Electricity

CHAPTER 7

ELECTRICITY

Prepared by: Puan Farizah binti Ariffin

At the end of this topic, a student is able to;• describe what electrostatics is, • state the types of static electrical charges,• state the properties of static electrical charges,• describe how static electrical charges can be produced

in some materials,• describe ways to detect static electrical charges,• explain everyday phenomena caused by static electrical

charges,• state the safety measure to be taken when dealing with

static electricity.

7.1 Electrostatics

ELECTROSTATICS

• The study of static electrical charges• Some common phenomena:- Little crackles when we pull off a supermarket

plastic bag from a roll- When one’s hair is combed with a plastic comb,

tufts of hair will be attracted to various parts of the comb

- When a piece of transparency is placed on a piece of paper and rubbed against the underlying paper, it is difficult to separate the transparency from the paper.

• Everything around us is made up of atoms

• An atom is made up of:

- Electron (negative charge)

- Proton (positive charge)

- Neutron (neutral)

ELECTROSTATICS

• Static electric charges can be produced through friction

• When materials are rubbed together, electrons from the atoms of one materials may be transferred to the other.

• An atom that loses electron - positively charge (+)• An atom that gains/ accept extra electrons

– negatively charge (-)

ELECTROSTATICS

Examples

• Atoms of each material have equal numbers of electrons and protons.

• After rubbing, - Woolen cloth becomes negatively charged- Acetate strip becomes positively charged

Examples

Atoms of each material have equal numbers of electrons and protons.

After rubbing, - Woolen cloth becomes positively charged- Polythene strip becomes negatively charged

GOLD LEAF ELECTROSCOPE

• Electroscope is a device to detect static charge

• Consists of a metal cap, metal plate and a gold leaf

• Gold leaf- detect small amount of electrical charges

HOW GOLD LEAF ELECTROSCOPE WORKS?

Metal cap of a neutral electroscope

Charged material

Gold leaf repell from the metal plate

Insulated metal rod becomes charged

Lightning- is an electric charge- When the clouds rubs each

other, - negative charges build up at

the bottom of the cloud, the top of the cloud becomes positively charged.

- The Earth’s surface (ground) and the buildings are positively charged.

- Lightning will occur when negative charges move to the positively charged areas of nearby cloud or negative charges jump to positively charged area on the ground or buildings.

Everyday phenomena related to electrostatics

Lightning Conductor: To reduce the production of lightning and if lightning strikes the lightning conductor, electrons will be flow to the ground.

Everyday phenomena related to electrostatics

AIRCRAFT SPARK PLUG

At the end of this topic, a student is able to;• give examples of sources of electrical energy used in

everyday life,• state what electricity is,• state what voltage is,• state what resistance is,• describe the directions of current and electron flow in an

electric circuit.

7.2 Electricity

7.2 Electricity

• Most common sources of electrical energy are generators and electric cells

• Generators: A device that converts mechanical energy to electrical energy

• Electric cells: A device that converts chemical energy to electrical energy

7.2 Electricity

• Current- Flow of electrons (negative

charges) through a circuit.- The rate of flow of charges

•Two forms of electricity that is electrical charges (electrostatic charges) and electric current

Device that can produce electric charges (electrostatic charges) ofvery high voltage on its dome.

Van de Graaff Generator

• When the generator is turned on, the rubber belt turns and rubs against both rollers. The dome becomes charged.• The generator dome positively charged.• A dry dome can hold its charges longer.

Electric Current

Electric Current

Voltage

• Electrical energy that is needed to enable electrons to flow from one point to another in a conductor.

• Voltage is the force that pushes electrons through a circuit to produce electric current.

• Measured by voltmeter, unit: voltan (V)

Resistance

• The characteristics of a material that opposes the flow of electrons.

• High resistance; allows small current to pass through it.

