Chapter 7: ElectricityChapter 7: Electricity

7.1 Electrostatics7.2 Electricity7.3 Electric Current, Voltage & Resistance7.4 Relationship Between Current, Voltage & Resistance7.5 Parallel & Series Circuits7.6 Magnetism7.7 Electromagnetism

(a) Charging by friction(a) Charging by friction

Electrostatics Electrostatics – the study of static electrical charges.– the study of static electrical charges. There are There are 2 types of electrical charges2 types of electrical charges – positive (+) & – positive (+) &

negative (-).negative (-). A material when rubbed (digosok), may be +ve/-ve A material when rubbed (digosok), may be +ve/-ve

charged according to the type of material it is made of.charged according to the type of material it is made of. +ve charges & -ve charges have the ability to attract or +ve charges & -ve charges have the ability to attract or

repel other charges.repel other charges. Charges of the Charges of the same type (like charges)same type (like charges) repelrepel one one

another.another. Charges of Charges of different types (unlike charges) attractdifferent types (unlike charges) attract one one

another.another.

7.1 Electrostatics

(b) (b) Production Of Static Electric Production Of Static Electric ChargesCharges

Static electric charges can be produced on a Static electric charges can be produced on a neutral object by neutral object by rubbingrubbing it with a different material it with a different material like like a woollen/silkclotha woollen/silkcloth..

When 2 different materials are rubbed, When 2 different materials are rubbed, electronelectron transfer occurs. One of the material receives transfer occurs. One of the material receives electrons & the other loses electrons.electrons & the other loses electrons.

The material that The material that receives electronsreceives electrons becomes –ve becomes –ve charged (more elecrons than protons).charged (more elecrons than protons).

The material that The material that loses electronsloses electrons becomes becomes +ve+ve charged (more protons than electrons)charged (more protons than electrons)

An object is An object is neutralneutral (the number of +ve charges (the number of +ve charges (protons) = the number of –ve charges(electrons))(protons) = the number of –ve charges(electrons))

Materials that are positively charged & negatively chargedMaterials that are positively charged & negatively charged

Material positively chargedMaterial positively charged Material negatively chargedMaterial negatively charged

GlassGlass Silk clothSilk cloth

Cellulose acetateCellulose acetate Silk cloth, woollen clothSilk cloth, woollen cloth

Silk cloth (kain sutera)Silk cloth (kain sutera) Hard rubberHard rubber

Rubber (balloon)Rubber (balloon) NylonNylon

Woollen cloth (kain bulu)Woollen cloth (kain bulu) Rubber (balloon), polythene Rubber (balloon), polythene (politena)(politena)

Examples of materials that are easily charged are cellulose acetate (+ve) & polythene (-ve). The type of charge produced depends on the types of material that are rubber together

(c)(c) Everyday Phenomena Related To Everyday Phenomena Related To Static Static Electrical Charges Electrical Charges

Occurrence(kejadian) of lightningOccurrence(kejadian) of lightning Spark plug (Palam pencucuh)Spark plug (Palam pencucuh) Nylon clothes (pakaian nilon)Nylon clothes (pakaian nilon) Combing hairCombing hair Electronic lighter (pemetik api elektronik)Electronic lighter (pemetik api elektronik)

(d) Safety Measures Related To (d) Safety Measures Related To Static Static

Electric ChargesElectric Charges Lightning conductor (Konduktor kilat)Lightning conductor (Konduktor kilat) Oil tanker (Lori tangki minyak)Oil tanker (Lori tangki minyak) Aeroplane (pesawat terbang)Aeroplane (pesawat terbang)

7.2 Electricity7.2 Electricity

Electricity – a form of energyElectricity – a form of energy

Sources of electrical energy:Sources of electrical energy: solar cell (sel suria)solar cell (sel suria) dry cell (sel kering/bateri) dry cell (sel kering/bateri) wet cell (accumulator)wet cell (accumulator) power generatorpower generator lithium iron batterylithium iron battery cadmium batterycadmium battery

Electric CurrentElectric Current Electric current/Arus elektrikElectric current/Arus elektrik ( ( I )I ) : : the rate of the rate of

flowflow of electrons (kadar pengaliran cas-cas –ve) of electrons (kadar pengaliran cas-cas –ve)

Voltage/Voltan ( Voltage/Voltan ( V V ):): potential difference between potential difference between 2 points/the electrical force needed to move 2 points/the electrical force needed to move electrons between 2 points (Tenaga elektrik yg electrons between 2 points (Tenaga elektrik yg diperlukan utk memblhkan elektron mengalir dr. diperlukan utk memblhkan elektron mengalir dr. 1 titik ke 1 titik yg lain).1 titik ke 1 titik yg lain).

Resistance/Rintangan ( Resistance/Rintangan ( R R ):): property of material property of material that prevents or resists the flow of electronsthat prevents or resists the flow of electrons through the material (Sifat bahan yang through the material (Sifat bahan yang menghalang pengaliran elektron).menghalang pengaliran elektron).

The flow of electrical charges produces electric current

7.3 Electric Current, Voltage, 7.3 Electric Current, Voltage, ResistanceResistance

Quantity Quantity Unit Unit Tools to Tools to measuremeasure

Electric currentElectric current Ampere (A)Ampere (A) AmmeterAmmeter

(Connect in (Connect in series)series)

Voltage Voltage Volt (V)Volt (V) VoltmeterVoltmeter

(Connect in (Connect in parallel)parallel)

Resistance Resistance Ohm Ohm (Ω)(Ω)

An ammeter must be connected in series with the electrical sources/component

7.4 Relationship Between 7.4 Relationship Between Current, Voltage & Current, Voltage &

ResistanceResistance

Ohm’s Law states that the current flowing Ohm’s Law states that the current flowing through a conductor is directly proportional through a conductor is directly proportional to its voltage. to its voltage.

