The moving charges (the microscopic particles) from the electric supply constitute an electric current

Conventionally:The direction of the electric current is taken to be the flow of positive charge (from negative terminal to positive terminal)Whereas, the electrons flow in the opposite direction to that of the electric current

THE DIRECTION OF THE FLOWING

ELECTRIC CURRENT

+ -

An electric current (I) can be stated as the rate of the flowing electric charge (Q) through a certain cross-section of a conductor

The instrument used to measure current is ammeter

Formula:

I = current, Q = charge, and t = time

tQ

=I

MEASURING CURRENT

Electric current is measured in amps (A)

using an ammeter connected in series in

the circuit.

A

One ampere is the amount of current that exists when a number of electrons having a total charge of one coulomb pass a given cross-sectional area in one second

circuit diagramcircuit diagram

cell switch

lamp

wires

Scientists usually draw electric circuits using symbols;

circuit symbolscircuit symbolsIn circuit diagrams components are represented by the following symbols;

cell battery

switch

lamp

motorammeter

voltmeter

buzzer

resistor

variable resistor

ELECTROMOTIVE FORCE (E.M.F)

A source of e.m.f is a device in which non-electrical energy such as chemical, mechanical, etc, is converted into electrical energy

Such as : electrical cells, solar cells, generators, etc

The CELLThe CELL

The cell stores chemical energy and transfers

it to electrical energy when a circuit is

connected. When two or more cells are

connected together we call

this a Battery.

The cells chemical energy is

used up pushing a current

round a circuit.

THE E.M.F OF A CELL

The e.m.f of a cell is defined as the energy converted from non-electrical forms to electrical form when one coulomb of positive charge passes through the cell

BATTERY

A battery is a voltage source that converts chemical energy into electrical energy

The way cells are connected, and the type of cells, determines the voltage and capacity of a battery

THE ARRANGEMENT OF BATTERIES

THE FORMULA OF E.M.FThe formula used to

calculate e.m.f is:

E = e.m.f

W = converted energy

Q = positive chargeTherefore, the SI unit of

e.m.f is joule/coulomb, which is volt (V)

QW

=E

POTENTIAL DIFFERENCE (p.d) The potential difference

between two points can be defined as the energy converted from electrical energy to other forms when one coulomb of positive charge passes through the cell

THE FORMULA OF P.DThe formula used to

calculate p.d is:

V = p.d

W = converted energy

Q = positive chargeTherefore, the SI unit of

p.d is joule/coulomb, which is volt (V)

QW

=V

1 VOLT• 1 volts is 1 joule of

energy converted from electrical to other forms if 1 coulomb of positive charge passes through it

• Schematic Symbol for the DC Voltage Source

THE MEASUREMENT OF E.M.F and P.D

• The electromotive force (e.m.f) can be measured by a voltmeter connected directly across the terminals of the source

• The potential difference (p.d) between 2 points separated by a load (such as resistor) can be measured by a voltmeter connected in parallel to the load

MEASURING VOLTAGE

The ‘electrical push’ which the cell gives to

the current is called the voltage. It is

measured in volts (V) on a voltmeter

V

Different cells produce different voltages.

The bigger the voltage supplied by the

cell, the bigger the current.

MEASURING VOLTAGE

Unlike an ammeter a voltmeter is

connected across the components

Scientist usually use the term Potential

Difference (pd) when they talk about

voltage.

VOLTMETER

Voltage:

Probes connect to either side of the resistor

Measuring Voltage

A circuit is the path that is made for an electric current.

Close circuitCircuit in which there is a continuous conducting path around which charge can flow continuously

Open circuitCircuit in which there is a break in the circuit to stop the current from flowing

A circuit that only has one path for current to flow through is called a series circuit.

Current is the same at all points in a series circuit

If the path is broken, no current flows through the circuit.

A type of circuit that has multiple paths for current is called a parallel circuit.

