Chapter 7: Electricity And Magnetism

CHAPTER 7CHAPTER 7

ELECTRICITY AND ELECTRICITY AND MAGNETISMMAGNETISM

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Charging by FrictionCharging by Friction

Electrostatics - study of static electrical charges. Electrostatics - study of static electrical charges. Two types of electrical charges:Two types of electrical charges:

positive (+) positive (+) negative (-). negative (-).

When two different materials are rubbed against one When two different materials are rubbed against one another, the friction that acts between the materials another, the friction that acts between the materials causes one of the materials to become positively causes one of the materials to become positively charged and the other to become negatively charged. charged and the other to become negatively charged.

Charging by FrictionCharging by Friction

Charging by frictionCharging by friction

The electroscopeThe electroscope

Used for the detection of electrical charges. Used for the detection of electrical charges. The steps in the use of an electroscope to The steps in the use of an electroscope to

detect an electrical charge. detect an electrical charge.

(a)(a) Bring the object under test close to the Bring the object under test close to the metal cap of a neutralmetal cap of a neutral electroscope.electroscope.

(b)(b) If the gold leaf is deflected the object is If the gold leaf is deflected the object is charged. If the gold leaf is not deflected as charged. If the gold leaf is not deflected as the object is neutral or not charged.the object is neutral or not charged.


The electroscopeThe electroscope

Detecting on electrical chargeDetecting on electrical charge

Charged objectCharged object Neutral objectNeutral object

Materials which have an equal number of Materials which have an equal number of positive and negative charges are classified as positive and negative charges are classified as neutral or not charged. neutral or not charged.

When two different materials rub against one When two different materials rub against one another, charging by friction occurs another, charging by friction occurs

During the charging by friction, negative During the charging by friction, negative electrical charges are transferred from one electrical charges are transferred from one material to another material. material to another material.

The material which gains negative charges The material which gains negative charges becomes negatively charged while the material becomes negatively charged while the material which loses its negative charges becomes which loses its negative charges becomes positively charged.positively charged.

If two charged objects are held close to each If two charged objects are held close to each other, there are force between themother, there are force between them

Like charge repelLike charge repel Unlike charges attractUnlike charges attract


Static electrical chargesStatic electrical charges

Everyday Phenomena Related to Everyday Phenomena Related to Static Electrical ChargesStatic Electrical Charges

LightningLightning To reduce electrical charges on tankers body, a To reduce electrical charges on tankers body, a

tanker has a metal chain attached to its back tanker has a metal chain attached to its back Electrical charges from the tanker flow into Electrical charges from the tanker flow into

the ground through the metal chain without the ground through the metal chain without causing any sparkcausing any spark

When an aeroplane is being refuelled, the When an aeroplane is being refuelled, the aeroplane has a metal cable connected to the aeroplane has a metal cable connected to the ground.ground.

Everyday Phenomena Related to Everyday Phenomena Related to Static Electrical ChargesStatic Electrical Charges

Sources of Electrical EnergySources of Electrical Energy

Electrical energy or electricity is a form of Electrical energy or electricity is a form of energy. energy.

Sources of electrical energy: Sources of electrical energy: the solar cellthe solar celldry cell, dry cell, wet cellwet cellpower generator, power generator, lithium ion battery lithium ion battery cadmium battery.cadmium battery.

Sources of Electrical EnergySources of Electrical Energy

Wet cellWet cell

Solar cellSolar cell

Electric CurrentElectric Current

A continuous flow of negative charges or electrons is A continuous flow of negative charges or electrons is needed to produces electric current.needed to produces electric current.

When a Van de Graaff generator is switched on, When a Van de Graaff generator is switched on, electrical charges will gather on the dome electrical charges will gather on the dome

If the charged dome is connected to a galvanometer If the charged dome is connected to a galvanometer and the Earth, electrical charges will then flow and the Earth, electrical charges will then flow through the galvanometer producing electric current through the galvanometer producing electric current which causes the pointer of the which causes the pointer of the galvanometer to galvanometer to deflectdeflect

The flow of electrical charges The flow of electrical charges produces electric currentproduces electric current

Van de GraffVan de Graff generatorgenerator

CurrentCurrentflowflowgalvanometergalvanometer

An electric current (I), is defined as the rate of An electric current (I), is defined as the rate of flow of electrons. flow of electrons.

Voltage (V): the electrical force needed to Voltage (V): the electrical force needed to move electrons between two points or move electrons between two points or potential difference between the two points.potential difference between the two points.

