Energy, and why perpetual motion is impossible
Energy is never used up. It is only ever moved somewhere less useful.
Energy is the capacity to do work, measured in joules. It comes in forms that convert into one another, and the total never changes. That last part is the most useful law in physics.
The forms, and they are all the same thing
- Kinetic - energy of motion, 1/2 m v squared
- Gravitational potential - energy of height, mgh
- Elastic potential - energy in a stretched or compressed spring
- Thermal - the kinetic energy of particles jiggling
- Chemical - energy stored in bonds, in food and fuel
- Electrical, sound, light, nuclear
Naming them separately is a convenience. Thermal energy is really kinetic energy at a small scale; chemical energy is really electrical energy in bonds. They are labels for where energy currently is, not different substances.
Conservation, and what it forbids
Energy cannot be created or destroyed, only transferred or converted.
So a swinging pendulum converts height into speed and back, endlessly in principle. In practice it slows, and the missing energy is not gone - it has become heat in the air and the pivot, spread out and unrecoverable.
This is why perpetual motion machines cannot work, and why patent offices refuse to examine them without a working model. A machine that runs forever needs energy from nowhere; a machine that produces more than it consumes needs more of it. Neither is an engineering difficulty to be overcome. Both are forbidden.
Nothing was lost. Ask 'where did it go?' rather than 'how much was lost?' and there is always an answer, which is what makes conservation a tool rather than a slogan.
Power is not energy
Power is energy per second, measured in watts. One watt is one joule per second.
Climbing stairs slowly and running up them uses the same energy - the height and your mass are the same. Running uses more power, because you do it in less time. This is why you can lift a heavy box given long enough but cannot throw it: the energy is achievable, the power is not.
A kilowatt-hour, the unit on an electricity bill, is a power multiplied by a time, so it is a unit of energy: 1,000 watts for one hour, or 3.6 million joules.
Why efficiency is never 100%
Every conversion produces some heat, and heat is the form energy tends to end up in because it is the most disordered. Energy spontaneously spreads out and does not spontaneously gather back.
- Petrol engine: roughly 25-30% to motion, the rest heat
- Filament bulb: about 5% to light, 95% heat - which is why they were banned
- LED bulb: roughly 40%, which is the whole point of them
- Electric motor: often above 90%
'Wasted' energy is a statement about usefulness, not about quantity. Every joule is still there; it is just spread thinly through the surroundings where nothing can be done with it.
This is the second law of thermodynamics. The first says energy is conserved. The second says it moves from concentrated to spread out, and not back on its own. Together they explain why engines need fuel, why the fridge warms the kitchen, and why time appears to have a direction.
Where the energy on Earth came from
Nearly all of it is the Sun. Sunlight drives photosynthesis, which makes plants, which feed animals - and which, buried and compressed for a few hundred million years, became coal and oil. Wind is air moved by uneven solar heating. Hydroelectric power is water lifted by evaporation, which the Sun did.
The exceptions are nuclear, which uses energy stored when the atoms themselves formed in dying stars, geothermal, which is heat left over from Earth's formation and radioactive decay, and tidal, which comes from the Moon's orbit. Everything else is sunlight, sometimes very old sunlight.