• Resistance depends on:

At the end of this topic, a student is able to;• identify the instrument for measuring current,• identify the instrument for measuring voltage,• state the unit for current,• state the unit for voltage,• state the unit for resistance,• measure current in an electric circuit.

7.3 Electric Current, Voltage and Resistance

7.3 Electric Current, Voltage and Resistance

Current- Measure by using ammeter- The ammeter must be connected in series with other

electrical appliances in a circuit- The SI unit is Ampere (A)

Voltage- Measure by using voltmeter- The voltmeter must be connected in parallel with other

electrical appliances in a circuit- The SI unit is volt (V)

Resistance- Measure by using resistor- The SI unit is ohm (Ω)

At the end of this topic, a student is able to;• design an experiment to study the relationship between

resistance and current,• carry out the experiment to study the relationship

between resistance and current,• describe the effects of the change in resistance on

current, • design an experiment to study the relationship between

voltage and current,• carry out the experiment to study the relationship

between voltage and current,• describe the effect of the change of voltage on current,• state Ohm’s Law

7.4 The Relationship Between Current, Voltage and Resistance


Resistor: - An electrical component

which resists the flow of electric current

- Two types: 1. Fixed resistor - resistances cannot be

changed2. Variable resistor (rheostat)- resistances can be changed to

control the amount of electric current

- The higher the resistance in a circuit, the lower the current flows and vice versa.

Relationship between resistance and current


• The higher the voltage, the greater the electric current that flows through the circuit.

Relationship between voltage and current

• Discovered by George

Simon Ohm in 1827• The current flowing through a

conductor is directly proportional to its voltage

• Voltage = Current x Resistance• V = IR


Ohm’s Law

At the end of this topic, a student is able to;• identify the components of an electric circuit and their

symbols, • draw a diagram of a complete circuit,• build a complete electric circuit• build a parallel circuit• compare and contrast the arrangement of components in

a series circuit and a parallel circuit.

7.5 Electric Circuit


• Electric circuit is a path along which an electric current is able to flow

• A simple electric circuit consists of a cell, a bulb and connecting wires

When the switch is turned on, the electrons will flow and the bulb will light up.The circuit is said to be a complete circuit or a closed circuit.

When the switch is turned off, the circuit breaks off and stops the flow of electrons. The bulb will not light up.The circuit is said to be an incomplete circuit or open circuit.

An electric circuit

Circuit Symbols


Series Circuit Parallel Circuit

Connect its electrical components in sequence from end to end

The current has only one path to flow through

The light bulbs connected in series do not glow as bright as those connected in parallel

If one light bulb is disconnected, the other will not light up because the circuit is incomplete.

Connect its electricalcomponents which are apart but parallel to each other

The current has more than one path to flow through

The light bulbs glow brighter than those connected in series

If one light bulb is disconnected, the other still glows.

At the end of this topic, a student is able to;• describe the current flowing though the components in a

series circuit,• describe the voltages across the components in a series

circuit,• describe the resistance in a series circuit,• explain the advantage and the disadvantage of a series

circuit.

7.6 Series Circuit

7.6 Series Circuit

I = I1 = I2 = I3

V= V1 + V2 + V3

R = R1 + R2 + R3

1. All electrical appliances in the circuit are controlled by one switch

- suitable for the uses of large places.

2. Cells connected in series supply more power.

- the current increases if the number of dry cells increases.

