Energy Conversions
Energy
Energy is one of the most fundamental concepts in physics, defined as the capacity to do work or cause change. The SI unit is the joule (J), equal to one newton-metre (N·m) or one watt-second (W·s), named after the English physicist James Prescott Joule who established the equivalence of mechanical work and heat in the 1840s. Energy manifests in many forms — kinetic, potential, thermal, chemical, electrical, and nuclear — and the law of conservation of energy states that it can neither be created nor destroyed, only converted from one form to another.
In everyday life, the most commonly encountered energy units are the kilocalorie (kcal) in nutrition — where food energy is expressed as Calories (note the capital C, always meaning kilocalories) — and the kilowatt-hour (kWh) on electricity bills. One kilowatt-hour equals exactly 3,600,000 joules (3.6 MJ). The British thermal unit (BTU) remains in widespread use in the United States for measuring heating and cooling capacity in HVAC systems. One BTU equals approximately 1,055 joules, the energy needed to raise one pound of water by one degree Fahrenheit.
At the subatomic scale, energy is measured in electronvolts (eV), where 1 eV equals 1.602 × 10⁻¹⁹ joules — the energy gained by a single electron accelerated through a potential difference of one volt. Particle physics experiments at facilities like CERN operate at energies measured in teraelectronvolts (TeV). The range of energy scales in physics — from 10⁻¹⁹ J for a single photon of visible light to 10⁴⁴ J released in a supernova — spans over 60 orders of magnitude, making unit conversion an essential skill across all branches of science.
Popular Comparisons
| J | kJ | cal | kcal | kWh | |
|---|---|---|---|---|---|
| Lighting a match | 1 | 0.001 | 0.239 | 0.000239 | — |
| Eating an apple | 218,000 | 218 | 52,100 | 52.1 | 0.0606 |
| 1 kWh | 3,600,000 | 3,600 | 860,421 | 860.4 | 1 |
| Daily food intake (2000 kcal) | 8,368,000 | 8,368 | 2,000,000 | 2,000 | 2.324 |
| 1 litre of petrol | 34,200,000 | 34,200 | 8,176,000 | 8,176 | 9.5 |
Frequently Asked Questions (FAQ)
How do I convert kilocalories (kcal) to kilojoules (kJ)?
Multiply kilocalories by 4.184 to get kilojoules. For example, a meal of 500 kcal equals 2,092 kJ. This conversion is most commonly needed when reading food labels — the European Union requires both kcal and kJ on nutrition labels, while the US uses only kcal (often written simply as "Calories" with a capital C, which always means kilocalories).
What is a kilowatt-hour (kWh) and how does it relate to joules?
A kilowatt-hour is the amount of energy consumed by a 1,000-watt device running for exactly one hour. It equals exactly 3,600,000 joules (3.6 MJ). It is the standard unit used on electricity bills worldwide. A typical smartphone battery holds about 0.01–0.02 kWh; an average European household uses approximately 3,500 kWh per year.