Ohm’s Law, Energy & Power 1 of 52 MECH1100 1 of 52 Chapter 3 – Ohms Law, Energy and Power Topics Ohm’s Law Energy and Power Power Ratings Energy Conversion

Ohm’s Law, Energy & Power

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MECH1100

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Chapter 3 – Ohms Law, Energy and Power

TopicsOhm’s Law

Energy and Power

Power Ratings

Energy Conversion and Voltage Drop

Power Supplies and Batteries

Troubleshooting


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Voltage is

Review of V, I, and R

the amount of energy per charge available tomove electrons from one point to another in a circuit and is measured in volts.

Current is the rate of charge flow and is measured in amperes (amps).

Resistance is the opposition to current and is measured in ohms.


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• Most important fundamental law in electronics

• Relates voltage, current, and resistance.

• Georg Simom formulated the equation

VI

R

What is the current in a circuit with a 12 V source if the resistance is 10 W? 1.2 A

Ohm’s law

I = CurrentV = Voltage or energy (E)R = Resistance


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If you need to solve for voltage, Ohm’s law is:

What is the voltage across a 680 W resistor if the current is 26.5 mA? 18 V

V IR


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If you need to solve for resistance, Ohm’s law is: V

RI

115 V

V

1 s

1 s

40 m A

1 0 A

C O M

Ra ng eAuto ra ng eTo uc h /Ho ld

Fuse d

O FF V

V

Hz

m V

A

What is the resistance of the bulb? 132 W


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IF: 1. Resistance is held Constant and2. Voltage Changes3. What happens to current?

VI

R


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IF: 1. Resistance Changes and2. Voltage is held Constant3. What happens to current?


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What is the System Current?


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Graphic aid for the Ohm’s law


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What is the system current

100 5.0

VI

R I = 310*5

100I = 20*10-3 amp or “X?” mAmp

State your knowns and value required!!


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What is the System Voltage?

Vs = IR Vs = 5 x 10-3 amp * 56 x 103 ohms

56k

Vs = 280 volts


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What is the Resistance of the Photocell?

9.0

R = 5696.2 ohms (5.69 kΩ)

s

s

I

VR


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The force must be measured in the same direction as the distance. The unit for work is the newton-meter (N-m) or joule (J).

Energy and Power

When a constant force is applied to move an object over a distance, the WORK (W) is the force times the distance.

Force

Distance


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One joule is the work done when a force of one newton is applied through a distance of one meter. A joule is a small amount of work approximately equal to the work done in raising an apple over a distance of 1 m.

1 n

1 mThe symbol for energy, W, represents work, but should not be confused with the unit for power, the watt, W.


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• Energy is closely related to work.

• Energy is the ability to do work.

1. Measured in the same units as work,

2. Newton-meter (N-m) (same as a joule (J)).

What amount of energy is converted to heat in sliding a box along a floor for 5 meters if the force to move it is 400 n?

W = Fd = (400 N)(5 m) = 2000 N-m = 2000 J


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Power is the rate of doing work.

• Rate means a time unit is required.

• Joule per second (J/s) = one watt (W).

Energy and Power

What power is developed if the box in the previous example is moved in 10 s?

WP

t

2000 J

10 s

WP

t 200 Watts

Work (energy in joules) not Watts


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Electrical Energy to Thermal Energy


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A loss of energy (voltage) by electrons (charge) as they flow through a resistance1. creates a voltage drop 2. because voltage equals energy divided by charge.


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The kilowatt-hour (kWh) is a much larger unit of energy than the joule.

• 3.6 x 106 J in a kWh.

• kWh is convenient for electrical appliances.

What is the energy used in operating a 1200 W heater for 20 minutes?

1200 W = ? kW

1.2 kW X 20minutes/60 minutes/hr = 0.396 kWh

1.2


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In electrical work, the rate energy is dissipated is dependent on the amount of resistance and the amount of current.

2P I R

2VP

R

Together, the three forms are called Watt’s law.

Thus: P = I*V V = I*R

Therefore:

or

and


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What power is dissipated in a 27 W resistor if the current is 0.135 A?

2

2(0.135 A) 27

0.49 W

P I R

Given that you know the resistance and current, substitute the values into P =I 2R.


