Measuring Heat


It's all very well to say that we have this thing called heat, and that it is a form of energy, but how do we measure it? These days, scientists have a very clear idea of how they measure energies, and there are well-defined units to allow us to do that; however, this has not always been the case. Even today, there are many different units that are used to measure engergy.

The Calorie

The calorie (symbol: cal) is the old unit for measuring energy, (French from Latin calor, meaning heat). Calorimetric means measuring heat. The calorie is a bit of a mess of a unit... there are many different definitions depending on the context. Essentially, it is the temperature required to raise 1 g of water 1 °C which is about 4.18 J. The calorie is still commonly used by chemical engineers in the US and dieticians. Note that the Calorie and the calorie are actually different! When spelt with an uppercase "C", the Calorie is actually 1000 calories... 1 Cal = 1000 cal. Go figure...

(more information on the calorie).

The British Thermal Unit

The British Thermal Unit (symbol: BTU) is another unit for measuring energy. It is approximately 1055 J (the temperature to raise 1 lb (1 pound = 2.2 kg) of water by 1 °F (Farenheit). The BTU is used by some chemical engineers to measure heat output and heat transfer rates.

(more information on the BTU).

The Joule

The joule is the SI unit for energy and is the standard unit on energy used by chemists today. A joule is defined as the work done by a force of 1 newton over 1 metre, i.e. 1 J = 1 N m = 1 kg m2 s-2. This definition of the joule means that it is internally consistent with the other units (it just requires the definitions of the kilogram, metre and second...).

The Kilowatt-Hour

The kilowatt-hour (symbol: kWh) is a quite bizarre form of energy measurement used only by electricity companies and some very strange electrical engineers. The watt is defined as a joule per second (1 W = 1 J -2) and as such is a rate of energy consumption (i.e. it is a measure of power). If we were to measure how long (in seconds) we had used a certain number of watts of power, then we could easily work out how many joules of energy we had used by multiplying the two numbers. Instead of doing this, however, the electricity companies take the number of watts and multiply it by the amount of time in hours and multiplying them to give watt-hours. A kilowatt-hour is just 1000 watt-hours.

If you want to work out how much electricity a light bulb in your house is using, this is quite easy. A 60 W globe running for 1 hour will consume 60 Wh = 0.060 kWh of energy. To convert this into joules, you'll need to convert the hours into seconds: 1 kWh = 3600 kJ, so the bulb consumes 216 kJ of energy in 1 hour. Have a look on the back of your breakfast cereal packet at how long that energy could keep your body going...