Unit Three: Ohm’s Law

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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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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)
• 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
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.
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
• Chapter 3: Ohm’s Law Pages 71-80
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
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
• 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
Graph of Data

V

Voltage

x

x

x

x

x

I – Current in Amps

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

V = I R

V = I ROhm’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
V = I ROhm’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 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

Practice
• V = I R
• What voltage (V) is needed to push a current of 2 Amperes (I) through a resistance of 18 Ohms (R) ?
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
? 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
19Examples

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

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
21Examples

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

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

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
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

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
Practice
• V = I R
• What current (I) flows through a resistance of 8 ohms when the resistor is connect to a 24 volt battery?
V = I RPractice
• 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
V = I RPractice
• What size resistor allows 2 amperes of current through it when it is connected to a 10 Volt power supply?
V = I RPractice
• 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
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
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
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

• 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

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 (–)

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
• 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
• 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
• 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

1. Ohm’s Law

2. Solving for voltage, current, or resistance in a one-load circuit

3. Ohm’s Law using metric prefixes