Energy

What it is

Energy is a quantity describing the capacity to do work. It is measured in joules (J): 1 J is the work done when a force of 1 N acts along 1 m. Energy comes in forms, and one form can be turned into another:

• Kinetic energy, E = mv²/2. A 1 000 kg car at 20 m/s has 1 000 · 20²/2 = 200 000 J = 200 kJ; see kinetic energy.
• Potential energy, E = mgh. A 2 kg ball 3 m up has 2 · 9.8 · 3 = 58.8 J; see potential energy. A compressed spring stores elastic energy; see elastic potential energy.
• Thermal energy, the heat in a substance; see the first law of thermodynamics.
• Electrical energy. One kilowatt-hour is 3.6 million joules; see electrical energy in kWh.
• Chemical energy in fuel and food, radiant energy in light and nuclear energy in atomic nuclei, where mass and energy are linked; see mass–energy equivalence.

Conservation of energy says the total energy of an isolated system, one that exchanges no energy with its surroundings, is constant. A 1 kg ball falling 5 m loses 1 · 9.8 · 5 = 49 J of potential energy and gains 49 J of kinetic energy: v = √(2 · 49/1) ≈ 9.9 m/s; see conservation of mechanical energy.

What is not energy:

• Power. A watt is a joule per second. A 10 W lamp converts 10 J every second; see mechanical power.
• Force. A newton is not a joule. Force is what pulls; energy is what changes when it pulls along a distance.
• Temperature. 20 °C does not say how much energy there is; that depends on the mass and the substance too.

Why it matters

Energy is what to count when you want to follow a process without following every force. It is enough to find where the energy is before and after: the speed at the bottom of a hill, how high a ball goes, what an appliance costs to run. Work is energy moved by a force; see work; and efficiency says how much of the energy ends up where you want it.

How a fall is worked out with energy is shown in energy conservation between two points; an electricity bill is the same quantity in kWh.

Common misconceptions

• “Energy gets used up.” It is converted. When a braking car stops, the kinetic energy has become heat in the brakes and tyres. It is still there, but cannot be used again the same way.
• “Energy and power are the same.” Power is energy per time. 1 kWh is energy, 1 kW is power: a 2 kW appliance running for 3 hours uses 6 kWh.
• “Doubling the speed doubles the kinetic energy.” It makes it four times as large, because v appears squared. 20 m/s has four times the energy of 10 m/s.
• “Conservation of energy fails with friction.” It holds for the isolated system, surface included; friction only converts part of the mechanical energy into heat. It is the mechanical energy that is not constant.

Common questions