Power calculations

You can compute the power, in watts, in a direct current circuit such as that shown in the Figure, by formula *P *= *E**I *or the product of voltage in volts and the current in amperes. You might not be given the voltage directly, but can compute it if you know the current and resistance.

Remember the Ohm's Law formula for obtaining voltage: *E* = *IR**. *If you know *I *and *R**, *but do not know *E**, *you can get the power *P *by means of formula *P *= *(IR)**I **= **I*^{2}*R. *That is, you take current in amperes, multiply this to itself, and then multiply the result by resistance in the unit ohms.

You can get the power if you are not given the current directly. Assume that you are given voltage and resistance, the Ohm's Law formula for obtaining current: *I* = *E /R. *Thus, *P *= *E (E/R)* = *E*^{2}*/R**. *Take the voltage, and multiply it to itself, and divide by the resistance.

These power formulas are as follows when stated all together:

*P *= *EI *= *I*^{2}*R *= *E*^{2}/*R*

Problem: 1

Suppose that voltmeter reads 12 V and ammeter shows 50 mA. What is power dissipated by potentiometer?

Use formula *P * = *EI**. *First, convert current to amperes, getting *I* 0.050 A. Then *P =* *E**I = *12 * 0.050 =0.60 W.

You may get the answer 600 mW, although that is to three significant figures. It's not easy to specify the number 600 to 2 significant digits without using a means of writing numbers called as *scientifi**c notation**. *That subject is beyond the scope of this discussion, so for now, you may get the answer "600 milliwatts, accurate to 2 significant figures."