What Is Energy? Stores and Forms

Energy is the capacity of a system to do work or cause change, measured in joules (J), and it exists in stores such as kinetic, gravitational potential, elastic potential, thermal, chemical and nuclear energy. In brief: energy is not a substance but a quantity that can be transferred between stores and objects (by forces, heating, waves or electric currents). It is never created or destroyed, only moved or changed from one store to another.

Moving objects, hot water, a charged battery and food all have energy. Energy explains why things happen, from a falling ball to a growing plant. This guide defines energy and the joule, describes the main energy stores and the ways of transferring energy, compares the stores in a table, works through a calculation with simple stores and corrects common mistakes.

What is energy?

Energy is a measure of the ability of a system to make changes happen: to move objects, heat things up or produce light. The SI unit is the joule (J). One joule is the energy transferred when a force of 1 N moves an object 1 m in the direction of the force (1 J = 1 N·m). Larger units are the kilojoule (kJ, 1,000 J) and megajoule (MJ). Food energy is often given in kilocalories (kcal): 1 kcal = 4,184 J. Energy is a scalar.

What are the energy stores?

• Kinetic: energy of moving objects, E_k = ½mv².
• Gravitational potential: energy due to position in a gravitational field, E_p = mgh.
• Elastic potential: energy stored in stretched or compressed objects, such as springs and rubber bands, E = ½kx².
• Thermal (internal): the total kinetic and potential energy of the particles in a substance. It rises with temperature.
• Chemical: energy stored in the bonds of substances such as food, fuels and batteries.
• Nuclear: energy stored in the nuclei of atoms, released in fission and fusion.
• Electrostatic and magnetic: energy of charges and magnets in fields.

How is energy transferred?

• Mechanically (by forces): when a force moves an object, work is done.
• Electrically: by an electric current in a circuit.
• By heating: from a hotter place to a colder one, by conduction, convection or radiation.
• By radiation: by light, sound and other waves.

Energy stores compared

Worked example: energy in everyday objects

1. Kinetic: a 1,000 kg car at 20 m/s: E_k = ½ × 1,000 × 20² = 200,000 J = 200 kJ.
2. Gravitational potential: a 2.0 kg book on a shelf 1.5 m high: E_p = 2.0 × 9.8 × 1.5 = 29.4 J.
3. Elastic potential: a spring with k = 100 N/m stretched by 0.20 m: E = ½ × 100 × 0.20² = 2.0 J.
4. Food energy: a snack with 100 kcal has 100 × 4,184 = 418,400 J = 418 kJ.
5. Comparison: the snack contains about 2 times the kinetic energy of the car in item 1 (418 ÷ 200 = 2.1). A small amount of food has a lot of chemical energy compared with everyday mechanical energy.

How did the concept of energy develop?

The word “energy” in its modern sense was introduced by Thomas Young in 1807. Gottfried Leibniz studied vis viva (½mv²) in the 1690s, and James Joule’s experiments in the 1840s showed that mechanical work and heat are equivalent, which led to the principle of conservation of energy, stated by Hermann von Helmholtz in 1847. The unit joule was named in his honor. Einstein’s equation E = mc² (1905) showed that mass is also a form of energy.

Which mistakes are common?

Mistake 1: Saying that energy is “used up”

Energy is transferred or dissipated, usually as thermal energy in the surroundings, but the total amount is conserved.

Mistake 2: Confusing energy and force

Force is a push or pull in newtons. Energy is in joules.

Mistake 3: Treating heat as a store

Heating is a way of transferring energy. The store is the thermal (internal) energy.

Mistake 4: Mixing joules and kilocalories

1 kcal (a “Calorie” on food labels) is 4,184 J.