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ET115 DC Electronics. Unit Three: Ohm’s Law. John Elberfeld [email protected] WWW.J-Elberfeld.com. 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)

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Unit three ohm s law l.jpg

ET115 DC Electronics

Unit Three:Ohm’s Law

John Elberfeld

[email protected]

WWW.J-Elberfeld.com


Schedule l.jpg
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


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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.


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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


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Reading Assignment

  • Read and study

  • Chapter 3: Ohm’s Law Pages 71-80


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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


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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


Ohms law l.jpg
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



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Graph of Data

V

Voltage

x

x

x

x

x

I – Current in Amps


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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


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Electronic Circuit

  • A battery with the voltage V pushes a current I through a resistor R

V = I R


Ohm s law l.jpg

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


Ohm s law14 l.jpg

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


Ohm s law15 l.jpg
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


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Practice

  • V = I R

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


Practice17 l.jpg
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


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? 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


Examples19 l.jpg

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


Examples20 l.jpg

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


Examples21 l.jpg

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


Examples22 l.jpg

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


Examples23 l.jpg

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


Examples24 l.jpg

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


Examples25 l.jpg

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





Practice29 l.jpg
Practice

  • V = I R

  • What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery?


Practice30 l.jpg

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



Practice32 l.jpg

V = I R

Practice

  • What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply?


Practice33 l.jpg

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



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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


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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


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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


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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


Lab 4 voltage measurement l.jpg
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 (–)


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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


Voltage notation l.jpg
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


Voltage l.jpg
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


Voltage differences l.jpg
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


Unit 3 summary l.jpg
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


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