Joule's Experiment

Mechanical Equivalent of Heat

Joule experiment

The original Joule experiment consists of a receptacle filled with water and a mechanism with spinning plates. The kinetic energy of the plates is transformed into heat, because the force of gravity performs work on the weight falling a distance . This gave an experimental confirmation of the equivalence between heat and work, now defined to be exactly 1 calorie for every 4.1855 joules and called a "thermochemical calorie".

 

How to do

• set the mass of the two suspended spheres in kg

• set the mass of water contained in the calorimeter (includes the equivalent mass in water of the calorimeter) in grams

• enter a value by moving  the sliders or by entering the numbers in the corresponding boxes

• press the "START" button, suspended objects will be free to fall under the action of gravity;

• at the end of the  fall the thermometer will mark a given temperature, greater than the initial one;

• press the "START" button to drop the masses again ;

• press the "RESET" button to reset all the initial conditions;

Proposed activity

• press the reset button

• record the initial temperature Ti

• drop the spheres n times

• record the final temperature Tf

• calculate the work done: L = 2mgh * n ( we have 2 falling masses and the  height h = 2m)

• calculate the product heat : Q =ma (Tf-Ti) * c (c: specific heat of water = 1cal / (g ° C),       ma = mass of water + equivalent mass in water of the calorimeter)

• calculate  j = L / Q

NOTE
mass of spheres  in kilograms
water mass in grams
sensitivity of the thermometer 0.1 C
length of the fall 2m ±1cm

 

James Prescott Joule (1818 – 1889) calculated in 1843 the mechanical equivalent of heat in a series of experiments. In the most famous apparatus he built for this end, now called the Joule apparatus (see image below), a descending weight attached to a string caused a paddle immersed in water to rotate and heat the water. Joule supposed that the gravitational potential energy lost by the weight in descending was equal to the thermal energy (heat) gained by the water by friction with the paddle.

In this experiment, the friction and agitation by the paddle-wheel of the body of water, trapped in an insulated barrel (calorimeter), caused heat to be generated which, in turn, increased the temperature of the water. The temperature change ∆T of the water and the height of the fall ∆h of the weight m*g were recorded. Using these values, Joule was able to determine the mechanical equivalent of heat.

Joule contended that motion and heat were mutually interchangeable and that, in every case, a given amount of work (motion) would generate the same amount of heat. Moreover, he also claimed that heat was only one of many forms of energy (electrical, mechanical, chemical) and only the sum of all forms was conserved. Otherwise the calculated mechanical equivalent of heat is meaningless.

In 1845, James Joule reported his experiment in a paper On the mechanical equivalent of heat for the British Association meeting in Cambridge.

 

 

 

 

The quantity of heat produced is given by Q = C (Tf - Ti) cal, in which (Tf - Ti) is the difference between the final and initial temperature and C it is the total amount of heat of the system, given from Ma*ca +Mr*cr, in which Ma, ca and Mr, cr are respectively the masses and the specific heats of the distilled water and the calorimeter.

 

( please insert here some problems and/or exercises)