1 electrical power electronics copyright © texas education agency, 2014. all rights reserved

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Electrical Power

Electronics

Copyright © Texas Education Agency, 2014. All rights reserved.

Electrical Power

Electrical power is the rate of electrical energy consumed by an electrical circuito Shows how fast energy is used

Mechanical power is the rate at which work is done

A rate is like a speedoTypical values for speed: mi/hr or m/s

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Power and Work

Mechanical work has units of force times distanceo Something has to move for work to be doneo The thing that moves for electrical work is the

electron

Electron movement is measured by the amount of current

The force that makes electrons move is voltage

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Watt’s Law

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P = V • IP = ΔV • I


Electrical power is the product of current times voltage

o Use change in voltage only when a portion of the available voltage is used by a circuit or component

The unit of electrical power is the watt (W)

A watt has units of joules per second (J/s)

Power and Energy

Power is a rate of energy usageo Utility companies charge for energy used, not for

power used

Energy is a product of power and timeo For example, if one watt is used for one second,

one joule of energy is consumed

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Household Energy Use

Utility companies use kilowatt hours (kW•h) to measure units of energy in houses

How many joules are in one kW•h?o3,600,000 joules or 3.6 MJ (mega joules)

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Other Power Formulas

You may alternate between versions of the power formula depending on the known circuit values

These two versions result from substituting Ohm’s Law for either voltage or current

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Abbreviations for Power

Power = P

Watt = W

Kilowatt = kW

Kilowatt hours = kW•h


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Summary of Power Formulas

When current and voltage are known, use P=IV When current and resistance are known, use P=I2R When voltage and resistance are known, use P=V2 / R


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Using a Wattmeter

To measure power with a wattmeter:

1. Turn off circuit power

2. Connect the current (I) terminals of the wattmeter in series with the circuit load

o CAUTION - Observe polarity. The positive lead must be toward the positive side, and the negative lead toward the negative side.


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Using a Wattmeter (cont’d.)

3. Connect the voltage (V) terminals of wattmeter across the load

4. Turn on circuit power

5. Read and record power value in watts indicated on wattmeter

6. Turn off circuit power and disconnect wattmeter


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Wattage Rating of Resistors

Resistors have ohm values and wattage ratings

Resistor size generally indicates wattage rating

Wattage rating indicates the maximum amount of power

o Wattage rating should be double the expected power level of the circuit


Calculate PT, P1, and P2

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VS =15 VR1 = 100 Ω

R2 = 200 Ω

Practice Problem 1: Power in a Series Circuit


Calculate PT, P1, and P2oThis problem can be solved in several ways

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VS =15 VR1 = 100 Ω

R2 = 200 Ω




First, write the equation(s) that solve the problem

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VS =15 VR1 = 100 Ω

R2 = 200 Ω



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VS =15 VR1 = 100 Ω

R2 = 200 ΩP = V • I so PT = VT • IT, P1 = V1 • I1, P2 = V2 • I2



Second, look for what you need to solve oThe information you need may or may not be given

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VS =15 VR1 = 100 Ω

R2 = 200 ΩP = V • I so PT = VT • IT, P1 = V1 • I1, P2 = V2 • I2

Calculate the currento Current is the same everywhere because this is a

series circuit

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VS =15 VR1 = 100 Ω

R2 = 200 Ω



Calculate the currento Current is the same everywhere because this is a

series circuit

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VS =15 VR1 = 100 Ω

R2 = 200 Ω



Now, calculate total power

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VS =15 VR1 = 100 Ω

R2 = 200 Ω



You don’t have voltage for the individual resistors

You do have current and resistance values for the individual resistors

o Use an alternate power formula 22

VS =15 VR1 = 100 Ω

R2 = 200 Ω

P = I2 • R



Calculate power used by each resistor

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VS =15 VR1 = 100 Ω

R2 = 200 Ω



Calculate power used by each resistor

Combine the power of these resistors to get the total power 24

VS =15 VR1 = 100 Ω

R2 = 200 Ω



Calculate PT, P1, and P2oThis problem can be solved in several ways

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VS =18 V R2 = 300 ΩR1 = 200 Ω

Practice Problem 2: Power in a Parallel Circuit



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VS =18 V R2 = 300 ΩR1 = 200 ΩP = V • I so PT = VT • IT, P1 = V1 • I1, P2 = V2 • I2

Practice Problem 2: Power in a Parallel Circuit


Method 1To Calculate Power, Solve for Current


Current can be calculated using Ohm’s Law

Voltage is the same across each parallel path27

P = V • I so PT = VT • IT, P1 = V1 • I1, P2 = V2 • I2I=VR


VS =18 V R2 = 300 ΩR1 = 200 Ω

Method 1

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VS =18 V R2 = 300 ΩR1 = 200 Ω


Method 1

First, calculate individual power

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VS =18 V R2 = 300 ΩR1 = 200 ΩP1 = V1 • I1 = 18 V • 90 mA = 1.62 WP2 = V2 • I2 = 18 V • 60 mA = 1.08 W


Method 1

Now, calculate total powero Total power is the sum of the power for each resistor

Conclusion: power adds in a parallel circuit

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VS =18 V R2 = 300 ΩR1 = 200 Ω

PT = VT • IT = 18 V • 150 mA = 2.7 W


Method 2

You have voltage and resistance for this circuito Use the alternate power formula

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VS =18 V R2 = 300 ΩR1 = 200 Ω

P =


Method 2

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VS =18 V R2 = 300 ΩR1 = 200 Ω


Method 2

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VS =18 V R2 = 300 ΩR1 = 200 Ω


Method 1 and Method 2 have the same result

Application of Watt’s Law

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How much power will be expended to operate the electric toaster?

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Power Formula Proportions

Direct proportion - a change in one quantity produces the same direction of change in another quantity

o Example: In the power formula P=VI, if the current remains constant but the voltage is decreased, power is also decreased

• Inverse proportion - a change in one quantity produces the opposite direction of change in another quantity

o Example: In the power formula P= V2/R, if voltage remains constant but resistance is increased, power is decreased


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Terms and Definitions Mechanical power - the rate at which work is being done Electrical power - the rate of electrical energy used by

an electrical circuit Watt - the unit of measurement for power; one volt

times one amp Kilowatt - 1,000 watts Kilowatt hours - a unit of electrical energy Fuse - a non-resettable electrical device which protects

a circuit from excessive current Circuit breaker - a resettable electrical switch which

protects a circuit from excessive currentCopyright © Texas Education Agency, 2014. All rights reserved.

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Electrical Power Safety Precautions

Circuit safety precautionso Never install a fuse or circuit breaker if current rating

is higher or if voltage rating is lower than specified for a particular circuit

o Never bypass or defeat a fuse or circuit breaker

Worker safety precautions with live circuitso Work with well-insulated tools

o Avoid completing a circuit through the body


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Summary

Electrical power overview Formulas used to compute electrical power Wattmeter and sequence for power

measurement Power calculations and applying Watt’s Law Wattage rating of resistors Electrical power safety precautions Terms and definitions


1 electrical power electronics copyright © texas education agency, 2014. all rights reserved

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