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Energy: Part 0.3
Examples of types of energy store conversions in systems and closed systems
Doc Brown's Physics exam study revision notes
INDEX physics notes: Types of
energy and energy stores, energy transfers and selected energy
calculations
0.3
Examples of types of energy store conversions in systems
Energy can be transferred between
energy stores in four principal ways
1.
By heating
- the transfer
of thermal energy from a hotter material/object to a cooler
material/object.
You can only have a net heat
flower from a higher temperature to a lower temperature region.
2.
By radiation of a wave -
sound wave vibrations transfer energy and electromagnetic radiation.
There seven types of EM radiation
including microwaves, infrared and visible light.
Infrared radiation is referred to
as thermal radiation - its a means of net transfer of thermal energy
from a higher to a lower temperature material.
3.
Flow of electrically -
energy is transferred by electrical charge (electrons) moving around an
electrical circuit down
a potential difference i.e. moving from a higher to a lower potential
energy, releasing energy in the process e.g. in the form of heat or
light.
See also
Usefulness of electricity and energy transfer for lots of
examples starting with electricity
4.
Mechanically - anything
that is moved by a force acting on it involves a mechanical transfer of
energy - anything rotating, pushed, squashed or pulled etc.
Reminder of the law of conservation of energy
No matter the nature of an energy
store or an energy store transfer, the following law applies ...
Energy can be (i )stored, (ii)
changed from one form to another and (iii) dissipated, but the total
energy of a closed system is constant and you cannot create or
destroy energy
There is no net change in
total energy, no matter what energy transfers take place.
Dissipated usually
means wasted energy like heat spreading out to increase the
thermal energy store of the surroundings.
In the 19th century scientist began to
realise that mechanical work generated heat, and so concluded that there was
a connection between them e.g. heat is generated by friction when a
mechanical device does work.
Work done in joules = force in
newtons x distance through which force acts,
which means the same amount of work
transfers the same amount of energy, but not all usefully.
Electrical heating elements transfer
energy from an electrical energy store to the thermal energy store of
the material being heated e.g. oil in a heater, water in a kettle.
Moving objects have kinetic energy.
If you want to slow a moving object down you must reduce its kinetic
energy store e.g. car brakes by operating a force. In doing so work must
be done and heat from friction is released to increase the thermal
energy store of the surroundings.
Whenever you get an increase in
temperature of a system, energy must be transferred from one energy
store to another.
A system is the physical components
involved with the energy transfer conversion. e.g. your hand and winding up a
clock, a car engine, your muscles being used to lift a weight, a photocell, an
electrical appliance, objects colliding.
A closed system is a
collection of objects you treat on their own and energy can be
transferred between different energy stores of the system, but not
between energy stores outside of the system - in other words no energy
leaves or enters the defined closed system.
e.g. a hot drink standing on the
table - the system includes the surroundings too, because thermal energy
is readily transferred to the surrounding air - its an 'open' system..
However, if the hot drink is in a
well insulated thermos flask, then this can be considered a closed
system.
When a system changes it involves an
energy transfer. Energy might be transferred into or out of a system between the
different components of a system or between different types of energy stores.
You can transfer energy mechanically when a
force is applied to do work e.g. winding up a clock, dragging a heavy box across
the floor.
You can transfer energy electrically when
work is done by moving charge e.g. an electrical heater, electric motor.
When you raise the temperature of a material
(e.g. from a flame, electrical supply, electromagnetic radiation) you increase
the thermal energy store of that material.
When your vocal chords vibrate you are
transferring kinetic energy into sound energy
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 expressing this law to say
energy is never lost but transferred between different energy stores
often involving different objects or materials.
However, energy is only useful
if it can be converted from one form to another.
Be careful in using the term energy
loss!
The phrase 'energy loss' is used in the
context of energy transfer when not all the energy is transferred into a
useful form e.g. some energy from a car fuel is 'lost' in the friction of
moving parts, i.e. some chemical energy ends up as heat or sound rather than
kinetic energy to move the car.
Examples - from a suitable
energy source ==> useful form of energy (plus waste in most cases)
Energy is usually transferred by
radiation, electricity flow, heating or doing work in a mechanical sense.
When a gun fires chemical energy
is converted into heat energy, sound energy, light energy and mainly kinetic
energy. When the bullet embeds itself into some material the kinetic energy
of movement is converted into some sound energy, but mainly heat energy.
Some of the energy is wasted but most of the chemical energy stored in
cartridge is converted to the useful kinetic energy of the bullet.
 Photovoltaic
solar panels convert light energy into electrical energy. If this
electrical energy is used to charge up a battery then you increase
chemical energy store of the battery.
 We
use a large number of electrical devices in the home eg
TV converts electrical energy
into useful light and sound, but some waste heat
A charged mobile phone battery
converts chemical energy into electrical energy, which in turn is converted
into useful light and sound energy.
In the charging process you are
increasing the energy store of the mobile phone battery.
 Wind
turbines convert kinetic energy into electrical energy
The kinetic energy store of the
wind is decreased, whereas that of the turbine blades is increased,
initially an energy transfer involving the same type of energy
store.
The start of making a cup of tea
is a system. You use an electrical heating element in the kettle
base to boil water by transferring energy to water via the conversion of
electrical energy to thermal energy. The water increases in temperature and
therefore its thermal energy store is increased. The water and heating
element constitute a system.
Whenever an electrical current
flows in a circuit, work is done against the resistance of the wire.
This is all to do with the behaviour of the electrons moving due to
an electrical potential difference.
It doesn't seem the same as say,
pushing a lever to operate the machine, but you are applying a force
to move the lever against a mechanical resistance. In fact the work
done or energy transferred is equal to the force applied x the
distance through which the force operates (see calculations further
down the page).
When the car of a 'big dipper'
fun ride is raised to the top of a loop using an electric motor you
are converting electrical energy mechanically into the car's kinetic
energy store.
As the car increases in
height its gravitational potential energy store (GPE) increases.
When the car rolls down the
other side its GPE store decreases and its kinetic energy store
increases.
See also
Conservation of energy,
more on energy transfers-conversions, efficiency
INDEX ENERGY: Types, stores, transfers, energy
calculations
Keywords, phrases and learning objectives
on energy
Be able to describe, analyse and explain examples types of energy store conversions
involving closed
systems, heating by thermal energy transfer, thermal energy radiation transfer,
electrical current energy transfer and mechanical work done energy transfer.
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INDEX ENERGY: Types, stores, transfers, energy
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