The resistance The resistance (R): (R): the property of a material the property of a material that prevents or resists the flow of electrons that prevents or resists the flow of electrons through the materialthrough the material


The electric currentThe electric current

ELECTRIC CURRENT, VOLTAGE ELECTRIC CURRENT, VOLTAGE AND RESISTANCEAND RESISTANCE

The S.I. unit of electric current is ampere (A).The S.I. unit of electric current is ampere (A). Electric current is measured using an ammeter Electric current is measured using an ammeter An electrical source or component, must be An electrical source or component, must be

connected in series to measure the electric connected in series to measure the electric current that flows through the electrical source current that flows through the electrical source or componentor component

The S.I. unit of voltage is The S.I. unit of voltage is volt volt (V). Measured (V). Measured using a using a voltmeter voltmeter To measure the voltage of an electrical To measure the voltage of an electrical

source or component, a voltmeter must be source or component, a voltmeter must be connected in connected in parallel parallel across the electrical across the electrical source or component source or component

An electrical conductor has the characteristic An electrical conductor has the characteristic of resisting the flow of electric current. The of resisting the flow of electric current. The S.I. unit of resistance is the S.I. unit of resistance is the ohm ohm ((ΩΩ).).

Voltmeter Voltmeter Ammeter Ammeter


RELATIONSHIP BETWEEN CURRENT, RELATIONSHIP BETWEEN CURRENT, VOLTAGE AND RESISTANCEVOLTAGE AND RESISTANCE

The resistance of a material is the ability of the The resistance of a material is the ability of the material to resist the flow of electric current material to resist the flow of electric current through it.through it.

Standard resistor - type of electrical Standard resistor - type of electrical component that has a fixed resistance component that has a fixed resistance

Variable resistor or rheostat has a resistance Variable resistor or rheostat has a resistance that can be changed that can be changed used to control the brightness of a bulb, the used to control the brightness of a bulb, the

speed of a fan and the loudness of a radio.speed of a fan and the loudness of a radio.

Standard resistorsStandard resistors Variable resistorsVariable resistors

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

Ohm's law is given by the following formula:Ohm's law is given by the following formula:

Voltage (V)Voltage (V)

Current (A)Current (A)

R is a constant known as the resistance of the R is a constant known as the resistance of the conductor.conductor.

= Resistance (Q)= Resistance (Q) V = RV = R II

Voltage versus current graphVoltage versus current graph



Gradient of graph:Gradient of graph:= = voltagevoltage currentcurrent= resistance= resistance

Example:Example:

1.1. What is the resistance of a resistor in an What is the resistance of a resistor in an electric circuit if the dry cell supplies 1.5 V electric circuit if the dry cell supplies 1.5 V and the ammeter gives a reading of 0.5 A?and the ammeter gives a reading of 0.5 A?

Step 1: R= Step 1: R= VV II

Step 2: R= Step 2: R= 1.51.50.50.5

Step 3: R= 3 Step 3: R= 3 ΩΩ

PARALLEL AND SERIES PARALLEL AND SERIES CIRCUITSCIRCUITS

An electric circuit is the path along which electrical An electric circuit is the path along which electrical charges flow. charges flow.

It consists of a source of electrical energy such as:It consists of a source of electrical energy such as:dry cell, connecting wires and one or more dry cell, connecting wires and one or more

electrical components such as switches, resistors, electrical components such as switches, resistors, ammeters, voltmeters or bulbs.ammeters, voltmeters or bulbs.

In a closed circuit, electric current flows out from the In a closed circuit, electric current flows out from the source, round the circuit and back to the source. source, round the circuit and back to the source.

If there are any breaks in an electric circuit, it is If there are any breaks in an electric circuit, it is known as an open circuit. known as an open circuit.

Symbols of some common Symbols of some common electrical componentselectrical components

Electrical Electrical componentcomponent

Symbol Symbol

1. Electric cell1. Electric cell

2. Fuse2. Fuse

3. Swith3. Swith

4. Bulb 4. Bulb

5. Resistor5. Resistor

Electrical Electrical componentcomponent

Symbol Symbol

6. Variable 6. Variable resistorresistor

7. Rheostat7. Rheostat

8. Voltmeter8. Voltmeter

9. Galvanometer9. Galvanometer

10. Ammeter 10. Ammeter

Series and Parallel CircuitsSeries and Parallel Circuits

A series circuit:A series circuit:The electric current from The electric current from

the battery flows through the battery flows through bulb A, then bulb B bulb A, then bulb B followed by bulb C and followed by bulb C and then returns to the battery. then returns to the battery.

The electric current flows The electric current flows through one path only.through one path only.

AA

AA

++

++ --

--


The Advantage and Disadvantage The Advantage and Disadvantage of a Series Circuitof a Series Circuit

The advantage :The advantage :each bulb lights up with equal brightness- because each bulb lights up with equal brightness- because

the amount of current that flows through each bulb the amount of current that flows through each bulb is the same.is the same.

Disadvantage:Disadvantage:If one or more bulbs burn out, the whole circuit is If one or more bulbs burn out, the whole circuit is

disconnected. disconnected. If the number of bulbs is increased, the brightness If the number of bulbs is increased, the brightness

of the bulbs decreases. of the bulbs decreases.

