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Basic Electrical Engineering

What is it and where does it come from?

Sources of Electrical Energy

Electricity with electrons flowing in only one direction is called Direct Current

DIRECT CURRENT (DC)

ALTERNATING CURRENT (AC)

Electricity with electrons flowing back and forth, negative - positive- negative, is called Alternating Current, or AC

Generation of Electricity

Generation, Transmission and Distribution of Electrical Energy

Voltage

The ‘electrical push’ which the cell gives to the electrons is called the voltage

Measurement of Voltage

It is measured in volts (V) by a voltmeter

V V

Current

An electric current is a flow of microscopic particles called

electrons flowing through wires and components.

+ -

Measurement of Current

Electric current is measured in amps (A) using an

Ammeter connected in series in the circuit.

A A

Resistance

R

Resistance is the property of material by virtue of which it opposes the flow of Electrons i.e. current

Resistance

(a) Conductors (b) semiconductors (c) insulators

Factors affecting Resistance of a conductorFactors affecting Resistance of a conductor

• Resistance depends on– Temperature– Length– Cross-sectional area – Type of Material

The resistance of a metallic conductor increases as the temperature increases e.g. copper

The resistance of a semiconductor/insulator decreases as the temperature increases e.g. thermistor.

Factors affecting Resistance of a Factors affecting Resistance of a conductorconductor

1. Temperature

Resistance of a uniform conductor is directly proportional to its length.


A

1

=

l

h

w

R

2. Length

i.e. R L

Resistance of a uniform conductor is inversely proportional to its cross-sectional area.i.e. R 1/A

3. Cross-sectional area


Simple circuitsSimple circuits

Here is a simple electric circuit. It has a cell, a lamp

and a switch.

To make the circuit, these components are connected

together with metal connecting wires.

cell

lampswitch

wires

Circuit diagramCircuit diagram

cell switchlamp wires

Scientists usually draw electric circuits using symbols;

Simple circuitsSimple circuitsWhen the switch is closed, the lamp lights up. This is

because there is a continuous path of metal for the

electric current to flow around.

If there were any breaks in the circuit, the current could not flow.

Circuit diagramsCircuit diagramsIn circuit diagrams components are represented by the following symbols;

cell battery

switch

lamp

motorammeter

voltmeter

buzzer

resistor

variable resistor

Ohms Law

+_V

I

unknown resistive

element

Assume that the wires are “perfect conductors” and Physical conditions remaining constant.

The unknown circuit element limits the flow of current.

A simple relationship exists between voltage, current, and resistance in electrical circuits. Understanding this relationship is important for fast, accurate electrical problem diagnosis and repair.

Voltage

Current When R is Constant

R

EI

Ohms Law

The current in a circuit is directly proportional to the applied voltage and inversely proportional to the amount of resistance. This means that if the voltage goes up, the current flow will go up, and vice versa. Also, as the resistance goes up, the current goes down, and vice versa.

EI = — R

E = I R

ER = — I

Three Algebraic Forms of Ohm’s Law

Voltage = Current x Resistance

Power

Power = Voltage X Current

R

V

R

VxVP

2

RIIxRxIP 2

WattVxIP

1)Find the resistances of following copper wires

i) 1 mm2 cross sectional area, 100 m long

ii) 25 cm2 cross sectional area, 200 m long

Given that ρ = 1.73 µ Ω- cm

73.1

101

101001073.1)

2

26

x

xxx

A

lRi

001384.025

102001073.1)

26 xxxRii

2) Calculate the Resistance of 100m length of wire having uniform cross-sectional area of 0.01 mm2 and having resistivity of 50 µ Ω-cm. If the wire is drawn three times its original length, calculate the resistance.

Given: ρ = 50 µ Ω-cm = 50 x 10-6 Ω-cml = 100 m = 100 x 102 cm, area= a = 0.01 mm2 = 0.01 x 10-2 cm2

5000

1001.0

101001050

2

26

x

xxxR

New length l’= 3 x l, New cross section area a’

Since the volume remains same

After drawing, volume = c/s area x length = a x l = a’ x l’ = a’ x 3 x l

a’ = a/3 l’ = 3 x l = 100 x 102 x 3 = 300 x 102 cm

a’ = a/3 = 0.01 x 10-2 / 3 cm2

45000

1001.0

103001050

2

26

'

'

x

xxx

a

lRnew

Simple circuitsSimple circuitsWhen the switch is closed, the lamp lights up. This is

because there is a continuous path of metal for the

electric current to flow around.

If there were any breaks in the circuit, the current could not flow.

Types of the circuitTypes of the circuit

There are two types of electrical circuits;

SERIES CIRCUITS PARALLEL CIRCUITS

The components are connected end-to-end, one after the other.

They make a simple loop for the current to flow round.

SERIES CIRCUITS

If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.

