T 2010 - Unit 3 Work,Energy & Power Page 1

Work, Energy and Power. Work - Definition Of Work

Work is the transfer of energy.

If you put energy into an object, then

you do work on that object. If an

object is standing still, and you get it

moving, then you have put energy

into that object.

If you lift a suitcase to place it in its

compartment, we say that you are

doing work. Work is done when a

force moves against an opposing

force. The opposing force is often

gravity or friction. You can only do

work if you have some energy. If the

suitcase falls you will feel this energy!

Energy is the capacity for doing work.

Both the Work Done and the Energy used to do the work

are measured in Joules (J).

The amount of work is calculated by multiplying the force

times the displacement. That formula looks like this:

Work Done = Force x distance moved

(In the direction of the force)

or W = F x s

The unit of Work is the Nm or Joule (J).

The work done depends on the size of the force and distance.

No work is done when there is no motion.

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Power

Work has nothing to do with the amount of

time that a force acts to cause a

displacement.

Sometimes, the work is done very quickly

and other times the work is done rather slowly. For example,

two pupils can take different times to climb a stair. The two

people might do the same amount of work. However the pupil

who does the work in considerably less time has a greater

power rating than the slower one.

Power is the rate at which work is done. It is the

work/time ratio.

The standard metric unit of power is the Watt.

A Watt is equivalent to a Joule/second. (J/s)

For historical reasons, the horsepower is occasionally used to

describe the power delivered by a machine. One horsepower

is equivalent to approximately 750 Watts. Most machines are

designed and built to do work on objects. All machines are

typically described by a power rating.

Energy

Everything happens because of energy. Energy is a

quantity that is often understood as the ability to perform

work. Without energy we could not live or move. We use

energy to keep alive, for entertainment and transportation.

Work Done = Energy transferred

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DIFFERENT FORMS OF ENERGY

Energy comes in many different

forms. We cannot create nor

destroy energy. The only thing

which is possible is for energy to

change from one form into another.

This is known as the Principle of

Conservation of Energy which

states that:

Energy can neither be created nor destroyed. It can

only be changed from one form to another.

When a light bulb is

switched on, electrical

energy is used by the bulb

and this form of energy is

converted to light ( and

some heat). Changes from

one form of energy to

another can be shown by

energy flow diagrams.

People have learned how to change energy from one form to

another so that we can do work more easily and live more

comfortably. Note that the following forms of energy are all

measured in

Joules.

This flow

diagram

represents the

energy

changes in a

mobile phone.

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1. Kinetic Energy (KE)

This is the energy an object

has because of its motion.

For example, a moving train, a

moving ship and a moving lorry

all have Kinetic Energy.

The K.E. of a moving object can

be calculated using the following

formula.

KE = ½ x mass x velocity2

or KE = ½ m v2

K.E. is the Kinetic Energy in Joules (J),

m is the mass of the object in kg ,

v is the velocity/speed of the object in m/s .

If an object is not moving its Kinetic Energy is zero.

2. Potential Energy (Stored Energy)

Potential Energy [measured in J], is the energy stored in an

object. There are 3 kinds of Potential Energy or PE:

A) CHEMICAL PE

Fuels (wood, petrol, etc.), food and batteries

are all examples of chemical energy. They have

energy stored within them.

Fuels release energy when they are burnt to give heat and

light energy.

Batteries have stored energy which is converted to electrical

energy.

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B) ELASTIC PE

A stretched elastic band or spring

has stored energy called elastic

PE. This energy is released as

motion when the stretching force

is removed.

C) GRAVITATIONAL PE

Potential energy is energy that is stored

within a system. The further away the

body is from the surface of the earth,

the larger would be its potential energy.

Potential Energy (P.E.)

The gravitational potential energy can

be calculated using the following

equation.

PE = mass x gravity x height

or PE = m g h Where

m is the mass of the object in kg,

g is the acceleration due to gravity (on earth g =10

N/kg),

h is the height of the object above the earth surface

in metres (m).

The following represents the PE / KE changes for a

rollercoaster.

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3. Heat Energy (Thermal Energy)

Heat energy is the internal energy in substances.

It depends on the vibration and movement of the

atoms and molecules within substances.

4. Electrical Energy

Electrical energy is the movement of

electrical charges. Everything is made of

tiny particles called atoms. Atoms are

made of even smaller particles called

electrons, protons, and neutrons.

Applying a force can make some of the

electrons move. Electrical charges

moving through a wire is called

electricity. Lightning is another example

of electrical energy.

5. Nuclear Energy

Nuclear Energy is energy stored in the

nucleus of an atom– the energy that

holds the nucleus together. The energy

can be released when the nuclei are

combined or split apart. Nuclear power

plants split the nuclei of uranium atoms

in a process called fission. The sun

combines the nuclei of hydrogen atoms in a process called

fusion.

