It’s the Law … Joule’s Law

In the last tutorial, we discussed Ohms law and we saw the relation between Voltage, Resistance and Current.

Joules, an English physicist and brewer (beer and science: the perfect couple 😉 ), found out that something else had a direct relation to current and resistance: heat.

Joules law states that the heat produced by an electric current is equal to the product of the resistance of the conductor, the square of the current, and the time for which it flows.

or as a formula:        Q = I²·R·t

where Q is the produced heat (unit = Joule), I is the current (unit = Ampere), R is the resistance (unit = Ohm) and t is time (unit = seconds).

“What does this have to do with electronics?” I hear you ask. “I want to use leds not an electric heater”. Well there is another unit that is directly related to Q: the Watt or W.

W is the unit of Power and is defined as one Joule per second and we can write that as:

P = Q/t

With a bit of mathematical magic we can deduct the equation for Q:

Q = P·t

Now we can insert this in Joules’ equation:

P·t = I²·R·t

When the same factor is applied on both sides of an equation, we can just drop it:

P = I²·R

I² is the same as I·I so:

P = I·I·R

Haven’t we seen I·R somewhere before? Indeed, Ohm’s law or V = I·R so we can replace I·R by V :

P = I·V

Now that’s a handy formula for electronics. If we want to know the power consumption of a circuit, we only need to know the Voltage and the current.

Or if we know the power consumption of a circuit and the voltage, we can calculate the current:

I =P/V

Joules law applies to all the circuits that follow Ohm’s law and when you have both equations together, you have a real problem-solving twin for a lot of your circuits.


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