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1 ET115 DC Electronics Unit Three: Ohm’s Law John Elberfeld [email protected] WWW.J-Elberfeld.com 2 Schedule Unit Topic Chpt Labs 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 1 2 3 3 4 5 6 6 6 7 Quantities, Units, Safety Voltage, Current, Resistance Ohm’s Law Energy and Power Series Circuits Exam I Parallel Circuits Series-Parallel Circuits Thevenin’s, Power Exam 2 Superposition Theorem Magnetism & Magnetic Devices Course Review and Final Exam 2 (13) 3 + 16 5 (35) 6 (41) 7 (49) 9 (65) 10 (75) 19 (133) 11 (81) Lab Final 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 Current (x) Voltage (y) 0A 0V 2A 6V 4A 12 V 6A 18 V 8A 24 V 10 Graph of Data V V o l t a g e x x x x x I – Current in Amps 11 Reasoning V=IR • 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=IR 13 Ohm’s Law V=IR • This is the BIG IDEA for the day (year)! •V=IR • What if we divide both sides by R? • V=IR R R • But R/R = 1, so we don’t need to write it down: • I= V I=V/R R 14 Ohm’s Law V=IR •V=IR • What if we divide both sides by I? • V=IR 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=IR 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=IR • V = 2 A x 18 Ω • V = 36 V 18 Examples • Ohms Law: V = I R k = 103 μ = 10-6 • How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow? • V=IR ?V 1.2k Ω 575 μA 19 Examples • Ohms Law: V = I R k = 103 μ = 10-6 • How much voltage must be connected across a 1.2 k Ω resistor to cause 575 μA of current to flow? • V=IR • V = 575 μA 1.2 k Ω • V = .69V = 690 x 10-3V = 690 mV ?V 1.2k Ω 575 μA 20 Examples 103=k 10-3 = m 10-6 = μ • Ohms Law: V = I R • How much current flow through a 25 Ω resistor with 10 V across it? • V=IR I=V/R 10 V 25 Ω 21 Examples 103=k 10-3 = m 10-6 = μ • Ohms Law: V = I R • How much current flow through a 25 Ω resistor with 10 V across it? • V=IR 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 22 Examples 103=k 10-3 = m 10-6 = μ • 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=IR R=V/I 35 V 250 mA 23 Examples 103=k 10-3 = m 10-6 = μ • 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=IR R=V/I • 35 V = 250 mA R • R = 35 V / 250 ma • R = 140 Ω 35 V 250 mA 24 Examples 103=k 10-3 = m 10-6 = μ • Ohms Law: V = I R • How much current flow through a 3.3k Ω resistor with 4.5 mV across it? • V=IR I=V/R 4.5 mV 3.3k Ω 25 Examples 103=k 10-3 = m 10-6 = μ • Ohms Law: V = I R • How much current flow through a 3.3k Ω resistor with 4.5 mV across it? • V=IR I=V/R • 4.5 mV = I 3.3k Ω • I = 4.5 mV / 3.3k Ω • I = 1.36 μ A 4.5 mV 3.3k Ω 26 DIRECT AND INVERSE RELATIONSHIPS • I=V R 27 DIRECT AND INVERSE RELATIONSHIPS • I=V R 28 And Still More Practice V=IR I=V/R 1.6 mA 250 μA 500 mA 850 μA 75 μA 3 mA 5 μA 2.5 A R=V/I 2.2 kΩ 1.0 kΩ 1.5 MΩ 10 MΩ 47 Ω 27 kΩ 100 MΩ 47 Ω 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 Practice V=IR • What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery? • V=IR I=V/R • 24 V = I x 8 Ω I = 24 V / 8 Ω • I = 24 V / 8 Ω I=3A • I=3A 31 And Still More Practice V=IR 40 V 1 kV 66 kV 12 V 25 V 5V 15 V I=V/R R=V/I 68 kΩ 2 kΩ 10 MΩ 10 Ω 10 kΩ 2.2 MΩ 1.5 kΩ 32 Practice V=IR • What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply? 33 Practice V=IR • What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply? • V=IR R=V/I • 10 V = 2 A x R R = 10 V / 2 A • R = 10 V / 2 A R=5Ω • R=5Ω 34 And Still More Practice V=IR 500 V 50 V 1 kV 6V 8V 12 V 39 V I=V/R 250 mA 500 μA 1 mA 2 mA 2A 4 mA 150 μA R=V/I 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 A V • You must BREAK the circuit to measure current 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