85 generation of electricity 09

Sek Men Sains Muzaffar Syah

160

Renewable energy Non-renewable energy

Coal-firedpower station

Gas-fired powerstation

1. Electricity is produces using generators2. A generator has a huge magnet that is turned by a turbine.3. As the magnet turns inside a coil of wire, electricity is produced by

electromagnetic induction.4. Many sources of energy are used to turn these turbines.

1. Coal is burned and the heat energy is used to produce high pressuresteam.

2. The steam produced is used to drive the generators to produceelectricity.

3. The energy changes: chemical energy to heat energy to kinetic energy toelectric energy

1. The burning of natural gas produces high pressure steam that is used todrive turbines.

2. The energy changes: chemical energy to heat energy to kinetic energy toelectric energy

8.5

Electricity is generated in power stations, transmitted (sent) through long-distance cables,and then distributed to consumers.

Listsources ofenergyused togeneratedelectricity

Renewable energy sources : an energy resource that is continuallyreplaced and will not run out.

Non-renewable sources: an energy resource that cannot bereplaced once it has been used.

Describethe variousways ofgeneratingelectricity


161

Hydro-electricpower station

Nuclear powerstation

Biomass

Solar

Wind generator

1. A huge dam is built across rivers to contain water.2. When the water in the dam is allowed to flow down to a lower level, the

kinetic energy of the moving water is used to drive water turbines, whichin turn drive the generators to produce electricity.

3. The energy changes: potential energy to kinetic energy to electricenergy

1. Energy from nuclear fission (splitting of uranium 235 atoms) is used toproduce high pressure steam to drive turbines

2. The energy changes: nuclear energy to heat energy to kinetic energy toelectric energy

1. Biomass includes rotting plants and agricultural wastes such as oil palmkernels and household sewage that are used to produce methane gasand alcohol.

2. Heat energy from the burning of biomass is used to generate electricity

1. Solar energy originates from the sun2. Solar cells use sun energy to generate small electrical energy to power

calculators.3. Solar panels use sun energy to generate heat energy to heat water at

home.

1. High speed wind can be used to turn the blades of large windmills whichwill in turn drive generators to produce electricity.


162

Electrical energy is transmitted from power station to the consumer usinglong transmission cables or power lines.

Electrical energy is transmitted at a ________ voltage and use______________ current.

A step-up transformer which __________ the voltage to as high as110,000 or 132, 000 V.

Step-down transformers are used to ___________ the voltage beforebeing delivered to the consumers.

The long tick cables used as transmission lines are made of copper oraluminium

The heating effect due to the ____________ of the cables causes loss ofelectrical energy as heat energy.

When a current flows in a cable, the power loss, P through heating isP = I

2R where R is the resistance of the cable.

The power loss can be reduced by(a) reducing the resistance of the cables(b) reducing the current in the cables

In order to reduce resistance of the cables, a thick cable is used egcopper or aluminium. But thick cables are very expensive to install andwill be too heavy to be supported securely.

The loss of power in the transmissions of electricity is reduced byreducing the current in the cables.

The power to be transmitted by the cables isP = VI where I = current in the cables,

V = voltage of the cables.

The current in the cables: V

PI

This means that the current in the cables is inversely proportional to thevoltage of the cables for a certain value of power transmission.

When power is transmitted at lower voltage, the current in the cables islarge.

The greater the current in the cables, the greater the power loss.

Describe thetransmissionof electricity

Describe theenergy lossin electricitytransmissioncablesdeduce theadvantage ofhigh voltagetransmission


163

Activity 1: Solve problems involving electricity transmission

1. Find the power loss in a transmissioncable when 20 kW is transmittedthrough a cable of 1.5 Ω(i) at a voltage of 200 V(ii) at a voltage of 10 kV.

What is the effect of the energy lossesin case (i) and (ii)?

2. Electric power is transmitted from apower station to a town by atransmission cables with a totalresistance of 50 Ω. If the powerstation generates 8 MW of power,calculate the power loss in the cablesif power is transmitted(a) at 80 kV(b) at 400 kV

Transmission issues

Alternating current, a.c An alternating current is used in the transmission of electricalenergy at a high voltage because its voltage can be easilyincreased or decreased with transformers.

A transformer cannot function with a direct current.High voltage or lowvoltage?