Series and Parallel CircuitsSeries and Parallel Circuits A Parallel Circuit:A Parallel Circuit:

electric current from the battery electric current from the battery can flow through bulb A and can flow through bulb A and then return to the battery. then return to the battery.

Similarly, the electric current Similarly, the electric current from the battery can also flow from the battery can also flow through bulb B or bulb C and through bulb B or bulb C and then return to the battery. then return to the battery.

The electric current flows The electric current flows through three different paths.through three different paths.

++ --

AA

BB

CC

The Advantage and Disadvantage The Advantage and Disadvantage of a Parallel Circuitof a Parallel Circuit

The advantages:The advantages:The bulbs can be controlled separately. The bulbs can be controlled separately. If one bulb burns out, the other bulbs are not If one bulb burns out, the other bulbs are not

affected. affected. If the number of bulbs connected in parallel is If the number of bulbs connected in parallel is

increased, the brightness of the bulbs is not increased, the brightness of the bulbs is not affected. affected.

The disadvantage: The disadvantage: The current produced is the same as the current The current produced is the same as the current

produced by only one cell. produced by only one cell.

I I totaltotal

Differences between series and parallel Differences between series and parallel circuitscircuits

Physical quantityPhysical quantity SeriesSeries ParallelParallel



Resistance (Resistance (ΩΩ))

II11 II22

IItotal total = I= I11 = 1 = 122

VVtotal total = V= V11 + V + V22

RRtotal total = R= R11 + R + R22

IItotal total = I= I11 + 1 + 122

VVtotal total = V= V11 = V = V22

1/R1/Rtotal total = 1/R= 1/R11 + 1/R + 1/R22

II11

II22

IItotaltotal

VVtotaltotalVVtotaltotal

VV22VV11

VV11

VV22

RR11

RR11

RR22

RR22

RRtotaltotalRRtotaltotal

Test Test

1.1. An ammeter in a circuit gives a reading of 4 An ammeter in a circuit gives a reading of 4 A. Both bulb have 1A. Both bulb have 1 ΩΩ and 2 and 2 ΩΩ resistance resistance respectively. What is the voltage value?respectively. What is the voltage value?

2.2. What ampere value will be recorded in the What ampere value will be recorded in the ammeter for the circuit above?ammeter for the circuit above?


Magnetic FieldMagnetic Field The region around a magnet The region around a magnet

where its magnetic forces act. where its magnetic forces act. The magnetic field pattern of The magnetic field pattern of

a bar magnet is shown in a bar magnet is shown in Figure Figure a side.a side.

Magnetic field lines:Magnetic field lines: lines lines that represent the direction of that represent the direction of the magnetic fieldthe magnetic field. . Run from the north pole to Run from the north pole to

the south pole of a magnet. the south pole of a magnet.

Pattern and Direction of Pattern and Direction of Magnetic FieldMagnetic Field

Can be mapped using iron filings. Can be mapped using iron filings. The pattern of the magnetic field around a bar The pattern of the magnetic field around a bar

magnet is shown by the arrangement of the magnet is shown by the arrangement of the iron filings. iron filings.

The pattern and direction of a magnetic field The pattern and direction of a magnetic field can be mapped using aplotting compass. can be mapped using aplotting compass.

The patern and direction The patern and direction of a magnetic fieldof a magnetic field

Right-hand grip ruleRight-hand grip rule

To determine the direction To determine the direction of the magnetic field around of the magnetic field around a wire.a wire.

When the thumb points When the thumb points along the direction of the along the direction of the current, the fingers wrapped current, the fingers wrapped around the wire point to the around the wire point to the direction of the magnetic direction of the magnetic field.field.


ElectromagnetElectromagnet Produced when an electric current flows through Produced when an electric current flows through

a conductor. a conductor. An electromagnet loses all its magnetism when An electromagnet loses all its magnetism when

no electric current flows through the conductor. no electric current flows through the conductor. Used in cranes to lift heavy iron and steel loads Used in cranes to lift heavy iron and steel loads

such as cars. such as cars. Used to sort out scrap iron or steel from non-Used to sort out scrap iron or steel from non-

magnetic materials at scrap yardsmagnetic materials at scrap yards

TESTTEST

1.1. If a 2 If a 2 ΩΩ resistor is used, what will be the resistor is used, what will be the reading recorded in the ammeter if the dry reading recorded in the ammeter if the dry cell supplies a voltage of 3 V?cell supplies a voltage of 3 V?

R=V/IR=V/I

I=V/RI=V/R

I=3/2I=3/2

=1.5 A=1.5 A

2.2. If 0.5 A of current flows through a 2 If 0.5 A of current flows through a 2 ΩΩ resistor, calculate the voltage in the electric resistor, calculate the voltage in the electric currentcurrent

R=V/IR=V/I

V=I x R\V=I x R\

= 0.5 X 2= 0.5 X 2

= 1.0 V= 1.0 V


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Chapter 7: Electricity And Magnetism