ELECTRICITY AND CIRCUITRY

PDT 180ENGINEERING SCIENCE

MISS MUNIRA MOHAMED NAZARISCHOOL OF BIOPROCESS ENGINEERING

UNIMAP

SESSION 2012/20132

TOPIC OUTLINEConcepts of electricity, Insulators and conductors, Electric fields, Coulomb’s Law. Concepts of electric current, Ohm’s Law, Electric power, Series and parallel wiring

3 SESSION 2012/2013

BACIS CONCEPTS OF BACIS CONCEPTS OF ELECTRICITYELECTRICITY

OBJECTIVES-To define electricity- To identify electrical charges- To define electrical fields

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Concept of ElectricityConcept of Electricity

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Electricity

If an amber rod is rubbed with a piece of cloth, the amber attracts small pieces of leave or dust.

If u rub a plastic ruler and bring it close to some tiny pieces of papers, the ruler will eventually attract the papers.

Greek word electrons which mean “amber” which is a petrified tree resin.

This phenomenon is called “static electricity”This phenomenon is called “static electricity”

Concept of ElectricityConcept of Electricity

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Object can be charged by rubbing.

Electric charge

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Electrostatics is the study of interaction between electric charges which are not moving.

Two types of electric charges:

Electric charge

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The force between the charges can be either attraction or repulsion.

Unit for charge is Coulomb, C where;

Mass of electron =Mass of proton =

protonsor electrons 10 x 6.25 Coulomb 1 8

coulomb 10 x 1.6 e 1 -19

or

kg 10 x 11.9 -31

kg 10 x .6721 -27

Conservation of electric charge

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Law of conservation of electric charge states that the net amount of electric charge produced in any process is zero.It means that in any process, electric charge

cannot be created or destroyed.It can be transfer to one object to another.For example, when a plastic ruler is rubbed

with a paper towel, the plastic acquires a negative charge and the towel an equal number of positive charges. The charges are separated but the sum of the two is zero.

Conservation of electric charge

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Can be illustrated using a simple model of atom.

A neutral atom contains an equal number of protons and electrons.

If the number of protons and electrons are not same, the atom will have a net positive or negative charge. This atom is called an ion.

Insulators and conductors

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Materials can be either insulating or conducting materials.

Insulating

Conductors

Semiconductors

Material contains electrons which are not free to move through the material. Eg: glass & woodMaterial contains electrons are free to move in the material. Eg: steel & gold

Material in which there are a few free electrons and the material is a poor conductor or electricity. Eg: silicon & carbon.

Coulomb’s law

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Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.

Coulomb’s Law

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Coulomb’s Law states that two point charges exert a force (F) on one another that is directly proportional to the product of the magnitudes of the charges (Q) and inversely proportional to the square of the distance (r) between their centers.

We can rewrite the Coulomb’s Law in term of equation as:

k is a constant which has a value of

rQ1 Q2

22 1

r

QQk F

229 C/Nm 10 x 8.988

Coulomb’s Law

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The force is along the line connecting the charges and is attractive if the charges are opposite, and repulsive if they are the same.

Example 1

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Find the magnitude and direction of the force on the electron.

22 1

r

QQk F

229 C/Nm 10 x 8.988 k

m 10 x 0.53 r -10

N 10 x 8.2 F -8

Example 2

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Which charge exerts the greater force?

These two forces have equal magnitude. F21 = F12.

The force on Q2 exerted by Q1 is the same as the force on Q1 exerted by Q2 except that Q1 and Q2 are reversed.

Example 3

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Three charges in a line.Three charged particles are arranged in a

line as shown below. Calculate the net electrostatic force on particles 3 (the -4.0 µC on the right) due to the other two charges.

-

-

+

Q1 = - 8.0 µC

0.30 m 0.20 m

Q3 = - 4.0 µCQ2 = +3.0 µC

F = - F32 + F31 = -2.7 N + 1.2 N = - 1.5 N

Electric field

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An electric field present if an electric charge experiences an electric force at any particular point in space.

In order to visualize the path taken by a charged particle in an electric field; electric field lines (lines of force) are drawn.

These line start on a positive charge and end on negative charge.

Electric field

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The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge.

The electric filed is stronger where the field lines are closer together.

Electric field

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Electric dipole : two equal charges, opposite in sign.

The electric field between two closely spaced, oppositely parallel plates is constant.

Electric field

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Summary of field lines:Field lines indicate the direction of the field;

the field is tangent to the line.The magnitude of the field is proportional to

the density of the lines.Field lines start on positive charges and end

on negative charges; the number is proportional to the magnitude of the charge.

BACIS CONCEPTS OF BACIS CONCEPTS OF CIRCUITRYCIRCUITRY

OBJECTIVES-To define electric current, voltage, resistance and electric power- To define electric circuit- To analyze the series and parallel wiring.

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Concepts of Electric CurrentAn electric current exists whenever electric

charge flows from the battery terminals through a region or circuit like a light bulb circuit as shown in figure below.

