2. Einstein’s equivalence
S Perhaps the most famous equation of all: E = mc2
S Energy and mass are interconvertible… with a large
number (9 x 1016) showing that a small amount of mass
is equivalent to a large amount of energy
S From a general relativity perspective we can think about
‘mass-energy’ or ‘matter-energy’, as we also think about
‘space-time’
3. Energy
S Energy can be defined as the ability to do work
S When energy is converted from one form to another,
work is typically done
S Work can include heating, emission of light or movement
of an object, among other things
S The unit of energy is the joule (J). We often use kilojoules
(kJ) for the energy in food.
4. Forms of Energy
S Two major categories of energy:
S kinetic energy: energy that something possesses
due to its motion
S potential energy: energy that something
possesses due to its position in a field or its
chemical composition or to a distortion (e.g.
compression of a spring)
5. Examples of Energy
S Chemical potential energy in flammable and reactive substances
S Bond energy in all bonded substances
S Spring potential energy
S Gravitational potential energy (but the gravitational field is not itself
a form of energy)
S Electrical potential energy
S Kinetic energy in moving objects (but friction is not itself a form of
energy)
S Kinetic energy in sound waves and ocean waves
6. Energy Conversions
S Electrical potential energy converted into light energy in a
light bulb (in which more later)
S Gravitational potential energy converted into kinetic
energy when skydiving
S Chemical potential energy in petrol turning into kinetic
energy of the car
8. Efficiency
S Some energy is always lost at each energy conversion
S No conversion is 100% efficient (this explains the
impossibility of a perpetual motion machine)
S The lost energy is lost as heat
S On earth, all energy ends its travels as heat, which is
radiated off into space
9. Heat Energy
S Heat is a measure of the vibratory energy in the particles
in an object. The unit of heat is the joule (J), the same as
the unit for other forms of energy.
S Heat always flows from a hotter to a cooler object
S Heat can travel by conduction (direct contact), convection
(heated air (or liquid) currents) and radiation (infrared (or
shorter-wavelength) radiation)
10. Sources of Energy
S The great majority of all the energy we use on earth
comes, one way or other, from the sun
S The only real exception is nuclear energy
S Solar energy is radiated to the earth as short wavelength
visible light. It fuels photosynthesis and weather, and is
converted into many forms, ending up as long
wavelength infrared radiation that radiates into space.
11. Electrical Potential
S We need a distinction between potential and potential
energy
S Potential is measured in volt (V), potential energy in joule
(J)
S A battery has different electrical potential at each of its
terminals. A circuit connects the battery to circuit
elements that convert the electrical potential energy
given to the electrons to other forms of energy.
12. S (play with simple series and parallel circuits for a while
here)
