7.1A. Energy conservation and a closed system
Reminders:
Law of conservation of energy -
energy cannot be created or destroyed.
BUT, not all the energy in a system
is useful, there is always some wasted or dissipated energy and their
relative values can be quite different.
Know and understand that energy can be transferred
usefully from one form to another, or stored, or dissipated, but energy cannot be created or destroyed.
This is the law of
conservation of energy.
Another way of stating the law is to say
the ...
'total energy of a system remains
constant no matter what energy conversions take place'.
However, energy is only useful
if it can be converted from one form to another.
Examples - from a suitable
energy source ==> useful form (plus waste in most cases)
Energy stores have been described in
Types of energy store - a comparison with examples explained
Here we are interested in how
efficient are energy transfers to produce useful energy.
What do we mean by conservation of energy and a closed system?
A system is an object or objects
involved in a particular situation which can be described in its own context
without any energy being exchanged with its surroundings - the system does not
absorb energy from, or lose energy too, the surroundings.
If energy is gained or lost by the
system, it cannot be closed.
However, you can make an existing system
into a closed system by including other things as part of it.
e.g. a car and its kinetic energy is
limited because energy is being lost by friction (wheel-road) and air
resistance (car body-air), but if you include the air around it and the road
(the environment) you have 'constructed' a closed system.
Theoretically, irrespective of form, if
the total energy at the start of a process doesn't equal the total energy at
the end, that process can't happen.
You can use the work (E = Fd) and power
(P = E/t) formulae to calculate an energy transfer-conversion from one
energy store to another, but the formulae do not explain why the transfer
takes place, nor does it tell you how much of the energy involves useful
work or wasted energy. E can represent energy or work done - its all the
same!
See
Types of energy store - a comparison with examples explained,
mechanical work done, power calculations
If there are no frictional forces
operating, the work done on the object equals the work done = the energy is
transferred to a useful energy store.
However, this is rarely the case
since resistive forces are often present e.g. surfaces scraping against
each other, air resistance etc.
Work must be done against these
resistive forces which causes energy to be wasted - dissipated to the
surroundings as heat due to friction - increasing the thermal energy
store of the surroundings.
Under these circumstances, the energy
transferred to the useful energy store as useful work, will be less than
that transferred from the original energy store.
If work is done by the object itself,
the work done does actually equal the energy transferred from the objects
useful energy store.
See
Types of energy store - a comparison with examples explained,
mechanical work done, power calculations
7.1B. Types of energy transfer:
What do energy transfers-conversions involve?
You need to be a bit more specific than just
saying 'energy transferred' from x energy store to y energy store!
Energy is transferred
electrically
when a moving electrical charge (the current) is doing work against a
resistance.
e.g. a battery providing electrical
energy via the circuit to light a torch bulb.
Operating any electrical appliance or
device from mains circuit electricity.
Energy is transferred by
heating when
a hotter object or material transfers heat energy to a colder object or material.
e.g. boiling water in an electric kettle
- the hot element transfers heat to the water.
When you first put something to bake
in an oven, heat is transferred from the hot air to the colder baking
tin and contents.
Using a soldering iron to make an
electric circuit link.
Energy is transferred
mechanically
when a force acts on an object to move it or change its shape e.g. pushing,
pulling, stretching-expanding or squashing-compressing.
e.g. kicking a football - the force you
exert from your leg accelerates the football and give it kinetic energy.
A car engine moving the wheels of a
car.
Cutting a piece of cake!
Energy is transferred by
radiation,
usually the waves of electromagnetic radiation.
e.g. The Earth receives visible light,
infrared and ultraviolet radiation from the Sun.
Infrared radiation from an electric
fire.
Radio waves from a transmitter to a
receiver.
7.1C.
Some 'domestic' examples of energy transfers
Cooking 1: Our electricity supply is initially powered
from either a fossil fuel chemical energy store, a nuclear energy store or a
renewable energy store (hydroelectric, wind or solar etc.).
The initially energy store decreases and the
kinetic energy store of the turbine blades and generator increases.
The generator converts the kinetic energy
into electrical energy, which becomes thermal energy in a toasted sandwich maker the heat is
transferred to the bread by conduction.
The electrical energy is converted into
heat/thermal energy by the insulated electrical resistors in the iron cased
toaster (iron is a good conductor of heat) - the thermal store of the heating
elements is increased.
The thermal energy is transferred by
conduction to the iron grill and the sandwich, increasing both their energy
stores and cooking the sandwich.
See also
The Usefulness of electricity, transferring
electrical energy and cost calculations gcse physics revision notes
Gas
fire appliance: The
natural gas (methane) is a chemical energy store.