The magnitude of the current is measures in amperes (A).

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Concepts of Electric Current

Above figure represent symbol of electric current flow.

What actually happened is that free electron charges moving from the battery terminal and flow or crossing through the wire. The flow charge is known as electric current.

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Concepts of Electric CurrentDefinition

Electric current in a wire is defined as the net amount of charge that passes through the wire per unit time at any point.

The average current is defined as:

Unit for electric current = ampere (A) (coulomb/sec)

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Example : Electric charge flowA steady current of 2.5 A flows in a wire for

4.0 min. Calculate charge passed through any point in the circuit. How many electrons would this be?

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Ohm’s LawOhm’s law discusses on resistance in electrical

circuit.Electric resistance is the resistance of electrical

current flow. Electrical resistance is important since it can control the amount of current flow. Ohm defines resistance, R as the ratio of the voltage applied across the circuit.

Resistance, R of the piece of material or wire is given as

Unit for resistance, R = ohm (Ω)SESSION 2012/201327

Example : Resistance in a bulbA small flashlight bulb draws 300 mA

current from 1.5 volt battery. Determine the resistance of the flashlight bulb.

R = V/I = 1.5 V / 0.30 A = 5.0 Ω

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Ohm’s LawResistors

All electrical devices have resistance to the flow of current.

For instance, the connecting wires of a circuit have resistance. However, in electronic devices, resistors are used to control the amount of current.

The types of resistor varied from fixed resistor to variable resistor. When we draw a circuit diagram, we indicate the resistance or the resistor in the circuit by the symbol:

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Fixed Resistor Variable Resistor

Ohm’s LawResistance factors in a wire.

Wire in an electric circuit can be thick or thin wire and the resistance is not the same. There ate 3 factors that affect the resistance of a wire which are:Length of wireCross sectional areaResistivity of the wire.

It was found that the relationship of these factors is as

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Ohm’s LawThe longer the wire, the higher the resistance

and if the cross sectional areas of the wire is smaller, the resistance is also higher.

Meanwhile, the symbol ρ represents the resistivity of the material. Resistivity of the material depends on the property of the material.

For conductors, the resistivity value is higher compared to insulators.

Meanwhile, resistivity for semiconductor materials depends on the temperature of the material.

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Electric PowerPower as in kinematics, is the energy

transformed by a device per unit time.

Unit for power = watt, W

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Electric PowerIf the devices have resistors, then the

electrical power of the device is transformed into other form of energy.

For instance, a toaster, irons, stoves, etc become hot when provided with electrical power.

If we want to express the power in term of resistance, then the formula becomes as:

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Example : FlashlightDetermine the resistance of the bulb

Determine the power if V = 1.2 volt

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V = 1.5 Volt P = 0.15 Watt

I = P/V = 0.1 A R = V/I = 15 Ω

P = V²/R = 0.096 Watt

Series and Parallel WiringThere are 2 methods by which connection

can be made:Series wiringParallel wiring

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Series and Parallel WiringSeries wiring

Means that the devices are connected in such a way that there is the same amount of current flow through each device.

In the series wiring, if one of the devices is disconnected, the current will not be able to flow through another device, which means that the device is interrupted too.

Because of the series wiring, the voltage supplied by the battery id divided between the devices.

For instance, if we have 3 resistors, then, the voltage is divided between the 3 resistors.

SESSION 2012/201336

Series and Parallel WiringVoltage, V of the battery,

V = V1 + V2 + V3 = IR2 + IR3

V = I (R1 + R2 + R3)

Equivalent resistance in the circuit,

Req = R1 + R2 + R3

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Series and Parallel WiringParallel wiring

Means that the devices are connected in such a way that the same voltage is applied across each device.

Parallel wiring is the most popular wiring method. This is because, if the current in one of the devices is interrupted (by opened or broken wire), then the current in the other devices are not interrupted.

In parallel circuit, the total current I that leave the battery break into each branch.Eg: If we have two branches, then the current is I1 and

I2.

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Series and Parallel WiringTotal current is

Devices in parallel circuit each experience the same voltage from the main voltage (battery). Therefore, each of the current can be represented as

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I = I1 + I2

I1 = V/R1 and I2 = V/R2

Series and Parallel WiringSimilar to series circuit, we need to

determine the equivalent resistance of circuit. This can be done by adding all the current in each branch.

For instance, in a 3 branch parallel circuit:

Therefore

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I = I1 + I2 + I3

= V/R1 + V/R2 + V/R3

= V(1/R1 + 1/R2 + 1/R3)

1/Req = V(1/R1 + 1/R2 + 1/R3)

Example Two resistors of 200 Ω are connected (a) in

series and (b) in parallel to a 24.0 volt battery. Determine the equivalent resistance and the current through each resistor.

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Summary A battery is a source of constant potential

difference.Electric current is the rate of flow of electric

charge.Conventional current is in the direction that

positive charge would flow.Resistance is the ratio of voltage to current:

Power in an electric circuit:

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