Different paths may contain different current flow. This is also based on Ohms Law.

If one part of the path is removed, the current continues to flow through the other paths of the circuit.

By Analogy: Series Vs Parallel

E I

E R1

R2

R3

I1

I2

I3

R1R2

RESISTANCE Resistance is a property of a

material that resist the movement of free electrons in the material

It determines the size of the current which can pass in the material

Current through an ideal conductor is proportional to the applied voltage

Conductor is also known as a resistor

An ideal conductor is a material whose resistance does not change with temperature

For an ohmic device,

THE FORMULA OF RESISTANCETHE FORMULA OF RESISTANCE

The formula used to calculate The formula used to calculate resistance is:resistance is:

VV = p.d= p.d (V)(V)

I I = electric current= electric current (A)(A)

RR = resistance= resistance ((ΩΩ)) Therefore, the SI unit of Therefore, the SI unit of

resistance is volt/ampere which resistance is volt/ampere which is ohm (is ohm (ΩΩ))

IV

=R

RESISTORRESISTOR It is used to provide a certain value of It is used to provide a certain value of

resistance in a circuitresistance in a circuit Its function is to control the size of the current Its function is to control the size of the current

that flows in a circuit or divide voltage, and in that flows in a circuit or divide voltage, and in some cases, generate heatsome cases, generate heat

There are 2 types of resistor:There are 2 types of resistor:

Fixed ResistorFixed Resistor

Variable Resistor (rheostat)Variable Resistor (rheostat)

FIXED RESISTORFIXED RESISTOR

The types are:The types are:

Carbon film resistorCarbon film resistor

Carbon composition resistorCarbon composition resistor

Tin oxide resistorTin oxide resistor

Wire – wound resistor used in high power Wire – wound resistor used in high power ratingrating

The common electric symbol is The common electric symbol is

Common Fixed ResistorsCommon Fixed Resistors

Color-code Bands on a ResistorColor-code Bands on a Resistor This coding is used for 5%, 10%, and 20% tolerance This coding is used for 5%, 10%, and 20% tolerance

resistorsresistors • • The first three bands are used to denote the rated The first three bands are used to denote the rated

valuevalue • • The fourth denotes how much the device may vary The fourth denotes how much the device may vary

from the rated valuefrom the rated value 1st band is the first digit of the resistance value1st band is the first digit of the resistance value 2nd band is the second digit of the resistance value2nd band is the second digit of the resistance value 3rd band is the multiplier (number of zeros)3rd band is the multiplier (number of zeros) 4th band indicates the tolerance4th band indicates the tolerance

Color-code Bands on a ResistorColor-code Bands on a Resistor

Color Coded Precision ResistorsColor Coded Precision Resistors

Alphanumeric LabelingAlphanumeric Labeling Two or three digits, and one of the letters Two or three digits, and one of the letters R, R,

K, or M K, or M are used to identify a resistance valueare used to identify a resistance value The letter is used to indicate the multiplier, The letter is used to indicate the multiplier,

and its position is used to indicate decimal and its position is used to indicate decimal point positionpoint position

VARIABLE RESISTORVARIABLE RESISTOR

Its function is to vary the current flowing Its function is to vary the current flowing in a circuitin a circuit

The common electric symbol is The common electric symbol is

Variable ResistorsVariable Resistors

Variable resistors are designed so that their Variable resistors are designed so that their resistance values can be changedresistance values can be changed

• • A potentiometer is a three terminal A potentiometer is a three terminal variable resistor used to divide voltagevariable resistor used to divide voltage

• • A rheostat is a variable resistor used to A rheostat is a variable resistor used to control currentcontrol current

Examples of Variable ResistorsExamples of Variable Resistors

Ohmic ResistorsOhmic Resistors

Metals obey Ohm’s Law linearly so long as Metals obey Ohm’s Law linearly so long as their temperature is held constanttheir temperature is held constant Their resistance values do not fluctuate with Their resistance values do not fluctuate with

temperaturetemperature i.e. the i.e. the resistanceresistance for each resistor is a constant for each resistor is a constant

Most ohmic resistors will behave non-Most ohmic resistors will behave non-linearly outside of a given range of linearly outside of a given range of temperature, pressure, etc.temperature, pressure, etc.

