What is energy?
In science, energy is the ability to do work. Work is done when a force causes an
object to move in the direction of the force. Energy is expressed in units of
joules (J). A joule is calculated as force x distance, so 1J = 1Nm
Energy is conserved in all cases. Because no exception to this rule has been found,
this rule is described as a law. According to the law of conservation of energy,
energy cannot be created or destroyed. The total amount of energy in a closed
system is always the same.
Kinetic energy is the energy an object has due to its motion. Kinetic energy
depends on both mass and velocity. The faster an object moves, the more kinetic
energy it has. The greater the mass of a moving object, the more kinetic energy it
has.
Potential energy is stored energy. Potential energy can be stored in the chemical
bonds of fuel, the nucleus of atom, foods, electric charge, and in other factors.
Potential energy can also be stored because of the work done on it. For example:
stretching a rubber band, winding a watch, pulling back on a bowstring, and lifting a
brick high in the air.
When a raised object is released gravitational potential energy is converted into
kinetic energy. If there is no significant air resistance, then conservation of
energy results in gravitational potential energy being converted into kinetic energy
as an object falls. The gain in kinetic energy equals the loss of gravitational
potential energy.
Roller coasters work because of the energy that is built into the system. Initially,
the cars are pulled mechanically up the tallest hill, giving them a great deal of
potential energy. From that point, the conversion between potential and kinetic
energy powers the cars throughout the entire ride.
Equations:
Kinetic energy = KE
Potential Energy = PE
KE = (mass x velocity2) ÷ 2
PE = force x distance
KE = (mv2) ÷ 2
PE = Fd
m = ( 2KE) ÷ v2
GPE = mgh
v = √((2KE) ÷ m)
w = GPE ÷ h
or
GPE = wh
h = GPE ÷ w
What is the kinetic energy of a car that has a mass of 2,400kg and is moving at
20m/s?
What is the kinetic energy of a 4,000kg elephant that is running at 2m/s?
If a bus is moving at 30m/s and has a mass of 2,000kg, what is it’s kinetic energy?
What is the gravitational potential energy of a cat that weighs 40N standing on a
table that is 0.8m above the ground?
What is the gravitational potential energy of a diver who weighs 500N standing on
a platform that is 10m above the pool?
What is the kinetic energy of a 400,000kg jet flying at 72m/s?
What is the gravitational potential energy of a cantaloupe with a mass of 4kg and a
height of 12 meters?
Distinguish among forms of energy
All matter is made of particles that are always in motion. Thermal energy (heat) is
the internal energy in substances; it is the vibration and movement of atoms and
molecules within substances. Objects with a higher temperature have more thermal
energy because their particles are moving faster. However, objects with a greater
mass, but a lower temperature, may have more thermal energy because they have
more particles. Think of an iceberg versus a cup of hot chocolate. Geothermal
energy is an example of thermal energy.
Chemical energy is the energy that is stored in the chemical bonds of atoms and
molecules. As the bonds are broken, energy is released. Nuclear energy is the
energy stored in the nucleus of an atom. It is the energy that holds the nucleus
together. The nucleus of a uranium atom is an example of nuclear energy.
Radiant energy is the electromagnetic energy that travels in transverse waves.
Radiant energy includes visible light, x-rays, and radio waves. Solar energy is an
example of radiant energy. Sound is the movement of energy through substances in
longitudinal (compression/rarefaction) waves. Electrical energy is the energy of
moving electrons. As the electrons move, they can be used to do work with
electrical devices. Lightning and electricity are examples of electrical energy.
Mechanical energy is the total energy of motion and position of an object. Both
potential energy and kinetic energy are kinds of mechanical energy.
Mechanical energy = potential energy + kinetic energy.
Elastic potential energy is stored mechanical energy in objects by the application
of force. Compressed springs and stretched rubber bands are examples of elastic
potential energy.
Gravitational potential energy is the energy stored when an object is lifted
upwards. This energy is released when the object falls back to its initial position.
Water in a reservoir behind a hydropower dam is an example of gravitational
potential energy. When the water is released to spin the turbines, it becomes
kinetic energy. The movement of objects or substances from one place to another
is kinetic mechanical energy. Wind and hydropower are examples of kinetic
mechanical energy.
Forms of Energy
All forms of energy fall under two categories
Potential Energy
Kinetic Energy
Potential energy is stored energy and energy of
position
Kinetic energy is energy of motion. It is the
motion of waves, electrons, atoms, molecules,
and substances.
Chemical Energy
Radiant Energy
The energy stored in the bonds of atoms and
molecules. Includes biomass, petroleum,
natural gas, and coal.
Electromagnetic energy that travels in
transverse waves. Includes light, x-rays, and
radio waves.
Nuclear Energy
Thermal Energy
The energy stored in the nucleus of an atom.
It is the energy that holds the nucleus
together. Includes the nucleus of the uranium
atom.
The energy or heat found in the internal energy
in substances; it is the vibration and movement
of atoms and molecules within substances.
Includes geothermal energy.
Stored Mechanical Energy
Motion
The energy stored in objects by the application
of force. Includes compressed springs and
stretched rubber bands.
The movement of objects or substances from
one place to another is motion. Includes wind
and hydropower.
Gravitational Energy
Sound
The energy of place or position. Water in a
reservoir behind a dam and a book on a shelf
are examples.
The movement of energy through substances in
longitudinal waves (compression and
rarefaction) waves.
Electrical Energy
The movement of electrons. Lightning and
electricity are examples.
Energy resources
When the nucleus splits (fission), nuclear energy is released in the form of heat
energy and light energy. Nuclear energy is also released when nuclei collide at high
speeds and join (fusion).
Wind power is used to rotate a turbine (the blades) which turns an electrical
generator.
Hydroelectric power uses falling water is used to drive a turbine which rotates an
electrical generator.
Tidal Power uses moving water caused by the tides to drive a turbine directly
which rotates an electrical generator.
Geothermal Energy is thermal energy caused by the heating of the Earth’s crust.
Solar Energy can be used as thermal and radiant energy. Solar heating is when
thermal heat energy from the sun is focussed onto pipes containing water. The
water boils producing steam. The steam is then used to drive turbines which turn
electrical generators. Solar cells produce electricity directly from the sun’s radiation.
Fossil Fuels are an energy resource formed from the remains of organisms that
lived long ago, includes coal, oil, and natural gas.
Biomass is biological material derived from living, or recently living organisms.
The vital difference between biomass and fossil fuels is one of time scale.
Most of our electricity is produced by thermal power stations. These use the heat
produced by burning fuels or the fission of uranium to generate electricity.
The heat produced is used to change water into high pressure steam. This steam is
used to turn a turbine which in turn rotates an electrical generator.
Over 90% of our electricity is generated by using thermal power stations.
U.S. Energy Consumption
Energy Resources
Gas 28%
Coal 18%
Petroleum 35%
Nuclear 8%
Renewable 10%
Biomass 5%
Wind & Solar 2%
Hydro 2.6%