How We Measure Energy
A good definition of energy is ‘the ability to do work’. When we measure energy, what we want to know is: how much energy was required to achieve certain results?
Finding a method of measuring energy that would enable meaningful comparisons to be made between the results of different experiments, was one of the challenges that the British scientist James Joule (born 1818) set himself. By the early 19th century, many experiments had already been conducted in subjects such as electromagnetism, heat and energy, and by the end of the century the science of thermodynamics was developing. It was James Joule who established a connection between heat and mechanical energy. His findings led to the rejection of an earlier theory about energy, the ‘caloric theory’.
According to the caloric theory, heat was an actual substance called ‘caloric’, which was either a liquid or a weightless gas; it was thought that caloric flowed from hotter bodies to colder ones, and passed in and out of the pores in solid matter. James Joule concluded that heat is not a solid substance, but is produced by motion. He also worked out a system of measuring energy by establishing the exact mechanical equivalence of heat; and he saw the need for a related system of electrical measurement that quantified the chemical action producing the electric current. The unit of measurement he established, the joule, is still used today as a unit of measurement for energy.
One Joule (J), also called a Newton-Metre (N.m), is the work done or the energy expended by a force of one Newton moving an object for a distance of one metre. In electrical terms, a joule (J) is a unit of electrical energy equal to the work done when a current of one amp passes through a resistance of one ohm for one second. A joule is also the same as a Watt-second, i.e. the work done to produce one watt continuously for one second.
Another system of measuring energy is in calories. Unfortunately this has evolved into a somewhat confusing system. The calorie was originally established as a measurement of heat, and it was then applied to energy as well, calculated on the basis that one calorie is equal to about 4.2 joules. However, the exact equivalence of calorie to joule varies according to temperature.
Specifically, one calorie is amount of heat required to raise the temperature of water of 1 gram of water by 1 degree Celsius, at a fixed atmospheric pressure. But in fact the amount of heat needed increases slightly as the temperature rises, so that the calorie representing a rise in temperature from 14.5 to 15.5 degrees is marginally larger than the calorie that represents a rise in temperature from 1.95 to 20.5 degrees; whereas a joule is a constant measurement.
When performing precise calculations involving calories, it is therefore necessary to clarify the precise joule equivalence. A further caveat when using calories in calculations is the existence of a ‘large calorie’ or ‘kilogram calorie’, which is the energy needed to raise the temperature of 1kg of water by 1 degree C – in other words, 1,000 times the value of a ‘small calorie’.
Calories or kilocalories are traditionally the most widely used measurement of the energy in food, but increasingly the kilojoule equivalent is being stated as well.
Measuring Specific Types of Energy
There are other measurements that relate specifically to each separate type of energy. Common electrical measurements are amps, volts, watts and ohms, as well as joules which have been discussed above. Amps, volts, watts and ohms all relate to different aspects of the electrical current; they measure the amount of electricity being generated and the power and flow of the electricity supply but they are not, strictly speaking, measurements of energy.
The heating power, or Calorific Value (CV), of gas can be expressed either as megajoules per cubic metre (MJ/m3), or as therms. The calorific value – the amount of energy released by a set quantity of gas when combusted under set conditions – depends on the chemical composition of the gas. Each constituent of natural gas, for example butane or carbon dioxide, has a different CV, so the CV of natural gas in a pipeline can be calculated by analysing the composition of the gas.
Gas usage in the UK, like electricity usage, is metered in kiloWatt/hours.
Measuring Energy to Monitor Consumption
Measuring the energy we use is the first step towards reducing our consumption, as it helps us identify the savings we can make by adopting less wasteful habits, buying less energy-hungry appliances and replacing outdated equipment with more energy-efficient systems.