3. All the bulbs share equally the current supplied by dry cells.

- all the bulbs will be equally bright

1. If one bulb fuses, the other bulbs will not light up

- Incomplete circuit2. Each bulb does not receive

the full voltage from the electrical source

3. The current that flows decreases if more electrical components are connected in series.

7.6 Series Circuit

Advantages Disadvantages

At the end of this topic, a student is able to;• describe the current flowing through the components in a

parallel circuit,• describe the voltage across the components in a parallel

circuit,• describe the resistance in a parallel circuit,• explain the advantage and the disadvantage of a parallel

circuit,• compare an contrast a series circuit and a parallel circuit

in terms of current, voltage and resistance

7.7 Parallel Circuit


I = I1 + I2

V= V1 = V2

1. Current flows through more than one path.

2. If one bulb fuses, the other bulbs still light up

- complete circuit

3. Each bulb receives the full voltage supply from the electrical source.

1. Each eIectrical appliances is controlled by different switch.

- Not suitable for the uses of large places

2. Overloading can happen at the main supply if too many electrical appliances are connected in parallel.


Advantages Disadvantages

Comparison Between Series Circuit and Parallel Circuit

Series Circuit Characteristic Parallel Circuit

One Pathways for current to flow

Two or more

The current that flows through any points is the same

I = I1 = I2 = I3

Current

The current that flows through the main path is the sum of the current in all the paths

I = I1 + I2

The total voltage is the sum of the voltages across each bulb

V= V1 + V2 + V3

Voltage

The voltage across each bulb is the same

V= V1 = V2

The effective resistance is the sum of the resistances in all the bulbs

R = R1 + R2 + R3

Resistance

The effective resistance for all bulbs is less than the value of the resistance for one bulb

At the end of this topic, a student is able to;• describe what a magnetic field is, • draw the magnetic field of a bar magnet• draw the directions of the magnetic field.• relate magnetic filed lines and strength of

magnetic field,• explain the use of the a magnet in a compass.

7.8 Magnetism

Characteristics of A Magnet

Only attract magnetic materials (that are made of iron, nickel and cobalt)eg: paper clips, iron nails, needles

Cannot attract non-magnetic materialseg: glass, plastics, paper, wood

• Area around the magnet where its magnetic force can be felt.

• The pattern of a magnetic field consists of curved lines of magnetic force called magnetic field lines

• Magnetic field lines represent the field of a magnet• The pattern of the magnetic field around a magnet can

be shown by using iron filings• The direction of the magnetic field can be shown by

using a compass

Magnetic Field

Characteristics of Magnetic Field Lines

• Begin at the north pole and end at the south pole of the magnet

• The lines do not cut across one another• The magnetic field lines from the same pole repel each

other• The magnetic field is strongest at the poles of a magnet• The stronger the magnetic field, the closer the magnetic

field lines

Magnetic Field of A Bar Magnet

The Compass

• Has a magnetised needle, fixed at its centre of gravity• This enable the compass needle to turn freely on its

spindle, in a horizontal plane• A magnet placed close to a compass causes the needle

to rotate until it is aligned with the magnetic field line that passes through the compass

• Earth’s magnetic field also causes a compass needle to rotate

• The north pole of the compass needle points towards Earth’s magnetic pole that is near the geographic north pole.

• This magnetic pole is actually a magnetic south pole.

At the end of this topic, a student is able to;• relate the current flow through a conductor with

magnetism.• Describe what an electromagnet is.

7.9 Electromagnetism

• The study of the relationship between electricity and magnetism

• When electric current flows through a conductor, a magnetic field is produced around the conductor.

• When the current is turned off, the magnetic field disappears.

Electromagnetism

Electromagnet

• A temporary magnet• A conductor that has similar characteristics with a

magnet when an electric current flows through it• The magnetism of an electromagnet is produced by the

flow of electric current• The electromagnet is usually in the form of coiled wires

(solenoid) that is wound around a soft iron core• Soft iron core: To strength the magnetism of the solenoid• The pattern of magnetic field depends on the shape of

the conductor• The direction of the current determines the direction of

the magnetic field produced

The strength of the magnetic field in an electromagnet can be increased by:

• Using the solenoid with smaller diameter

• Increasing the electric current

• Using more turns of the solenoid

• Using soft iron as core

Magnetic Field Around A Straight Wire

Right Hand Grip Rule

To determine the direction of the magnetic field based on the direction of the current

That’s all for now….

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Electricity