uss module 4 chpt 4 electrcity

Module 4:Gadgets Work Wonders (II)Chapter 4 Electricity

© Copyright Star Publishing Pte Ltd

Chapter 4 Electricity4.1 How can electric circuit symbols be

used to draw circuit diagrams?4.2 How are current, voltage and resistance

measured?4.3 What are the differences between series

and parallel circuits?4.4 What are the applications of series and

parallel circuits in daily life?4.5 How do we use electricity safely? 4.6 How can information on electric power be

used?© Copyright Star Publishing Pte Ltd

ObjectivesDraw simple series and parallel

circuits for daily applications (e.g. table lamps use series circuits, ceiling lamps use parallel circuits)

Label and interpret circuit diagrams which include cells, switches, resistors (fixed and variable),

voltmeters, ammeters, bells, bulbs, lamps and fuses


What are circuit diagrams?Circuit diagrams show how electrical

components are connected together.

In circuit diagrams, electrical components are drawn using symbols.


a circuit diagram

Circuit symbols


Circuits and their circuit diagrams


Circuits and their circuit diagrams


Activity Book LinkActivity 4.1Using Circuit Symbols

Chapter 4 Electricity4.1 How can electric circuit symbols be used

to draw circuit diagrams?4.2 How are current, voltage and

resistance measured?4.3 What are the differences between series




ObjectivesMeasure current, voltage and

resistance using a multimeterState that a voltage is required to

cause a current flowState that the effective resistance

increases when in series circuitsState that the effective resistance

decreases when in parallel circuit


CurrentIn the circuit, the bulb

lights up because there is an electric current flowing in theclosed circuit.

Conventionally, current is described as flowing from the positive terminal of a battery to the negative terminal.


current flows in a closed circuit

What is current?Electric current is due to tiny

electric charges flowing in the circuit.

When the electric charges are flowing at a fast rate, the current will be big.

Therefore, current is the rate of flow of electric charge.© Copyright Star Publishing

Pte Ltd

Symbol and SI unit of currentThe symbol for current is I.

The SI unit of current is the ampere (A). A current of 2 amperes is written as I = 2 A.

A smaller unit of current is the milliampere (mA) where:


How do we measure current?Current can be measured using an

ammeter connected in series.


current can be measured using an ammeter

circuit diagram

Measuring currentBesides the conventional type of ammeter,

another instrument called the multimeter can be used as an ammeter to measure current.


a multimeter

How to use a multimeter as an ammeter To use a multimeter as an ammeter,

(a) insert the black lead into the 'com' terminal,(b) insert the red lead into the 'A' terminal,(c) turn the knob to 'A'.

The value of the current will be displayed on the LCD panel.


using a multimeter as an ammetercircuit diagram

Activity Book LinkActivity 4.2Measuring Electric Current Using a Multimeter

What is Voltage?Voltage is needed for current to flow. The

dry cell provides the voltage, therefore there is a current.


What is Potential Difference? When we measure voltage across

two points, we measure the energy needed to move one unit of electric charge between the two points.

The voltage across these two points is also known as potential difference.


SI unit of voltageThe symbol for voltage is V.

The SI unit of voltage is the volt (V). A voltage of 3 volts is written as V = 3 V.

A smaller unit of voltage is the millivolt (mV) where


How do we measure voltage?Voltage can be measured using a

voltmeter connected in parallel.


measuring voltageacross a bulb with a voltmeter

circuit diagram

How to use a multimeter as a voltmeter To use a multimeter as an ammeter:

(a) insert the black lead into the 'com' terminal,(b) insert the red lead into the 'V.Ω' terminal,(c) turn the knob to 'V'.

The value of the voltage will be displayed on the LCD panel.

When a multimeter is used as a voltmeter, its symbol will be


measuring voltage using a multimeter

circuit diagram

Activity Book LinkActivity 4.3Measuring Voltage Using a Multimeter

What is Resistance?Resistance opposes the flow

of current.

The higher the resistance, the smaller the current.


SI unit of resistanceThe symbol of resistance is R.

The SI unit of resistance is the ohm (Ω).

A resistance of 5 ohms is written as R = 5 Ω.

All electrical components have resistance.


What are resistors?Resistors are electrical components that

provide resistance.

There are two types of resistors:

(a) Fixed resistor: The resistance of a fixed resistor is fixedand cannot be changed.

(b) Variable resistor:The resistance of a variable resistor can be adjusted, usually by turning a knob or moving a slider.


Types of resistors


Resistors in seriesWhen resistors are connected in series, the

effective resistance becomes bigger.

For example, when two10 Ω resistors are connected in series, the effectiveresistance becomes 20 Ω.


Resistors in parallel When resistors are connected in parallel,

the effective resistance becomes smaller.

For example, when two 10 Ω resistors are connected in parallel, the effective resistance becomes 5 Ω.