On combustion the fuel's chemical energy
store decreases and the heat/thermal energy store of the waste gases increases.
The hot gases from a gas fire will always
rise due to the immediate formation of a rising convection current which carries
the heat around the room.
The decrease in the chemical energy store of
the fuel gas therefore increases the thermal energy of the contents of the room,
mainly by convective heat transfer.
The heat is also transferred by infrared
radiation emitted from the hot flame - from the higher temperature flame to the
lower temperature room.
Car headlamp
A car battery is a potential chemical
energy store. When using the car, the chemical energy store of the battery
decreases as electrical energy is produced to operate lights (visible EM
radiation), ignition systems (heat), wipers (KE) etc.,
In the case of the headlamps the thermal
energy store of the metal filament is increased (not LED lamp)
so some of the thermal energy becomes visible light and
infrared radiation ...
... hence an early morning case of infrared radiation! Unlike 'modern'
LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat
radiation (from the increased thermal energy store of the filament). You can detect this with a frosty car where the central portion
of the ice has melted on the transparent headlamp cover. Filament bulbs only
convert ~10% of the electrical energy into visible light energy, most of the
rest is converted into infrared EM radiation. The ice absorbs infrared
equivalent to the latent heat of fusion (melting) and changes to liquid
water. You can see this (in the above photographs) after the
headlamps have been switched on for a few minutes on a frosty morning. The
concentrated IR beam increases the thermal energy store of the plastic cover
and the ice and the central part of the lamp cover warms up first
melting the ice.
Energy store changes: The chemical
energy store of the battery decreases as it is converted into
electrical energy. The electrical energy increases the thermal
energy store of the metal filament of the bulb. The thermal energy
store of the filament decreases as it emits visible and infrared EM
radiation. The absorbed EM radiation increases the thermal energy
store of the headlamp cover and ice - causing the latter to melt.
Eventually all the energy involved from the battery increases the
thermal energy store of the surroundings.
A 'green' note: If there
is, and its happening now in the UK and other countries, a change
from very inefficient filament light bulbs to very efficient low
energy LED light bulbs, there will be quite a reduction in the
domestic demand for electricity. This reduced demand will help, on
closure fossil fuel power stations, reduce CO2 emissions,
reducing the greenhouse effect, and allow renewable energy resources
to take over more of our electricity generation.
From the efficiency of light production old
fashioned tungsten filament bulbs used in the home are very
inefficient - ~5% of the electrical energy is transferred as visible
light energy. The other ~95% of the energy ends up as thermal energy
i.e. increase the thermal energy store of the room and its contents.
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 |
Cooking 2:
The initial energy store to produce
electricity has already been described with the toaster (and elsewhere on
this page). The electrical current does work on the resistance convert
electrical energy into heat energy - increasing its thermal energy store.
The electrical resistance elements of a cooker ring or a toaster
become hot enough to emit a strong beam of infrared radiation to heat
the contents of a pan or grill the toast.
Electrical energy is converted
into heat/thermal energy which increases the thermal energy store of the
heating elements and then increases the thermal energy store of the pan and
contents or bread being toasted.
Eventually all the heat is
wasted/dissipated to the surroundings, slightly heating up the kitchen.
See also
The Usefulness of electricity, transferring
electrical energy and cost calculations
Using a mobile phone
The battery is a chemical energy
store (after charging).
The chemical energy is transferred as
electrical energy.
The electrical energy is converted
into light energy and sound energy.
There is also a little wasted thermal
energy from the electrical circuits operating the phone.
A HiFi system
Electrical energy is converted into
kinetic energy as the cones in the speakers are made to vibrate - their
kinetic energy stores are increased.
The kinetic energy of the vibrating
cones causes waves of sound energy to spread into the room.
Sound waves are vibrations in the
air - a sort of kinetic energy.
Much of the sound energy is absorbed
by objects in the room, so it dissipated-wasted as heat energy.
A small % of the sound energy
vibrates your ear drums, increasing their kinetic energy store, and the
ear 'system' sends electrical signals to the brain.
7.1D. More varied examples of
conservation of energy transfers between energy stores
In all these examples you must treat them as
far as possible as a closed system (so include the surroundings) and be able to
account for all the energy transfers involved.
To put it as simply as possible:
Total energy input = Useful energy
output + Wasted (dissipated) energy
There is a separate section on
EFFICIENCY -
calculations and Sankey diagrams which deals with numerical
calculations and details of useful input/output data.
1.