Electrical power is transmitted at a high voltage so that The current in the cables is smaller The loss of power due to heating of the cables is minimized.

Overhead orunderground?

High voltage cables are the cheapest way of sending power overlong distances.

However, to prevent sparking, the only effective way of insulatingthe cables is to keep huge air spaces around them

The cable have to be suspended from pylons.Cost of cables Copper cables have low resistance but are of high cost

Aluminium cables are usually used as they are light, have lowresistance and cost less.

Charge leakage betweenthe cables and the earth

To prevent charge leakage, the aluminium cables are supported byhigh metal pylons.

The metal support of the pylon is earthed so that it is safe toworkers and the public who come into contact with them.

Danger of being struckby lightning

The pylons carry lightning conductors that are properly fixed intothe ground

The cables are properly fixed with porcelain support so that thecables do not touch the pylons.

Danger of being struckby light aircraft

Lights and special markers must be attached to the pylons.

Theft Stricter laws should be enforced to deter thieves from stealing thealuminium cables.


164

National Grid Network

The National Grid Network is a network oftransmissions lines which link all the major powerstations in the country with all the major users suchas our homes, offices, cities and factories.Electrical energy generated in power stations whereits voltage is increased by using a step-uptransformer to reduce energy loss in the cable beforetransmission via the national grid.At a sub-stations, before reaching the variousconsumers , the high voltage is reduced by usingstep-down transformer.

Advantages of the National Grid Network

1. Easy to manage.Regional control and switching centres enable power to be sent where and when it is needed.

2. Less interruptions and continuouslyRegional control and switching centres allow some stations and lines to be shut down formaintenance work without cutting off the consumers.

3. Reducing cost of generationHigh voltage transmission can reduce the power loss in transmission cables. The cost ofproduction is reduced.

4. Easy to control and regulateAt peak periods of electricity usage , more generators can be switched on. During periods oflow demand ,some generators can be turned off.

Non-renewable sources of energy such as crude oil, coal and natural gasesare fast depleting. Alternatives must be found to replace such sources toensure continuous supply of power in the future.

Renewable energy sources are those which originate from the sun or theearth and will last as long as the Solar System itself.

From the sun: solar panels, solar cells, biomass, wind and wave energy,hydroelectric power

From the earth: geothermal energy, energy from the tides. Renewable sources of energy do not pollute the environment. For renewable sources of energy to be widely used, many hurdles must be

overcomed, especially those related to the economical production anddistribution of power generated from such sources.

Explain theimportanceofrenewableenergy

State the importance of the National Grid Network


165

Energyresource

Advantages Disadvantages

Hydro Renewable energy Water is freely available in large

quantities Clean and does not pollute the

environment Water stored in dams can be used

to irrigate farms Dams can be used to control floods Dams can be promoted as

recreational centre

Building of dams can upset thebalance of the ecosystem

Not suitable in areas wherenatural disasters such asearthquakes may occur

Not suitable in flat and dryregions.

Oil andnatural gas

At present it is still available in largequantities

Can be transported to the locationwhere it is to be used

Non-renewable energy Discharges harmful gases that

can pollute the environment High cost

Solar Renewable energy Freely available Clean and no pollution

Requires very large space tocollect sufficient energy

Lower efficiency in the generationof electricity

Intensity of sunlight is dependentof seasonal changes, climate andlatitude of the region.

High costCoal Plenty of resources available for up

to 200 years Can be transported to the location

where it is to be used

Non-renewable energy Discharges harmful gases that

can pollute the environment

Nuclear A small amount uranium canrelease a large quantity of energy

Minimum discharge of carbondioxide

Reactor in nuclear power stationscan be used to produce usefulradioisotopes to be used inindustry, medicine and agriculture

High cost of building Waste in the form of used fuel

rods which are very hot andhighly radioactive

Hot water discharged causesthermal pollution to theenvironment

Risk of accidents which may leadto the leakage of large amounts ofradioactive substances to theenvironment.

Biomass Renewable energy Reduces problem of disposal of

organic waste

Requires large storage space farfrom human population if thebiomass is animal dung orsewage.

Wind Renewable energy Clean and does not pollute

environment

Requires many windmills whichcover a larger area

Generates high level of noise

Explain the effects on the environment caused by the use of various sources to generateelectricity.