How to use a multimeter as an ohmmeter To use a multimeter as an ohmmeter:

(a) insert the black lead into the 'com' terminal,(b) insert the red lead into the 'V.Ω' terminal,(c) turn the knob to 'Ω'.

The value of the resistance will be displayed on the LCD panel.


measuring resistance of a bulb using a multimeter

Activity Book LinkActivity 4.4Measuring Resistance Using a Multimeter


to draw circuit diagrams?4.2 How are current, voltage and resistance


and parallel circuits?4.4 What are the applications of series

and parallel circuits in daily life?4.5 How do we use electricity safely? 4.6 How can information on electric power be


ObjectivesIdentify situations in which series

and parallel circuits are used in daily electrical circuitry systems


Identifying Series and Parallel Circuits We use electrical gadgets ( e.g. torch lights and

calculators ) at home every day.

These electrical gadgets may be connected as series or parallel circuits.

(a) The diagram below shows the connection of the batteries, switch and light bulb in a torchlight.

We know that this is a series circuit becausethere is only one path for current to flow.


Identifying Series and Parallel Circuits(b) The figure shows a simplified diagram of the

circuitry used in homes for lighting. We know that this is a parallel circuit because there is

more than one path for current to flow.


ObjectivesIdentify the potential dangers in the

use of electricity at home, such as (i) damaged insulation (ii) overheating of cables (iii) poor or loose connections

State precautions to be taken for safe electrical use at home, such as (i) use of fuses (ii) earthing metal casings (iii) double insulating wires (iv) use of circuit breakers


Electrical HazardsElectricity is very useful but if it is not used properly

and correctly, electrical hazards (dangers) can happen.

Two common electrical hazards are:

(a) electric shock/electrocution(b) electrical fire


warning of electric shock

What is an electric shock?An electric shock happens

when an electric current flows through the body of a person.

Electrocution is death as a result of electric shock.

A current of about 0.1 A may be enough to kill a person.


Causes of Electrical HazardsElectric shock and electrical

fire can be caused by:

(a) damaged insulation(b) overheating of cables(c) damp conditions(d) poor or loose connections


Damaged Insulation To prevent people from touching

bare wires, a layer of insulation is used to cover the wires.

When the insulation is damaged, the wires will be exposed.

A person touching the exposed wires may get an electric shock or be electrocuted.

For safety, damaged insulation should be replaced immediately by a qualified electrician.


damaged insulation

Overheating of Cables

When many appliances are plugged into a single mains socket, the current flowing out of the socket will become large. This may lead to overheating of the cables which may in turn lead to electrical fire.


overloaded power socket

Overheating of Cables

Overheating can also occur if a cable is used to carry a current larger than its current rating. The current rating of a cable is the maximum current that the cable is meant to carry, e.g. a cable with a current rating of 12 A is meant to carry a current of up to 12 A.


fire caused by overheating

Damp Conditions

Water is a conductor of electricity. When a person uses an electrical appliance with wet hands, the water may connect him to high voltage. When this happens, the person will get an electric shock.

Wet skin reduces the resistance of the human body. If a person accidentally touches a high voltage when his skin is wet, a large current will flow through his body, causing severe injury or death.


touching a switch with wet hands can

cause electric shock

Poor or Loose Connections

Poor or loose connections can also cause overheating, especially when exposed wires touch each other (short circuit).

Sparks may also be produced at the points of loose connections. Both overheating and sparks can lead to electrical fires.


badly wired plug

Activity Book LinkActivity 4.7Home Circuitry &Safety Precautions

Safe Use of ElectricityThere are a number of safety features

and devices used to protect us against electrical hazards.

Examples of such safety features and devices are:

(a) fuse(b) circuit breaker(c) earth wire(d) double insulating wire


fuses melt and break thecircuit when current exceeds

their rating

Fuse

A fuse is a safety device which prevents an overly large current from flowing.

It is usually made up of a thin piece of copper wire housed in a case.

The rating of a fuse is the largest current that can flow through it without melting the fuse wire.

When the current becomes larger than the fuse rating, the fuse wire will melt and the circuit will be opened. For example, a fuse rated 5 A will melt when a 6 A current flows through it.


What is the correct fuse to use?The correct fuse to use is one with a rating

just higher than the current that an electrical appliance normally uses.

For example, if a rice cooker uses a current of 4 A, a suitable fuse will be one that is rated 5 A and not one rated 10 A.

In home circuitry, the fuse should be placed along the live wire. In this way, the appliance will be disconnected from high voltage when the fuse is blown.


Circuit Breaker Another safety device found in

all houses in Singapore is the circuit breaker.

The circuit breakers prevent an overly big current from flowing. When the current through a circuit breaker becomes higher than its rating, the circuit breaker will ‘trip’ and cut off the electricity supply.

When the circuit breaker ‘trips’, electricity to some or all parts of the house will be cut off.


resetting a circuitbreaker that has tripped

Earth WireMany electrical appliances

have metal casings. If a fault occurs causing the live wire to touch the metal casing, the metal casing will have a high voltage.

