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TOPIC 2OHM’S LAW, ENERGY AND POWER

EGR2209ELECTRICAL CIRCUITS

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OHM’S LAW

Topic 2.1

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Introduction

Ohm’s Law describes mathematically how voltage, current and resistance in a circuit are related. Voltage is the amount of energy per charge available

to move 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.

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

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Effect On The Current Of Changing The Voltage With The Resistance At A Constant Value

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Effect On The Current Of Changing The Resistance With The Voltage At A Constant Value

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Ohm’s Law

VIR

V IRVRI

Example: Verify that the current through 10 Ω resistor increases when the voltage is increased from 5 V to 20 V.

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A Graphic Aid For The Ohm’s Law Formulas.

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Example

In the circuit, how much voltage is needed to produce 5 A of current?

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ENERGY AND POWER

Topic 2.2

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Introduction

When there is a current through a resistance, electrical energy is converted to heat or any other form of energy, such as light.

Example: Light bulb becomes too hot to touchElectrical components must be able to dissipate a

certain amount of energy in a given period of time.

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Energy

Energy is the ability to do work, and power is the rate at which energy is used.

tWP

P = Power, watts (W)W = Amount of energy, joules (J)t = Length of time, seconds (s)

One watt is the amount of power when one joule of energy is used in one second

Number of joules used in 1 s equals to the number of watts.

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Example

An amount of energy equal to 100 J is used in 5 s. What is the power in watts?

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POWER

Topic 2.3

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Power

When there is a current through a resistance, the collisions of the electrons as they move through the resistance give off heat, resulting in a conversion of electrical energy to thermal energy.

In electrical work, the rate energy is dissipated can be determined from any of three forms of the power formula.

2P I R P VI2VPR

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Example

Calculate the power in each of the three circuits.

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The Power Rating of Resistor

Resistor gives off heat when there is current through it.

There is a limit to the amount of heat that a resistor can give off, which is specified by its power rating.

Power rating is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat buildup Is mainly determined by the physical composition, size

and shape of the resistor.

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Relative sizes of metal-film resistors with standard power ratings of 1⁄8 W, 1⁄4 W, 1⁄2 W, and 1 W

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Example

Choose an adequate power rating (1⁄8 W, 1⁄4 W, 1⁄2 W, and 1 W) for each of the metal – film resistors represented in Figure.

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Example

Determine whether the resistor in each circuit of Figure has possibly been damaged by overheating.

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Voltage Drop in a Resistance

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

QWV

V = Voltage, volts (V)W = Energy, joules (J)Q = Charge, coulombs (C)

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CREDITS TO ELECTRONICS FUNDAMENTALS: CIRCUITS, DEVICES AND

APPLICATIONS THOMAS L . FLOYD & DAVID M. BUCHLA

The End