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Matter and Energy

Einstein’s equivalence

Perhaps the most famous equation of all: E = mc2

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

From a general relativity perspective we can think about ‘mass-energy’ or ‘matter-energy’, as we also think about ‘space-time’

Energy

Energy can be defined as the ability to do work

When energy is converted from one form to another, work is typically done

Work can include heating, emission of light or movement of an object, among other things

The unit of energy is the joule (J). We often use kilojoules (kJ) for the energy in food.

Forms of Energy

Two major categories of energy: kinetic energy: energy that something

possesses due to its motion 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)

Examples of Energy

Chemical potential energy in flammable and reactive substances

Bond energy in all bonded substances

Spring potential energy

Gravitational potential energy (but the gravitational field is not itself a form of energy)

Electrical potential energy

Kinetic energy in moving objects (but friction is not itself a form of energy)

Kinetic energy in sound waves and ocean waves

Energy Conversions

Electrical potential energy converted into light energy in a light bulb (in which more later)

Gravitational potential energy converted into kinetic energy when skydiving

Chemical potential energy in petrol turning into kinetic energy of the car

(play with pendulum for a while here)

Efficiency

Some energy is always lost at each energy conversion

No conversion is 100% efficient (this explains the impossibility of a perpetual motion machine)

The lost energy is lost as heat

On earth, all energy ends its travels as heat, which is radiated off into space

Heat Energy

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.

Heat always flows from a hotter to a cooler object

Heat can travel by conduction (direct contact), convection (heated air (or liquid) currents) and radiation (infrared (or shorter-wavelength) radiation)

Sources of Energy

The great majority of all the energy we use on earth comes, one way or other, from the sun

The only real exception is nuclear energy

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.

Electrical Potential

We need a distinction between potential and potential energy

Potential is measured in volt (V), potential energy in joule (J)

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.

(play with simple series and parallel circuits for a while here)