power | unit of power | commercial unit of power| relation between the kilowatt-hour and joules скачать в хорошем качестве

power | unit of power | commercial unit of power| relation between the kilowatt-hour and joules

Suppose an old man takes 10 minutes to do a particular “work” whereas a young man takes only 5 minutes to do the same work. It is obvious that the “rate of doing work” of the young man is more than that of the old man. The rate of doing work is known as power, so we can say that the power of young man is more than that of the old man. Thus, power is defined as the rate of doing work We can obtain power by dividing the ‘Work done’ by ‘Time taken’ for doing the work. That is, P =power W =work done t= time taken In other words, power is the work done per unit time or power is the work done per second. Please note that the value which we get by dividing ‘Work done’ by ‘Time taken’ actually gives us ‘Average power’. We know that when work is done, an equal amount of energy is consumed. So, we can also define power by using the term ‘energy’ in place of ‘work’. Thus, power is also defined as the rate at which energy is consumed (or utilised). We can also obtain power by dividing ‘Energy consumed’ by ‘Time taken’ for consuming the energy. We can now say that : Power is the rate at which work is done or energy is consumed. It is clear from the above discussion that we can write the formula for calculating power in terms of ‘work done’ or in terms of ‘energy consumed’. Power is a scalar quantity which has only magnitude but no direction. Power is obtained by dividing ‘work done’ by ‘time taken’ to do the work. Now, work is measured in the unit of ‘joule’ and the time is measured in the unit of ‘second’, so the unit of power is ‘joules per second’. This unit of power is called ‘watt’. Thus, the SI unit of power is watt which is denoted by the symbol W. We can now define the unit of power ‘watt’ as follows : 1 watt is the power of an appliance which does work at the rate of 1 joule per second. We can also define watt by using the term ‘energy’ as follows : 1 watt is the power of an appliance which consumes energy at the rate of 1 joule per second. We can write an expression for watt as follows : Watt is an important unit of power since it is used in electrical work. The power of an electrical appliance tells us the rate at which electrical energy is consumed by it. For example, a bulb of 60 watts power consumes electrical energy at the rate of 60 joules per second (60 J/s or 60 J s–1). Different electrical appliances have different power ratings. The greater the power of an appliance, and the longer it is switched on for, the more electrical energy it consumes. The unit of power called ‘watt’ is named after a Scottish inventor, engineer and designer James Watt who became famous for improving the design of steam engine. Watt is a small unit of power. Sometimes bigger units of power called kilowatt (kW) and megawatt (MW) are also used. A yet another unit of power is called ‘horse power’ (h.p.) which is equal to 746 watts. Thus, This means that 1 horse power is equal to about 0.75 kilowatt (0.75 kW) The unit called ‘horse power’ originated long back when steam engines first replaced ‘horses’ as a source of power. These days the powers of engines (of cars, and other vehicles, etc.) are expressed in the unit called ‘brake horse power’ (b.h.p.). Brake horse power is the unit of power equal to one horse power which is used in expressing power available at the shaft of an engine. The b.h.p. of Maruti-800 car is 37 whereas that of Maruti Zen is 60. The more powerful a car is, the quicker it can accelerate or climb a hill, that is, more rapidly it does work. The commercial unit (or trade unit) of energy is kilowatt-hour which is written in short form as kWh. Kilowatt-hour is usually used as a commercial unit of electrical energy. The SI unit of electrical energy is joule and we know that “A joule is the amount of electrical energy consumed when an appliance of 1 watt power is used for one second”. Actually, joule represents a very small quantity of energy and, therefore, it is inconvenient to use where a large quantity of energy is involved. So, for commercial purposes we use a bigger unit of electrical energy which is called “kilowatt-hour”. One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour. 1 kilowatt-hour is the amount of energy consumed at the rate of 1 kilowatt for 1 hour. That is, 1 kilowatt-hour = 1 kilowatt for 1 hour 1 kilowatt-hour = 1000 watts for 1 hour From this discussion we conclude that 1 kilowatt-hour is equal to 3.6 × 106 joules of electrical energy. It should be noted that watt or kilowatt is the unit of electrical power but kilowatt hour is the unit of electrical energy.