three: Ohm’s Law

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

2. Schedule Unit Topic Chpt Labs • Quantities, Units, Safety 1 2 (13) • Voltage, Current, Resistance 2 3 + 16 • Ohm’s Law 3 5 (35) • Energy and Power 3 6 (41) • Series Circuits Exam I 4 7 (49) • Parallel Circuits 5 9 (65) • Series-Parallel Circuits 6 10 (75) • Thevenin’s, Power Exam 2 6 19 (133) • Superposition Theorem 6 11 (81) • Magnetism & Magnetic Devices 7 Lab Final • Course Review and Final Exam

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

4. Unit 3 Objectives – II • 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. • Explain the Multisim workbench and show how to construct a basic circuit. • Test circuits by connecting simulated instruments in Multisim

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

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

7. Written Assignments • Answer all questions on the homework handout • 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: • Write down the formula • Show numbers in the formula • Circle answer with the proper units

8. Ohms Law • MEMORIZE: V = I R • Ohm’s Law • If 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. Experimental Results

10. Graph of Data V Voltage x x x x x I – Current in Amps

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

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

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

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

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

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

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

18. ? V 1.2k Ω 575 μA Examples • Ohms Law: V = I R k = 103μ = 10-6 • How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow? • V = I R

19. 19 Examples Ohms Law: V = I R k = 103μ = 10-6 How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow? V = I R V = 575 μA 1.2 k Ω V = .69V = 690 x 10-3V = 690 mV ? V 1.2k Ω 575 μA

20. 10 V 25 Ω 103=k10-3 = m10-6 = μ Examples • Ohms Law: V = I R • How much current flow through a 25 Ω resistor with 10 V across it? • V = I R I = V / R

21. 21 Examples Ohms Law: V = I R How much current flow through a 25 Ω resistor with 10 V across it? V = I R I = V / R 10 V = I 25 Ω I = 10 V / 25 Ω I = .4 A or 400 x 10-3A = 400 mA 10 V 25 Ω 103=k10-3 = m10-6 = μ

22. 35 V 250 mA 103=k10-3 = m10-6 = μ Examples • Ohms Law: V = I R • If a certain resistor allows 250 mA to flow when 35 V are across it, what is the resistance? • V = I R R = V / I

23. 35 V 250 mA 103=k10-3 = m10-6 = μ Examples • Ohms Law: V = I R • If a certain resistor allows 250 mA to flow when 35 V are across it, what is the resistance? • V = I R R = V / I • 35 V = 250 mA R • R = 35 V / 250 ma • R = 140 Ω

24. 4.5 mV 3.3k Ω 103=k10-3 = m10-6 = μ Examples • Ohms Law: V = I R • How much current flow through a 3.3k Ω resistor with 4.5 mV across it? • V = I R I = V / R

25. 4.5 mV 3.3k Ω 103=k10-3 = m10-6 = μ Examples • Ohms Law: V = I R • How much current flow through a 3.3k Ω resistor with 4.5 mV across it? • V = I R I = V / R • 4.5 mV = I 3.3k Ω • I = 4.5 mV / 3.3k Ω • I = 1.36 μ A

26. DIRECT AND INVERSE RELATIONSHIPS • I = V R

27. DIRECT AND INVERSE RELATIONSHIPS • I = V R

28. And Still More Practice

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

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

31. And Still More Practice

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

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

34. And Still More Practice

35. Lab 5 - Ohm’s Law • 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 circuits • Use the special handout to organize your information

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

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

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

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. Next Steps • 4. 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 meter • 34.67 mV, for example

41. Voltage Notation • Voltage is always the difference between TWO points. • Measure VBC by attaching the RED lead to B and the BLACK lead to C A B V D C

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 D is your reference point, VB is: A B D C V

43. Voltage Differences • If D is your reference point, then • VB is really VBD • VC is really VCD • Electrically, then • VBC = VBD - VCD • Voltage is the difference between two points • Choosing a different reference point does NOT change the real voltage

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