Power calculations

You can calculate the power, in watts, in a dc circuit such as that shown in Fig.1, by the formula P = EI or the product of the voltage in volts and the current in amperes. You might not be given the voltage directly, but can calculate it if you know the current and the resistance.
Remember the Ohm’s Law formula for obtaining voltage: E= IR. If you know I and R, but don’t know E, you can get the power P by means of the formula P= (IR)I= I²R. That is, you take the current in amperes, multiply this figure by itself, and then multiply the result by the resistance in ohms.
You can also get the power if you aren’t given the current directly. Suppose you’re given only the voltage and the resistance. Remember the Ohm’s Law formula for obtaining current: I = E/R. Therefore, P = E(E/R) = E²/R. Take the voltage, multiply it by itself, and divide by the resistance.
Stated all together, these power formulas are:

P = EI = I²R = E ²/R

Now you are ready to do some problems in power calculations. Refer once again to Fig. 1.

Fig. 1: Circuit for working Ohm’s Law problems.