When a gun fires, chemical energy
is converted into heat energy, sound energy, light energy, but mainly into
the kinetic
energy store of the bullet by way off the rapidly expanding hot gases from
the explosion.
When the bullet fires, the thermal
energy store of the gases produced is increased.
When the bullet embeds itself into some material the kinetic energy
of movement is converted into some sound energy, but mainly increases the
thermal energy store of the material it hits.
| Chemical
energy store of bullet |
thermal energy of hot gases ====> |
mechanical
conversion ====> |
KE store of
bullet (and recoiling gun) |
mechanical conversion
====> |
thermal
energy - thermal store of material |
+ |
sound,
light, friction and heat losses to surroundings |
2.
Photovoltaic
solar panels convert light energy directly into electrical energy
which may be stored in a battery.
The Sun's nuclear energy and thermal
energy stores are decreasing and the energy is transferred by radiation to
the solar panel.
The chemical potential energy store of the battery is
increased by the transfer of energy in the electrical charge of current.
3.
TV and mobile phone: We
use a large number of electrical devices in the home e.g.
A TV converts electrical energy
into useful light and sound, but some waste heat from the electrical
circuits is produced. Eventually all the electrical energy is dissipated
in some way to the surroundings increasing its thermal energy store.
In
charging a mobile phone battery
you convert mains electrical energy into useful chemical potential energy.
The electrical energy store decreases and the chemical potential energy
store increases.
When using your mobile phone the
useful chemical energy store decreases as it produces electrical energy, which in turn is converted
into useful light and sound energy when using the phone. However, there is
some wasted thermal energy added directly to the surroundings - you can
detect this wasted heat as your phone warms up as you are using it.
4.
Wind
turbines convert kinetic energy into electrical energy. The kinetic
energy store of the wind decreases, the kinetic energy store of the turbine
blades increases.
This kinetic energy is converted into electrical energy by a
generator.
The turbine does work in turning the generator and this kinetic
energy is converted into electrical energy.
Some energy is wasted due to friction
of moving parts - energy dissipated to increase the thermal energy store
of the surroundings.
5.
When
you wind up a clock you are converting chemical energy from your body
to mechanically create kinetic energy to increase the elastic potential
energy store of the clock spring.
The potential energy store of the spring
decreases as it moves the hands around giving them kinetic energy.
Suppose
instead of a spring you have a falling weight?
In this case the winding up
involves using kinetic energy to increase the gravitational potential energy
of the clock weight.
As the weight falls the GPE store decreases.
Although using the term 'kinetic
energy' seems ok, what you should appreciate that in winding up the
clock and when the clock is freely working, a force is acting through a
distance.
Since work = force x distance, your
bodies chemical energy used = work done in winding up clock = work done
in working the clock.
6.
In
some hydroelectric power schemes, excess 'off-peak' electricity is
used to pump water back up into the reservoir.
This is mechanically using
electrical energy to increase the kinetic energy of the pumped-water to move
it upwards against the force of gravity.
The gravitational potential energy
store is
initially increased as the water builds up behind the
dam and then decreased when the water flows down through the generators.
At peak demand times, extra water is released,
accelerated by gravity, and so the dam's GPE energy store decreases and the
kinetic energy (KE) store of the water increases.
The KE store of the
water then decreases as it is mechanically converted into the kinetic energy
of the generator which converts its KE into electrical energy (with some
loss in sound and heat from friction).
The 'peak time' energy store changes
can be expressed in a simple diagram
| GPE store of water |
gravitational
acceleration - mechanical conversion ====> |
KE store of
the flowing water |
KE of rotating generator -
mechanical conversion ====> |
electrical energy |
+ |
friction
and heat losses to surroundings |
7. When you are cooking you are
converting electrical energy (electric cooker) or chemical energy (gas
cooker) into thermal energy to increase the thermal energy stores of the
cooker ring and then the pan and
its contents.
There will be heat energy losses due to convection, conduction
or radiation to increase the thermal energy store of the surrounding air.
8. When you
brake to slow down a moving
car and bring it to a halt, the kinetic energy store of the car is
decreased and energy is lost as thermal energy (heat), created by the friction between the
brake pads and the discs on the wheels.
This is an example of a mechanical
transfer between energy stores - resulting in initially increasing the
thermal energy store of the brake pads.
Eventually this excess heat
increases the thermal energy store of the surrounding air.
You are
mechanically using your chemical energy (from food) to create the force of
friction and the brakes mechanically convert the KE of the car into thermal
energy - work is being done against the forces of friction.
A little sound energy is also involved - adding to the waste energy
dissipated to the surroundings.