166


167

TUTORIAL 8.5

1. The diagram shows how electricity frompower stations reaches our homes.

The electric current used in X and Y ……A must be d.c.B must be a.c.C can be either a.c. or d.c.D must be d.c. at X and a.c. at Y

2. The diagram shows a model oftransmission of electricity .

What is the type of transformer at P andQ?

P QA Step-up

transformerStep-up

transformerB Step-down

transformerStep-up

transformerC Step-up

transformerStep-downtransformer

D Step-downtransformer

Step-downtransformer

3. The function of a transformer in anelectrical energy transmission system isto … (2006)A increase the powerB reduce the resistanceC change the potential differenceD speed up the time of transmission

4. Which of the following is the reason whyis the alternating current is used totransmits the electricity power from astation?A The electrical power only can be

generated in form of alternatingcurrent

B All electrical appliances in thehouse use alternating current.

C The voltage of alternating currentcan be changed by alternatingcurrent

5. Diagram below shows a model of thetransmission of electrical energy andincludes three identical bulbs, P, Q andR.

Which comparison of the brightness ofthe bulbs is correct? (2007)

P Q RA Brightest Brighter BrightB Bright Brighter BrightestC Brighter Brightest BrightD Brightest Bright Brighter

6. Diagram 26 shows a system for thetransmission of electricity.

Which arrangement of transformers iscorrect? (2008)

TransformerX

TransformerY

TransformerZ

A Step down Step up Step downB Step up Step down Step upC Step up Step down Step downD Step down Step up Step up


7. The diagram shows part of ahydroelectric power station.

(a) State the changes in energy thatoccur during the generation ofelectricity power in the hydroelectricpower station.

_______________________________

_______________________________

(b) Given that 0.5 m3

s-1

of water flowsdown the pipe.[ Density of water = 1 000 kgm

-3]

Determine the power delivered tothe water-turbine, assuming that noenergy is lost in the pipe.

(c) State two advantages of usinghydroelectric power compared thefuel power.

_______________________________

_______________________________

_______________________________

8. The figure shows the stages by whichthe heat is transferred into electricalenergy.(a) Complete the diagram by filling in the

missing word.

(b) A fuel burning power station uses2000 J of fuel energy to generate600 J of electrical energy. The restis wasted as heat. Calculate theefficiency of the power station.

(c) List A gives three energy resourcesused to generate electricity.List B gives environmental problemsthat may caused by using differentenergy resources.Draw a straight line from eachenergy resource in List A to theenvironmental problem it may causein List B. Draw three lines only.

List A List BEnergy Environmentalresource problem that

may be caused

Wind

Oil

Hydropower

Destroy flora
and fauna
Producesa lot of
noise
Producesradioactive

168

Acid rain

radiation


169

9. SPM 2005 Section C(a) Most of our electrical energy comes

from hydroelectric power stations andthermal power stations. These powerstations are connected by cables totransmit electricity to users inindustries, offices, schools and houses.This system is called the national gridnetwork.

(i) State the transformations of energyin a hydroelectric power station.

(ii) Explain briefly the importance of thenational grid network system indistributing electrical energy to theusers.

(b) The Figure shows how a model of anelectric transmission system can be setup in a laboratory. The model consists ofa power station that generates 12 V ofalternating current (a.c) that transmitsthe electrical energy to the users usingtransmission wires and transformers P,Qand R.

You are required to set up a model as inthe Figure above, using the informationin the following table.

Using the information in the Figure andTable above determine:

(i) the number of turns in theprimary coil and the secondarycoil of transformers P,Q and Rand show your working.

(ii) the type of core you will use inall the transformers and justifyyour choice.

(iii) the material you will use for thetransmission wire and justifyyour choice.

(c) If the resistance of the transmissionwires between transformers P and Qin the model at Diagram above is 30Ω, calculate:

(i) the current that flows inthe transmission wire.

(ii) The power loss due to theheating effect of thecurrent in thetransmission wire.

Numberof turnsof the coil

1 2001 000

500400160120

Type oftransformerCore

Solidcuprum core

Laminatedcuprum core

Solid softiron core

LaminatedSoft ironcore

Material oftransmissionline

Constantan

Copper

Aluminium

Nichrome

85 generation of electricity 09

Documents