6. Light Energy

Light energy forms part of the

electromagnetic spectrum. Light is one type

of energy that we can see with our eyes.

Some of the energy coming from the Sun is

in the form of Light rays.

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7. Sound Energy

This is the movement of energy

through a medium by means of waves.

Sound is produced when a force causes

an object or substance to vibrate––the

energy is transferred through the

substance in a wave.

Falling objects

A football of mass 0.5kg is dropped from a height of 4m.

Find the velocity with which the ball hits the ground.

P.E. at the top = K.E. at the bottom

mgh = ½ mv2

v2 = 2gh = 2x10x4 = 80

v = √80 = 8.94m/s

From v2=2gh we note that neglecting air-resistance,

any two masses dropped from the same height will

reach the ground at the same time.

4m

2m

P.E. = 20J (mgh = 0.5x10x4 = 20J)

K.E. = 0J (ball is not moving)

P.E. = 10J (mgh = 0.5x10x2 = 10J)

K.E. = 10J (total energy must remain 20J)

P.E. = 0J (ball is on the ground)

K.E. = 20J (total energy must remain 20J)

P.E. at the top = K.E. at the bottom

4m

2m

P.E. = 20J (mgh = 0.5x10x4 = 20J)

K.E. = 0J (ball is not moving)

P.E. = 10J (mgh = 0.5x10x2 = 10J)

K.E. = 10J (total energy must remain 20J)

P.E. = 0J (ball is on the ground)

K.E. = 20J (total energy must remain 20J)

P.E. at the top = K.E. at the bottom

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Efficiency

A machine would be 100% efficient if all the input energy

would be changed to required useful output energy. However

no machine is 100 % efficient since some of the input energy

is lost as heat in the transformation process. For example a

car is only 25 % efficient since only 25 % of the input energy

is converted to kinetic energy while the rest is lost as heat

and sound from the engine.

An energy saving light bulb emits 2880 J of light out of the

total 3000 J of electrical energy it consumes. Calculate the

efficiency of the bulb.

Efficiency = Output Energy / Input Energy * 100%

= 2880 / 3000 * 100%

= 96 %

That means that the bulb gives 96 J of light for every 100 J of

electrical energy it consumes.

When a machine transfers energy from one form into

another, some of it is used up (useful energy) while some of

it is wasted (wasted energy) as heat or sound. This affects

the efficiency of the machine.

If a machine is 100% efficient, then

Energy input = Energy output

Renewable and non-renewable sources of energy

• Non-renewable sources of energy are sources of

energy that can be used only once.

•Renewable sources of energy are sources of

energy that can be used more than once.

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Renewable sources of Energy

These consist of energy sources generated from natural

resources. These energy sources can be used more than once

and we have unlimited reserves of these resources.

Wind power is the conversion of wind energy into a useful

form, such as electricity, using wind turbines.

Wave power is the transport of energy by ocean surface

waves, and the capture of that energy to do useful work.

Tidal power, sometimes called tidal energy, is a form of

hydropower that converts the energy of tides into electricity

or other useful forms of power. Tides are more predictable

than wind energy and solar power.

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Solar energy consists of the light and heat rays that come

from the Sun. Only a small fraction of the available solar

energy is used because most of it is radiated into the space

around the Earth. Solar power technologies provide electrical

generation by means of heat engines or photovoltaic’s. Solar

applications includes space heating and cooling through solar

architecture, potable water via distillation and disinfection,

day lighting, hot water, thermal energy for cooking, and high

temperature process heat for industrial purposes

Hydroelectricity is electricity generated by hydropower,

i.e., the production of power through use of the gravitational

force of falling or flowing water. It is the most widely used

form of renewable energy. Once a hydroelectric complex is

constructed, the project produces no direct waste, and has a

considerably lower output level of the greenhouse gas carbon

dioxide (CO2) than fossil fuel powered energy plants.

Geothermal power is power extracted from heat stored in

the earth. This geothermal energy originates from the original

formation of the planet, from radioactive decay of minerals,

and from solar energy absorbed at the surface. It is used to

generate electricity and for direct uses such as wintertime

heating.

Biomass, as a renewable energy source, refers to living and

recently dead biological material that can be used as Fuel. In

this context, biomass refers to plant matter grown to

generate electricity, example corn or produce for example

garbage such as dead trees and branches, yard clippings and

wood chips .It also includes energy obtained from landfill

gases and alcohol fuels.

Biofuel is defined as solid, liquid or gaseous fuel obtained

from relatively recently lifeless biological material. Also,

various plants and plant derived materials are used for biofuel

manufacturing.

