Environmental Engineering Reference
In-Depth Information
of 1 g of water by 1°C (e.g., from 13.5 °C to 14.5 °C). One calorie is equivalent to
4.18 J.
The time necessary to perform a given amount of work is of great importance.
For example, a man can lift 40 25-kg stones, one by one, from the ground and place
them into a cart. But he will be unable to lift all of them (1,000 kg) in a single op-
eration, despite the total work done in the two cases being exactly the same. The
amount of work performed per unit of time is called power (P), and it is measured
in watts (W). One watt is equal to 1 joule per second (J/s). The unit employed to
measure power in many countries is the “horsepower” (or HP, equivalent to about
746 W), which traditionally represented the “power” of a horse or 7.5 times the
power of a man.
The human being, on average, consumes energy at a power of about 100 W (the
power of an average incandescent light bulb), varying between 85 W during sleep
and 800 W or more during intense exercises.
Table 1.1 Units of work, energy, and power
A unit used frequently is the ton of oil equivalent (toe), which is equal to 10 mil-
lion kcal (10
10
cal), since 1 kg of oil contains 10,000 kcal. It is usual to convert the
energy of all other sources of energy to tons of oil equivalent, including electrical
energy, measured in kilowatt-hours.
Table 1.1
lists the commonly used units.