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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

Reading Assignment

- Read and study
- 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

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

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 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

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

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

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

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

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

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

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

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

DIRECT AND INVERSE RELATIONSHIPS

- I = V R

DIRECT AND INVERSE RELATIONSHIPS

- I = V R

Practice

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

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

Practice

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

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

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

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

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

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