Power Calculations:

You can compute the power P (in watts, represented by W) in a dc circuit like that shown in figure below by using the following formula:

P = EI

Here E is the voltage in volts and I is the current in amperes. You might not be given the voltage directly, though you can compute it if you know the current and the resistance.

Keep in mind the Ohm's law formula for getting voltage: E = IR. When you know I and R but do not know E, you can get the power P by means of this formula:

P = (IR) I = I²R

That is, acquire the current in amperes, multiply this figure by itself, and then multiply the outcome by the resistance in ohms.

You can also acquire the power if you are not given the current directly. Assume that you are given only the voltage and the resistance. Recall the Ohm's law formula for acquiring current: I = E/R. Hence, you can compute power by using this formula:

P = E (E/R) = E²/R

That is, take the voltage, multiply it by itself, and divide by the resistance. Taking all together these power formulas are as follows:

P = EI = I²R = E²/R

PROBLEM:

Assume that the voltmeter reads 12 V and the ammeter shows 50 mA. Calculate the power dissipated by the potentiometer?

SOLUTION:

By using the formula P = EI. At first, convert the current to amperes, getting I =0.050 A. Then P = EI = 12 x 0.050 = 0.60 W.