Page 142 - Class-11-Physics-Part-1_Neat

128 PHYSICS

We point out a common misconception created W
by newspapers and magazines. They mention P av =
food values in calories and urge us to restrict t
diet intake to below 2400 calories. What they The instantaneous power is defined as the
should be saying is kilocalories (kcal) and not limiting value of the average power as time
calories. A person consuming 2400 calories a interval approaches zero,
day will soon starve to death! 1 food calorie is dW
1 kcal. t P = dt (6.21)
The work dW done by a force F for a displacement
6.10.6 The Principle of Conservation of
dr is dW = F.dr. The instantaneous power can
Energy
also be expressed as
We have seen that the total mechanical energy
dr
of the system is conserved if the forces doing work P = F.
dt
on it are conservative. If some of the forces
involved are non-conservative, part of the = F.v (6.22)
mechanical energy may get transformed into
where v is the instantaneous velocity when the
other forms such as heat, light and sound.
force is F.
However, the total energy of an isolated system
Power, like work and energy, is a scalar
does not change, as long as one accounts for all 2 –3
quantity. Its dimensions are [ML T ]. In the SI,
forms of energy. Energy may be transformed from its unit is called a watt (W). The watt is 1 J s .
–1
one form to another but the total energy of an The unit of power is named after James Watt,
isolated system remains constant. Energy can one of the innovators of the steam engine in the
neither be created, nor destroyed. eighteenth century.
Since the universe as a whole may be viewed There is another unit of power, namely the
as an isolated system, the total energy of the horse-power (hp)
universe is constant. If one part of the universe
1 hp = 746 W
loses energy, another part must gain an equal
This unit is still used to describe the output of
amount of energy.
automobiles, motorbikes, etc.
The principle of conservation of energy cannot
We encounter the unit watt when we buy
be proved. However, no violation of this principle electrical goods such as bulbs, heaters and
has been observed. The concept of conservation refrigerators. A 100 watt bulb which is on for 10
and transformation of energy into various forms hours uses 1 kilowatt hour (kWh) of energy.
links together various branches of physics, 100 (watt) × 10 (hour)
chemistry and life sciences. It provides a = 1000 watt hour
unifying, enduring element in our scientific =1 kilowatt hour (kWh)
3
pursuits. From engineering point of view all = 10 (W) × 3600 (s)
6
electronic, communication and mechanical = 3.6 × 10 J
devices rely on some forms of energy Our electricity bills carry the energy
transformation. consumption in units of kWh. Note that kWh is
a unit of energy and not of power.
6.11 POWER
Often it is interesting to know not only the work
u Example 6.11 An elevator can carry a
done on an object, but also the rate at which
maximum load of 1800 kg (elevator +
this work is done. We say a person is physically
passengers) is moving up with a constant
fit if he not only climbs four floors of a building
speed of 2 m s . The frictional force opposing
–1
but climbs them fast. Power is defined as the
the motion is 4000 N. Determine the
time rate at which work is done or energy is
minimum power delivered by the motor to
transferred.
the elevator in watts as well as in horse
The average power of a force is defined as the
power.
ratio of the work, W, to the total time t taken

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