Experiments on Mechanical Equivalent of Heat
Accurate investigation of
the relationship between heat developed and mechanical work
spent was taken up by the British Scientist James Prescott Joule. The
main aim of his investigations was to determine exactly the
ratio between the work done and the quantity of heat produced.
Joule used different
arrangements for doing the work, W in different ways and
measured the corresponding amount of heat, H produced in each
case. In all the
cases he found that the expenditure of the same amount of work
always produced the same amount of heat.
Every time he found that 4186 joule of work was spent to
produce the same amount of heat which could raise the
temperature of one kg of water through 10C.
He established the relation, W/H to be a constant
quantity. The constant relation W/H was represented symbolically
by the letter ‘J’. J
is known as Joule's mechanical equivalent of heat.
‘Symbolically, we can write :
W/H = J or
W = JH
Thus he established
that heat is a form of energy.
Joule’s Method for
Experimental Determination of J.
Apparatus : The apparatus consists of a specially designed
calorimeter placed in a wooden box C with felt lining to avoid
heat losses to the surroundings. A number of vanes, V, V projects from the walls of the
calorimeter in its interior.
A spindle carrying a number of brass paddles P, P acts as
a churner and it is so pivoted at the bottom that the paddles P,
P are capable of turning between the fixed vanes V.
spindle can be attached to a drum D by means of screw S’
whenever desired. The
drum D can be rotated by the handle H or by the falling weights
W, W, which are attached to two pieces of string passing over
the pulleys P1 and P2 and wrapped round
the drum. Two
vertical scales are fixed to note the vertical distances through
which the weights fall.
: The two weights W, W are allowed to fall through a height
‘h’. In doing
so, they turn the spindle and thereby turn the paddles P, P
immersed in a known mass (m) of water contained in the
water is thus churned but not allowed to rotate due to fixed
vanes and the potential energy of the falling weights is
converted into kinetic energy of paddles.
Due to friction offered by paddles KE is changed into
heat and as a result of it, the temperature of the water in the
calorimeter rises. The
rise in temperature is measured by an accurate thermometer T
inserted in the calorimeter.
The process is rapidly repeated several times (every time
detaching the drum, wrapping the string, tightening the screws’
and allowing the weights to fall), (say n times) such that there
is an accurately measurable rise in the temperature of water.
experiments conclusively established that heat is a form of
energy. It is
not a material substance like caloric fluid.
the same amount of heat was produced by spending a given
amount of mechanical work.
It is immaterial what type of arrangement is used for
doing mechanical work.
Other alternative ways used by Joule for conversion
of mechanical work into heat were (i)
by mechanically stirring mercury, and (ii)
by rubbing two iron rings together.
Every time he found that W/H is
a constant quantity, i.e., when 4.186 Joule
converted to heat the temperature of 1 kg of water will rise