Voltage and Current Relationship Voltage and Current Relationship for Linear Resistorsfor Linear Resistors

Voltage versus Current for a 10 ohm Resistor

00.10.20.30.40.50.6

0 1 2 3 4 5 6

Voltage (V)

Cu

rre

nt

(A)

Voltage and current are linear when resistance is held constant.

Voltage versus Current for Constant Voltage versus Current for Constant ResistanceResistance

The light bulb does not have a linear relationship. The resistance of the bulb increases as the temperature of the bulb increases.

Resistance is affected by temperature. Resistance is affected by temperature. The resistance of a conductor increases The resistance of a conductor increases as its temperature increases.as its temperature increases.

Light bulbs Light bulbs do notdo not obey Ohm’s Law obey Ohm’s Law linearlylinearly As their temperature increases, the As their temperature increases, the

power dissipated by the bulb power dissipated by the bulb increasesincreases i.e. i.e. They are brighter when they are They are brighter when they are

hotterhotter

Ohm’s Law continuedOhm’s Law continued

Series Circuit

Current is constant

Why? Only one path for the

current to take

1 2 3

1 2 3

1 2 3

V V V V

I I I I

R R R R

V I R

Parallel Circuit

1 2 3

1 2 3 1 23

23 2 3

1 2 3

where

1 1 1 1

V V V V

I I I I I I

I I I

R R R R

Voltage is constant

Why? There are 3 closed loops in the circuit

V I R

Formula of a number of resistors in series and

in parallelThe total resistance of a circuit is

dependant on the number of resistors in the circuit and their configuration

1 2

1 2

...

1 1 1 1...

total

total

R R R R

R R R R

Series CircuitSeries Circuit

Parallel CircuitParallel Circuit

SeriesResistors are connected such that the current can only take one path

Parallel

Resistors are connected such that the current can take multiple paths

Kirchhoff’s Current Law

Current into junction = Current leaving junction

in outI I

Iin I1

I2

I2

I1

Iout

1 2

0

in out

in out

I I I I

I I

The amount of current that enters a junction is The amount of current that enters a junction is equivalent to the amount of current that leaves equivalent to the amount of current that leaves the junctionthe junction

THE RELATION BETWEEN THE RELATION BETWEEN RESISTANCE AND RESISTIVITYRESISTANCE AND RESISTIVITY

• The resistance (R) of a conductor depends on:

• Its length

• Its cross-sectional area

• The material used in resistance

• Its temperature

EXPERIMENTAL RESULTS EXPERIMENTAL RESULTS HAVE SHOWN THAT:HAVE SHOWN THAT:

• The larger the cross-sectional area of a wire, the smaller the resistance

• The longer the wire, the larger the resistance

A1R∝

lR∝

THE FORMULA OF THE FORMULA OF RESISTANCERESISTANCE

• Combining the two equations, we find:

• is resistivity and its SI unit is Ω m

Alρ=R

ρ

Electric Power

Moving charges do work We can heat the filament in a light bulb We can turn the rotor in a motor The rate at which work is done is

power Electric Power = current x voltage Units are watts = joules/sec = amps x

volts

Electric Power

RV

=RV

V=P

RI=IRI

=P

VI=P

2

2

Wires have some resistanceTo transport energy from Point

A to Point B, we connect wiresEach wire has resistanceEnergy Loss in one second is I2RMake I small to minimizeTransformers convert AC

Voltages

Limit the current that runs through wires in your house

These wires have some resistanceEnergy loss by I2R converts to heatHot wires can start a fireLimit the current with a fuse or

circuit breaker