If a person touches the metal casing, he or she will get an electric shock.

To prevent this, a safety feature called earth wire is used.


earthing an electricalappliance

The earth wire is connected to the metal casing to protect users


Double Insulating Wire

Double insulating wires are wires with two layers of insulation. The extra layer acts as a safety feature in case one layer of insulation is damaged.


double insulating wires as safety feature





parallel circuits in daily life?4.5 How do we use electricity safely? 4.6 How can information on electric

power be used?© Copyright Star Publishing Pte Ltd

ObjectivesRelate power to energy transferred and time

taken, using appropriate example, and the equation: Power =

Use information on a label of electrical appliance to determine its power consumption

Give examples of ways to reduce electrical energy wastage at home

Use the equations Power, P = V x I, Energy, E = P x t

Calculate the cost of using electrical appliance where the energy unit is kWh

Understand and use information of electrical bills© Copyright Star Publishing Pte Ltd

TimeEnergy

What is Electrical Power?Electrical power is the rate at which electrical

energy is converted to other forms of energy.

The SI unit of electrical power is the watt (W). larger unit of power is the kilowatt (kW).

An electrical appliance with a power of 800 W will convert 800 J of electrical energy to other forms of energy every second.


Electrical powerElectrical power can be

calculated using the formula:

where P = power (in W)E = energy converted

(in J)t = time (in s)


Worked ExampleAn electric hot plate uses 8000 J

of energy in 4 s. Calculate the power.


P = tE

= 2000 W4s

8000J =

How do we calculate energy? To calculate energy, the formula can also be

rewritten as:

A hairdryer with a power of 1 kW is used for 30 s.Calculate the electrical energy needed


P = E × t

= 30 000 J = 1000 × 30

Using Information on Labels ofElectrical Appliances

Electrical appliances usually come with a label that provides important information. The:

(a) voltage that the appliance should be used with,(b) power rating of the appliance.


the label on an rice cooker tells us that it should be usedwith a voltage between 230 V and 240 V and that the power rating is

630 W

Using Information on Labels ofElectrical Appliances An appliance with high power rating can help us get

things done faster.

E.g. An electric kettle with a power rating of 2000W can boil the same amount of water in a shorter time compared to an electrical kettle with a power rating of 800 W.

When used for the same period of time, an appliance with high power rating will consume more electrical energy and result in higher utility cost.

E.g. Using a 1000 W air-conditioner will consume 10 times more electrical energy than using a 100 W fan.


Energy efficiency labels From 2008, it became a must

for some electrical appliances (air-conditioners, refrigerators and clothes dryers) to carry an energy label.

The number of ticks on the label tells us the energy efficiency rating of the appliance.

Choosing appliances with high energy efficiency helps to reduce energy usage.


energy efficiency labels giveconsumers clear information on

appliances that save energy

The number of ticks indicates how energyefficient an applicance is.


Energy saving lampsThe power rating of the

energy saving lamp is 26 W but the amount of light it can give out is the same as a 150 W incandescent bulb.

Therefore, another way to reduce energy usage is to use energy saving lamps instead of incandescent bulb.


an energy saving lamp

Calculating PowerElectrical power can be

calculated using the formula below:

where P = power (in W),V = voltage (in V),I = current (in A).


Calculating EnergyCombining E = P × t and P = V × I, the

following formula for energy is obtained:

where E = energy (in J)V = voltage (in V)I = current (in A)t = time (in s)


Worked Example 1A light bulb is connected as shown.

Calculate(a) the power of the bulb,(b) the electrical energy used by the bulb in 20 seconds.


Solution:(a) Power = V × I

= 9 × 2 = 18 W

Solution:(b) Electrical energy = V × I × t= 9 × 2 × 20= 360 J

Worked Example 2The kettle is connected to the

240 V mains supply. Calculate the energy used when the kettle is switched on for 10 minutes.


Solution:E = P × t= 800 × (10 ×

60)= 480 000 J

Calculating Electrical ConsumptionAlthough the SI unit of energy is the joule (J),

electrical energy is sold in a more convenient unit called the kilowatt-hour (kWh).

Electrical energy in kWh can be calculated by substituting the power in kW and the time in h into the formula E = P x t.

If the cost of 1 kWh of electrical energy is known, the cost of using an appliance can be calculated.


Worked Example A 2 kW air-conditioner is switched on for 8 h.

(a) Calculate the energy used in kWh.(b) If electricity is sold at 20 cents per kWh, calculate the cost of using the air-conditioner for 8 h.


Solution:(a) E = P × t

= 2 kW x 8 h = 16 kWh

Solution:(b) Cost = 16 × $0.20

= $3.20

ExampleThe figure below shows the utility bill of a

household. The household used 396 kWh of electricity in the month. At a cost of $0.2628 per kWh, the total cost of electricity will be 396 x $0.2628 = $104.07.


Activity Book LinkActivity 4.8Reading Energy Labels and Utility Bills

uss module 4 chpt 4 electrcity

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