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Note on Power

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The rate of doing work is called power.

One watt power can be defined as that in which one joule work is done in one second.

Mathematically,

Or, $$P = \frac {F\times d}{t}$$

Or, P = F× v

The SI unit of power is joule per second(Js-1) which is called watt (w).

Power of lamp is 60 watt means that lamp can convert 60 J electrical energy into light and heat energy in one second.

Since the SI unit of power is joule per second, also known as Watt we have,

$$\therefore$$ 1 Watt = $$\frac{ 1\; joule}{1\; second}$$

Thus, a body is said to be one watt power if it can do one joule of work in one second. There are some bigger units of power like Kilowatt (kW), megawatt (MW) and Horsepower (HP). The relation of them with watt is given below,

i.e., 1kW = 103W
i.e., 1 MW = 106W or 103 kW
1 HP = 746 W (approx. 750 W)

Relationbetween Work and Energy.

Energy is the capacity of doing work. When the person does some work on a body, then the energy of the body increases. According to the principleof conservation of energy, the energy gain by the body is equal to the energy lost by the person. For example, when we push table, then we lost some muscular energy. But consequently, the table gains some kinetic energy while it is in motion. When spring is compressed, it stores some potential energy and we loss equivalent amount of muscular energy. We take food which contains chemical energy. The chemical energy is converted into muscular energy inside our body. Because of which we are able to do some work. But, we feel week when we do not take food and we are not able to do work. We feel hungry after having a meal when we sleep for 7- 8 hours. This is due to the work done inside the body like, heart pumps the blood, muscles stretch and relax, etc. The energy provided by the food is used in these internal processes of our body even though we do not perform any external work.

Comparison of work, energy and power

 S. N. work energy power 1 It is the product of force and displacement in the direction of force. It is the capacity of doing work It is the rate of doing work or rate of conservation of energy 2 Its SI unit is joule. Its SI unit is joule. Its SI unit is watt. 3 Its value does not depend on time. Its value does not depend on time. Its value depends on time. 4 In general, work is done against friction or gravity. It has different forms. It does not have any form.

• The rate of doing work is called power.
• The SI unit of power is joule per second (js-1) which is called watt (w).
• One watt power can be defined as that in which one joule work is done in one second.
.

Very Short Questions

The rate of doing work is called power. Its SI unit is watt (W). Mathematically, Power = =

60W written on an electric bulb means 60 joules of electrical energy is converted into light energy.

The relation between horsepower (hp) and watt(w) is 1hp = 746 watt = 750 Watt (approximately).

At a thermal power station, chemical energy is converted into heat energy and heat energy is converted into electrical energy.

Some mathematical relations of the powers are:

1.P = W/ t
2. P= F. d / t
3. P = F v
4. P = mgh/t

Here, Mass (m) = 100kg
Height (h) = 3m
Time (t) = 5s
Power (p) = ?

Here, the wprk is done against the gravity. Thus, work done (W) is given by,

i.e, P = $$\frac{W}{t}$$ = $$\frac{mgh}{t}$$
= $$\frac{100 \times 10 \times 3}{5}$$
= 600watt.

Hence, power of the man is 600W

0%
• Power is expressed as

work/time
time/work
joule
force X avg speed

capacity
power
energy
force
• The formula of power is

time taken/work done
work done/time taken
work done/velocity
force/area

erg
pascal
joule
watt
• If 1 Joule of work is done in 1 second, the power is said to be

1 watt power
1 joule power
3 pascal power
2 erg power
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