LEARNING OBJECTIVE : 3.9.A - Describe the transfer of energy into, out of, or within a system in terms of Power
The time element is not involved in the definition of work. The same amount of work is done in raising a given weight through a given height whether the work is done in one second, or one hour, or one year.
Suppose we need to tidy up our lawn. We can use any of the three mowing machines to perform this job. Machine A can complete the work in 2 hours, Machine B can complete the work in 3 hours and Machine C can complete the work in 6 hours.
Which one you will use? All three machines are performing the same job but only difference is how much time they will take to perform it. Surely in this case we will choose machine A, because it can save us 4 hours or it takes 66% less time compared to machine C. In other words, Machine A is more powerful than Machine B and C.
To characterise the operation of various machines, it is important to know not only the amount of work that can be done by a given machine but also the time during which this work can be done. This ultimately determines the efficiency of any machine.
The ratio of the work W done to the time t during which this work is done is known as power and is denoted by P = W / t .
Defining Power
Power is defined as the rate of doing work.
Given the unit of work is joules and that of time is seconds. Unit of power Joules/sec is called Watt in honour of James Watt , inventor of steam engines.
Unit Watt is of very small magnitude and usually in Physics and Engineering we use Kilo Watt ( 1000 Watt = 1 Kw and million watt = 1 Mega Watt = 1 Mw ). One kilowatt-hour is the work done in one hour by an agent working at the constant rate of one kilowatt.
Another useful unit of power is called horsepower which is
1 hp = 746 W, which is roughly the rate at which a horse can work.
One horsepower-hour is the work done in one hour by an agent working at the constant rate of one horsepower.
A common misconception is that there is something inherently electrical about a watt or a kilowatt. This is not the case. It is true that electrical power is usually expressed in watts or kilo watts, but the power consumption of a lamp could equally well be expressed in horsepower, or an automobile engine rated in kilowatts.
Note that multiplication of a unit of power by a unit of time gives a unit of energy.
An example of this is the kilowatt-hour (kWh), this unit is often used by electricity companies to bill our energy consumptions.
Scale of power
Machines of very low as well as very high power have been created by man. A spring clock mechanism has a power of the order of 10-7 W. The engines of a ships or large-scale electric power plant sometimes develop a power of hundreds of thousands of kilowatts, i.e. 1015 times that of the clock. The average power of a horse is about 800 W. The average
power developed by a man during a physical activity is about 50-100 W.
For a very short time, a sportsman may develop a power of several kilowatt. The ability to develop high power, if only for a very short time, is one of the main qualities a sportsman should have. This is especially important in sprinting,
