Monday, May 22, 2017

#557 : J - Joule Heating

Yet again another Physics concept made memorable by a teacher. We had basic physics as a part of our engineering course work for first two semesters. A new lecturer had joined us back then. It was his first day in class and he promptly wrote down the syllabus on the board and made sure we copied it down too. Days went by when he would do nothing but ask us to study from the text book and repeat writing the syllabus on the board. Initially, we presumed him to be one of these strict type of professors only to realize he was the opposite. I still remember him talking about Joule Thompson effect. He kind of spent a whole month just dropping by those two terms and other random stuff. He used to call us out and ask us to explain things, his accent and the way he pronounced names would be the highlight of the whole class. We found that extremely funny and often made fun of him. Amidst all this fun, for some odd reason, my brain associated him with this concept.


Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor produces heat.

The most general and fundamental formula for Joule heating is:

{\displaystyle P=(V_{A}-V_{B})I}

P is the power (energy per unit time) converted from electrical energy to thermal energy,
I is the current traveling through the resistor or other element,

The explanation of this formula

(Energy dissipated per unit time) = (Energy dissipated per charge passing through resistor) × (Charge passing through resistor per unit time)

When Ohm's law is also applicable, the formula can be written in other equivalent forms:


where R is the resistance.

We kind of spent two years of our coursework playing about with this formula.

So the practical applications of Joule heating effect is fusing electric circuits. A short piece of metal, inserted in a circuit, which melts when excessive current flows through it and thus breaks the circuit. It thus protects appliances. The material of a fuse generally has a low melting point and high conductivity.

Common domestic applications are the electric iron, bread toaster, electric kettle, heater, etc.

Electric heating is also used in producing light, as in an incandescent bulb. Here, the filament is made of a resistor that retains as much of the heat generated as possible. Then it can get very hot and emit light. It must not melt at the high temperature. Usually, tungsten is used for the bulb filament, as it has a high melting point (6116oF) and is a strong metal. A small amount of the power used by the filament appears as radiated light, but most of it appears as heat.


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