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What power is dissipated by a heater that draws 12 A of current from a 120 V supply?

12 A 120 V

1440 W

P IV

The most direct solution is to use P = IV.


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What power is dissipated in a 100 W resistor with 5 V across it?

2

25 V

0.25 W100

VP

R

The most direct solution is to use 2V

PR

=


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Power rating of a resistor

Small resistors operating in low voltage systems need to be sized for the anticipated power.

Amount of power a resistor can dissipate without being damaged by the heat generated


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Resistor failures• Resistor failures are unusual except when they have

been subjected to excessive heat. • Look for discoloration (sometimes the color bands

appear burned). • Test with an ohmmeter by disconnecting one end

from the circuit to isolate it and verify the resistance.• Correct the cause of the heating problem (larger

wattage resistor?, wrong value?).

Normal Overheated


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Metal film resistors come in standard power ratings: 1/8 to 1 watt

Always choose a resistor with a higher power rating than the power being used in the circuit so that the resistor isn't destroyed by excess heat!

W8

1

W4

1

W2

1

W1


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Choosing the correct resistor

What power rated resistor would you chose for each circuit?

0.833 W => 1 Watt 0.100 W => .125 Watt

A resistor on a control board is calculated to require a power of 0.25W. What power rated resistor would you choose?


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Troubleshooting

Some questions to ask before starting any troubleshooting are:

?

1. Has the circuit ever worked?

2. If the circuit once worked, under what conditions did it fail?

3. What are the symptoms of the failure?

4. What are the possible causes of the failure?


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Troubleshooting• Plan the troubleshooting by reviewing

pertinent information:

?

1. Schematics

2. Line drawings

3. Ladder logic

4. Instruction manuals

• Then review when and how the failure occurred

• Always, talk with the operator or anyone else having knowledge of the failure


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• OPEN or open circuitAn electric circuit in which the normal path of current has been interrupted, either by the disconnection of one part of its conducting pathway from another, or by the intervention of an electric component, such as a transistor.

SHORT or Short CircuitA faulty or accidental connection between two points of different potential in an electric circuit, bypassing the load and establishing a path of low resistance through which an excessive current can flow.


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Half-splitting method of troubleshooting


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Power Wheel


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Ohm’s law

Linear

Energy

Power

Joule

Characterized by a straight-line relationship.

A law stating that current is directly proportional to voltage and inversely proportional to resistance.

Key Terms

The ability to do work. The unit is the joule (J).

The rate of energy usage.

The SI unit of energy.


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Watt

Kilowatt-hour

Ampere-hour rating

Efficiency

A number determined by multiplying the current (A) times the length of time (h) that a battery can deliver that current to a load.

Key Terms

The ratio of output power to input power of a circuit, usually expressed as a percent.

The unit of power. One watt is the power when 1 J of energy is used in 1 s.

A common unit of energy used mainly by utility companies.


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1. When the current is plotted against the voltage for a fixed resistor, the plot is a

a. straight line with a positive slope

b. straight line with a negative slope

c. parabola

d. hyperbola


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2. For constant voltage in a circuit, doubling the resistance means

a. doubling the current

b. halving the current

c. there is no change in the current

d. depends on the amount of voltage


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4. If the current in a 330 W resistor is 15 mA, the voltage across it is approximately

a. 5.0 V

b. 22 V

c. 46 V

d. 60 V


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5. A unit of power is the

a. joule

b. kilowatt-hour

c. both of the above

d. none of the above


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6. The SI unit of energy is the

a. volt

b. watt

c. joule

d. kilowatt-hour


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7. If the voltage in a resistive circuit is doubled, the power will be

a. halved

b. unchanged

c. doubled

d. quadrupled


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8. The approximate power dissipated by a 330 W resistor with 9 V across it is

a. ¼ W

b. ½ W

c. 1 W

d. 2 W


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9. Before troubleshooting a faulty circuit, you should find out:

a. If the circuit ever worked

b. The conditions that existed when it failed

c. The symptoms of the failure

d. All of the above

Documents

Ohm’s Law, Energy & Power 1 of 52 MECH1100 1 of 52 Chapter 3 – Ohms Law, Energy and Power Topics Ohm’s Law Energy and Power Power Ratings Energy Conversion