Voltage (V)(V) = Electric current (A) x Resistance (Ω)(Ω)

Voltage (V) Resistance (Ω)(Ω) = --------------- Current (A)

7.5 Parallel & Series 7.5 Parallel & Series CircuitsCircuits

- A - A complete circuit (litar lengkap)complete circuit (litar lengkap) is a circuit is a circuit in which the swich is in which the swich is closed or ‘on’closed or ‘on’ so that so that the the current can flow continuously.current can flow continuously.

Open circuitOpen circuit – no current flows – no current flows Close circuitClose circuit – current flows – current flows

There are There are 2 types2 types of electric circuit: of electric circuit: (a) Series circuit (Litar bersiri)(a) Series circuit (Litar bersiri) (b) Parallel circuit (Litar selari)(b) Parallel circuit (Litar selari)

Symbols Of Electric Circuit ComponentsCircuit Component Symbol Use

Electric cell/series cells/parallel cells

Electric sources that supply electrical energy

Fuse Melts when excess current flows through it

Switch Completes/breaks a circuit

Bulb/lamp Light up when current flows through it

Fixed Resistor Reduces current flows

Voltmeter Measures voltage (V)

Variable resistor/rheostat Controls current flow

Galvanometer Detects current

Ammeter Measures current (A)

Earth connector Sends leaked (terbocor) current to the earth

(a) Series Circuit (Litar Bersiri) (Draw Figure 7.12 o page 125)

Characteristics: i) Current that flows through any point is the

same as the current flows in the circuit I total = I1 = I2 = I3

ii) Voltage across the circuit is equal to the sum of voltage across each resistor

V total = V1 + V2 + V3

iii) Total resistance, R, is equal to the sum of resistance in each resistor

R total =R1 + R2 + R3

(Refer to Table 7.2 on page 129 in the textbook)

Advantages & Disadvantages Of Series Circuit

Advantages Disadvantages

One switch control all appliances /bulbs

If a bulb is burnt, other bulb will not light up

Supplies more power

V= V1 + V2 + V3

If more bulbs added, it will become dimmer

Current increases when num. of dry cell increases

I = I1 = I2 = I3

Current decreases if more appliances/bulbs is connected

All bulbs to light up with the same brightness

Dry cell discharge quickly

(b) Parallel Circuit (Litar Selari)

Characteristics:a) Total current is equal to the sum of current passing through the components

I total = I1 + I2 + I3

b) Voltage across each component is the same as

voltage supplied by power source

V total = V1 = V2 = V3

c) Total resistance is less than the sum of resistances of the components

1/Rtotal = 1/ R1 + 1/R2

(Refer to Table 7.2 on page 129 in the textbook)

Advantages & Disadvantages Of Parallel Circuit

Advantages Disadvantages

Bulbs can be controlled separately.

Current produced is the same as the current produced by one cell

If one bulb is burnt, the other bulbs still light up.

Not suitable for used in large hall

The brightness of bulbs is not affected even if more bulbs added

The voltage doen’t +/- if cells of the same type are connected

Parallel cells last longer

7.6 Magnetism (Kemagnetan)

Magnetic Field(Medan Magnet)

The region around a magnetIn which the magnetic effect can be felt

Pattern & direction magnetic field lines that can be

(a) Traced by using iron filings(a) Detected by using a compass

The pattern & direction of magnetic field

7.7 Electromagnetism

When a conductor is connected to an electric current, it can produce a magnetic effect. The conductor becomes an electromagnet.

Electromagnet refers to the magnetic properties shown by an electromagnet.

A straight wire carrying a current has a circular magnetic field around it.

The direction of a magnetic field is determined by the Right Hand Grip Rule.

A straight wire carrying a current has a circular magnetic field around it.

The direction of a magnetic field is determined by the Right Hand Grip Rule.

The thumb points in the direction of the current (positive to negative). The four fingers show the direction of the magnetic field.

current

thumb

direction of magnetic field

Right Hand Grip Rule

Magnetic field around a straight wire

wire

magnetic field

Magnetic field around a wire carrying electric current using right-hand grip rule

A coil of current-carrying wire induces a magnetic field.

The coil which creates a type of electromagnet is called a solenoid (wire coil).

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

(a) increasing the number of turns of the solenoid (b) increasing the electric current that flows (c) reducing the diameter of the solenoid

Electromagnets are used in devices like telephones, electromagnetic cranes (kren), telegraph machines, electric bells & car horns.

A coil of current-carrying wire induces a magnetic field.

The coil which creates a type of electromagnet is called a solenoid (wire coil).

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

(a) increasing the number of turns of the solenoid (b) increasing the electric current that flows (c) reducing the diameter of the solenoid

Electromagnets are used in devices like telephones, electromagnetic cranes (kren), telegraph machines, electric bells & car horns.

Magnetic field induced by a coil of current-carrying wire

magnetic field

current flow

Current induces a magnetic field in the electromagnets of an electric bell.

The hammer is pulled up to strike the gong.

As the hammer moves, it breaks the circuit.

It falls back to its original position.

Current induces a magnetic field in the electromagnets of an electric bell.

The hammer is pulled up to strike the gong.

As the hammer moves, it breaks the circuit.

It falls back to its original position.

electromagnets

make-and-breakcircuit contact

An electric bell