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Non-Renewable Sources of Energy

These refer to Energy sources that can be used only once.

Fossil Fuel – Oil & Natural Gas Crude oil is usually found in

underground areas called reservoirs. Scientists and engineers

explore a chosen area by studying rock samples from the

earth. Measurements are taken, and, if the site seems

promising, drilling begins.

Fossil Fuel – Coal is composed primarily of carbon along

with variable quantities of other elements, chiefly sulfur,

hydrogen, oxygen and nitrogen. Coal, a fossil fuel, is the

largest source of energy for the generation of electricity

worldwide, as well as one of the largest worldwide source of

carbon dioxide emissions. Coal is extracted from the ground

by mining, either underground or in open pits.

Nuclear Fuel is any type of nuclear elements that can be

made to undergo nuclear fission chain reactions in a nuclear

fission reactor. The most common fissile nuclear fuels are

235U (Uranium) and 239Pu (Plutonium), and the actions of

mining, refining, purifying, using, and ultimately disposing of

these elements together make up the nuclear fuel cycle,

which is important for its relevance to nuclear power

generation and nuclear weapons.

As the world's population increases and there is likely to be

demand for more electrical power. Every form of energy

generation has advantages and disadvantages as shown in the

table below.

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Source - Coal

Advantages Disadvantages

Inexpensive

Requires expensive air pollution

controls (e.g. mercury, sulfur

dioxide)

Easy to recover Significant contributor to acid

rain and global warming

Requires extensive

transportation system

Source - Nuclear

Advantages Disadvantages

Easy to recover Energy

generation is the most

concentrated source.

Requires larger capital cost

because of emergency,

radioactive waste and storage

systems.

Fuel is inexpensive. Potential nuclear proliferation.

Waste is more compact

than any source .Easy to

transport as new fuel

Requires resolution of the long-

term high level waste storage

issue in most countries

No greenhouse or acid

rain effects

Source - Gas / Oil

Advantages Disadvantages

Good distribution system

for current use levels.

Very expensive for energy

generation.

Easy to obtain

(sometimes).

Could be major contributor to

global warming.

Better as space heating

energy source.

Very limited availability.

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Source - Wind

Advantages Disadvantages

Wind is free if available. Limited to windy areas.

Good source for water

pumping demands of

farms .

Limited to small generator

size; need many towers.

Generation and

maintenance costs have

decreased significantly.

Highly climate dependent -

wind can damage equipment

during windstorms or not turn

during still summer days.

May affect endangered birds;

however tower design can

reduce impact.

Source - Hydro-electric

Advantages Disadvantages

Very inexpensive once

dam is built .

Very limited source since

depends on water elevation.

Produce no waste. Many dams available are

already built.

Dam collapse usually leads to

loss of life. Dams have

affected fish (e.g. salmon

runs) .

Environmental damage for

areas flooded (backed up) and

downstream.

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Source - Solar

Advantages Disadvantages

Sunlight is free when

available.

Limited to sunny areas. Demand

can be highest when least

available, e.g. winter solar

heating)

Costs are dropping. Does require special materials

for mirrors/panels that can affect

environment.

Current technology requires

large amounts of land for small

amounts of energy generation.

Source - Biomass

Advantages Disadvantages

Fuel can have low

cost.

Inefficient if small plants are

used.

Industry in its infancy.

Could be significant contributor

to global warming because fuel

has low heat content.

Source - Fusion

Higher energy output

per unit mass than

fission.

After ~40 years of expensive

research ,commercially available

plants not expected for at least

35 years.

Low radiation levels

associated with

process than fission-

based reactors.

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Source - Refuse Based Fuel

Advantages Disadvantages

Fuel can have low cost Inefficient if small plants are

used.

Low sulfur dioxide

emissions

Could be significant contributor to

global warming because fuel has

low heat content .

Flyash can contain metals as

cadmium and lead.Contain

dioxins and furans in air and ash

releases

Source - HYDROGEN

Hydrogen and tritium

could be used as fuel

source.

Takes more energy to produce

hydrogen then energy that could

be recovered.

Combines easily with

oxygen to produce water

and energy.

Very costly to produce .

Throughout the world, we need every energy source we

can get - including nuclear. As one can see from the table

above, all energy sources have BOTH advantages AND

disadvantages.

Even with conservation efforts, energy demand has been and

will continue to increase.In using each and every one of

these forms of energy production, we need to make sure we

conserve as much as we can so we leave sources for future

generations. Energy suppliers need to ensure that they do not

contribute to short and long-term environmental problems.

Governments need to ensure energy is generated safely to

that neither people nor the environment are harmed.