If you just take your foot of the
accelerator, on a level road, the car will eventually come to a halt due
to friction between the wheels and the road and the moving parts of the
engine, and also, air resistance as the air brushes over the car body.
BUT, the KE store of the car still decreases to the same amount as if
you were braking and the thermal energy store of the environment
increases the same amount too.
If a vehicles crashes into a
stationary object, the contact force causes energy to be
mechanically transferred from the vehicle's kinetic energy store to
elastic potential energy store of the crushed vehicle parts, the
thermal energy stores of the vehicle, object crashed into and the
surrounding air (including some sound energy too - which also ends up as
thermal energy!).
See also
Reaction times, stopping distances, safety
aspects. calculations including F=ma
gcse physics notes
9. When a
cricketer hits a cricket ball,
there are all sorts of energy changes going on. The bodies chemical energy
store is decreased as energy is used in the bodies muscles mechanical motion
on swinging the bat. Therefore the chemical energy is converted into increasing the kinetic energy store of
the bat as it is swung at the ball. When the ball is hit the kinetic energy store of the bat decreases
and the kinetic energy store of the moving ball is increased. However, some
of the kinetic energy of the 'bat and ball' is converted (wasted)
into sound and thermal energy - this eventually increases the thermal energy store of the
environment.
However, the wasted energy is NOT
wasted on the 3rd umpire in test cricket! The Umpire Decision
Review System is a technology-based system used in cricket to help the
match officials with their decision-making. The two on-field umpires or
players can choose to consult with the third umpire to consider a
decision of the on-field umpires. The technology used includes
microphones to detect small sounds (due to friction, from KE of
ball) made as the ball
hits bat or pad (or neither), and infra-red thermal imaging to detect temperature changes as
the ball hits bat or pad (heat from friction, from KE of ball).
If there is no contact between bat/glove & ball or ball & pad, there is
no increase above the background sound level and there is no 'hot spot'
due to friction,
The sound effect is mainly used in
the system these days?, so meet ...
... SNICKO the snickometer !!! The
sound (if any) of the 'snick' is detected by a sensitive microphone in
one of the cricket stumps.
The sound sequence of the movement of
the batsman and bat is then portrayed electronically on an
oscilloscope or a computer screen
linked to a piece of music technology software.
The sound trace is also synchronised
with a slow motion replay of the batsman's stroke.
10. Electrical machines that lift objects:
When a crane lifts an object, the motor usefully, and mechanically, converts
electrical energy into kinetic energy to lift the object. The lifted object
has increased its gravitational potential energy store. There will be
various losses due to - friction in the moving parts of the machine
producing heat and sound, heat losses from the resistance of the electrical
circuits (electrical store to thermal energy store).
| input of
electrical energy |
electric motor -
mechanical conversion ====> |
useful KE
of motor lifting object ====> |
useful GPE store of object
increased - mechanical conversion |
+ |
heat and
sound losses to surroundings |
11. As a
parachutist is
dropped from an aircraft, the person has its maximum gravitational potential
energy (GPE) store.
As the person falls, so does their
GPE store as the GPE is mechanically converted into kinetic energy.
However, other energy conversions
take place to. Some of the GPE is converted into heat and sound by
air resistance. This happens both before and after the parachute is
opened.
On landing on the ground the
parachutist's GPE is reduced to zero and the rest of the kinetic energy
is converted to heat and sound.
The energy conversions are:
The descent: GPE ====>
heat + sound + kinetic energy
On landing: kinetic energy
====> heat + sound
12.
A road vehicle
A non-electric car
Fuel (oil, diesel, hydrogen) is a
store of chemical potential energy.
The chemical potential energy is
converted to thermal energy when its burned in the engines
cylinders.
The thermal energy store of the
engine expands gas to drive the moving engine parts to increase
their kinetic energy store - so the car moves!
There will be thermal energy
losses from friction to the thermal energy store of the surrounding
air (via air resistance) and moving part and road surface frictions.
Wasted or dissipated thermal
energy which cannot be used or reclaimed.
An electric car
When the batteries are charged
with electrical energy they become a chemical potential energy
store.
When an electric car moves, the
chemical energy store is converted into electrical energy.
The electrical energy drives a
motor to move the car whose kinetic energy store is increased.
There will be thermal energy
losses from friction to the thermal energy store of the surrounding
air (via air resistance) and moving part and road surface frictions.
Wasted or dissipated thermal
energy which cannot be used or reclaimed.
The electric car has less
moving parts than a conventional fossil fuelled car, so the
thermal energy loss due to friction should be less.
