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T 2017 - 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.
T 2017 - Unit 3 Work,Energy & Power Page 2
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)
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.
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.
T 2017 - Unit 3 Work,Energy & Power Page 5
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.
T 2017 - Unit 3 Work,Energy & Power Page 7
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.
Ke = ½ mv2 = 20 J
v2=2 x Ke /m = 2 x20 / 0.5
v2 = 80
v =√80 = 8.94 m/s
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
T 2017 - Unit 3 Work,Energy & Power Page 8
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.
T 2017 - Unit 3 Work,Energy & Power Page 10
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.
T 2017 - Unit 3 Work,Energy & Power Page 11
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.
T 2017 - Unit 3 Work,Energy & Power Page 12
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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Renewable energy sources
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.