PARALLEL CIRCUITS

The current has a choice of routes.

The components are connected side by side.

If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.

Measuring current

A A

This is how we connect an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

Measuring currentSERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same

at all points in the

circuit.

2A 2A

2A

• current is shared

between the components

2A2A

1A

1A

Measuring voltage

V

This is how we connect a voltmeter in a circuit.


V

V

Measuring voltage

VV

V

Measuring voltage

V

This is how we connect a voltmeter in a circuit.


V

Series circuit

1.5V

• voltage is shared between the components

1.5V

3V

• voltage is the same in all parts of the circuit.

3V

Parallel circuit

3V

3V

Measuring current & voltage

copy the circuits on the next two slides.

complete the missing current and voltage readings.

remember the rules for current and voltage in series and parallel circuits.


V V

6V4A

A

A

a)a)


V

V

6V4A A

A

A

b)b)

Answers

3V 3V

6V

4A 4A6V

6V

6V4A 4A

2A

2A

4A

a)a) b)b)

Which of the two cases shown has the smaller equivalent resistance between points a and b?

Q.1

1. Case #1

2. Case #2

3. the equivalent resistance is the same in both cases

4. answer depends on the values of the individual resistances R1, R2, and R3

Which of the two cases shown has the smaller equivalent resistance between points a and b?

A.1

1. Case #1

2. Case #2

3. the equivalent resistance is the same in both cases

4. answer depends on the values of the individual resistances R1, R2, and R3

A 120-V, 60-W light bulb, a 120-V, 120-W light bulb, and a 120-V, 240-W light bulb are connected in series as shown.

If the voltage between points a and b is 120 V, through which bulb will there be the greatest voltage drop?

Q.2

1. the 120-V, 60-W light bulb



4. all three light bulbs have the same voltage drop

A 120-V, 60-W light bulb, a 120-V, 120-W light bulb, and a 120-V, 240-W light bulb are connected in series as shown.


A.2





A 120-V, 60-W light bulb, a 120-V, 120-W light bulb, and a 120-V, 240-W light bulb are connected in parallel as shown.


Q.3





A 120-V, 60-W light bulb, a 120-V, 120-W light bulb, and a 120-V, 240-W light bulb are connected in parallel as shown.


A.3





Three identical light bulbs are connected to a source of emf as shown. Which bulb is brightest?

Q.4

1. light bulb A

2. light bulb B

3. light bulb C

4. both light bulbs B and C (both are equally bright and are brighter than light bulb A)

5. all bulbs are equally bright

Three identical light bulbs are connected to a source of emf as shown. Which bulb is brightest?

A.4

1. light bulb A

2. light bulb B

3. light bulb C

4. both light bulbs B and C (both are equally bright and are brighter than light bulb A)

5. all bulbs are equally bright

You wish to study a resistor in a circuit. To simultaneously measure the current in the resistor and the voltage across the resistor, you would place

Q.5

1. an ammeter in series and an voltmeter in series

2. an ammeter in series and an voltmeter in parallel

3. an ammeter in parallel and an voltmeter in series

4. an ammeter in parallel and an voltmeter in parallel

You wish to study a resistor in a circuit. To simultaneously measure the current in the resistor and the voltage across the resistor, you would place

A.5

1. an ammeter in series and an voltmeter in series

2. an ammeter in series and an voltmeter in parallel

3. an ammeter in parallel and an voltmeter in series

4. an ammeter in parallel and an voltmeter in parallel

Problem-Solving Strategy

• A complicated circuit consisting of several resistors and batteries can often be reduced to a simple circuit with only one resistor

• Replace resistors in series or in parallel with a single resistor

• Sketch the new circuit after these changes have been made

• Continue to replace any series or parallel combinations

• Continue until one equivalent resistance is found

Problem-Solving Strategy

• If the current in or the potential difference across a resistor in the complicated circuit is to be identified, start with the final circuit and gradually work back through the circuits

• (use formula ΔV = I R and the procedures describe above)

Find the resistance between A and B

Will Either Bird be Shocked?

Connecting wire has no insulation and negligible resistance

4 + 6 = 10

1 / RCD = 1 / 9 + 1 / 8 + 1 / 10

RCD = 2.98

2.98 + 3 = 5.98

1 / RAB = 1 / 5.98 + 1 / 20

RAB = 4.60

What is the total resistance between Points A and B?

What is the potential difference between Points A and B? Also find current through 5.0 ohm resistance.

1 / RAB = 1/3 + 1/6

RAB = 2 ohm

Total: 6 ohm

I = 12 V / 6 ohm = 2 A

VAB = (2 A) (2 ohm ) = 4 V

I5 = 4 V / (5+1)ohm = 2 / 3 = 0.66 A

Find the current in the 12-Ω resistor in the Figure

I=0.428 Amp

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Documents

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