Joule's Experiment and the First Law of Thermodynamics

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Joule's experiment demonstrated the validity of the first law of thermodynamics. Electrical or mechanical energy can be converted into thermal energy, but the total amount of energy is conserved. In this Demonstration, the amount of heat in joules generated in a circuit element of resistance (immersion heater) is measured by a calorimeter. Assume that all the heat is absorbed by the water and measure the temperature increase from to . Then



Joule's law gives liberated by current flowing through a resistor with resistance for a time ,


Using Ohm's Law , we obtain ; therefore



The final temperature (in ºC) and the energy gained by the water (in J) are shown on the diagram.


Contributed by: Anping Zeng (June 2015)
Open content licensed under CC BY-NC-SA



Snapshot 1: only heating time into 240 s Snapshot 2: only voltage into 4.85 V Snapshot 3: only depth of water into 7.5 cm

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