Energy dissipation in resistors. Joule's Law

Energy and Power

Energy: is the capacity to do something (to perform some work).

For example, if you connect a battery to a light bulb, the energy is converted into light and heat.

The unit of energy is Joule (J). And the speed on which this energy is used is measured in joule per second. This is called: Power. Then

Power: Is the speed the energy is used

The formula is: P = J/T (power = energy per unit of time). If we use 1 Joule in 1 second, we can say that we have used 1 watt.

There are several formulas that help us to get the power that is dissipated in any element.

One of the best formulas known is: P = V x I.
Where:
- V is the voltage at the terminals of the element and ...
- I is the current flowing through it.
In the case of a resistor, we also can use the following formulas:

Alternate forms of Joule's Law for resistors

- P = V²/R: We use this formula if we know the resistors value and the voltage between its terminals. (but we don`t know the current)

- P = I²xR: We use this formula if we know the resistors value (resistance) and the current through the element. (but we don´t know the voltage)

For Example

If we connect a light bulb to a 12 volts car battery and the current through the light bulb is 2 amps, then the power that is dissipated by that bulb (heat and light) is: P = V x R = 12 x 2 = 24 watts

Using the same data and the power we found before, we can find the resistance (ohms) of the the light bulb using any of the following formulas:
P = V²/R or P = I²xR

Using the formula P = V²/R, and clearing R, you get:
R = V²/P = 144 / 24 = 6 ohms

Related Links
Electrical Resistance
Wire wound resistor
Photoresistor - LDR
Resistors in series and parallel
Continuous or direct current
Alternating current (AC)
Properties of the alternating current
Voltage division
Resistivity

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