Power in DC Circuits

Power is a measure of how much energy is given off or used up by an element in a circuit every second.

Power is measured in Joules per Second and uses the unit Watt (denoted W)

Power is simply the voltage across an element multiplied by the current through it: \(P = VI\)

Find the power across the resistor below.

From the circuit we can see that the voltage across the resistor \(V_r = 10\)V and from Ohm's Law we know the current is \(I_r = 100\)mA. So the power absorbed by the resistor is \(P_r = V_r\times I_r = 1\)W.

We can rearrange our basic power formula: \(P = VI\) using Ohm's Law to get a few other forms. By using \(I = \frac{V}{R}\) we get: \(P = \frac{V^2}{R}\) By using \(V = IR\) we get: \(P = I^2 R\)

Find the resistance of resistor R in the diagram below

We have voltage and power and we need resistance so, looking through our formulas, I'm going to pick the one that only uses voltage, power and resistance. \(P = \frac{V^2}{R}\) Now we'll plug in the values we have: \(1.4 = \frac{100}{R}\) Rearrange to solve for \(R\) and we get: \(R = \frac{100}{1.4} = 71.43\)