How to Calculate Current and Voltage

Ohm's law makes basic physics calculations easy to do.

High school physics is one of the more challenging classes that students must take before they graduate. These classes almost always include a unit on electricity that requires students to complete simple electrical calculations, such as finding the current and voltage of a given circuit. Learning the basic laws of electricity will help students to solve these problems.



In this case, the basic electrical law used to calculate current and voltage is Ohm's Law, which states that the voltage of a circuit is equal to its current multiplied by its resistance.

Things You'll Need

• Calculator

Instructions

Calculating Current

1. Write down the information that your problem gives you. In most cases, you will need the voltage and the resistance of your circuit to calculate current. Some more advanced problems might state the voltage as "potential difference," which can confuse beginners.

2. Check to make sure that you will use the resistance in your calculation. One very easy pitfall to avoid is using resistivity instead of resistance to calculate current. While these two concepts are similar, you will get the wrong answer if you calculate current using resistivity.

3. Divide the voltage by the resistance to calculate the current. For example, if your problem uses a 120V circuit with 10 ohms of resistance, you would divide 120 by 10 to get a current of 12 amperes.

Calculating Voltage

1. Write down all the information from your problem. In order to calculate the voltage, you will need to know both the current and the resistance of your circuit.

2. Check to see if the information from the problem uses the correct units. You should not have to worry about this when you calculate current, because professionals usually report voltage and current in whole units. However, you might have to check your units when you calculate voltage, because current is measured in milliamperes (mA) instead of amperes (A). For example, you might see a current of 120 mA.

3. Divide the current given to you in the problem by 1,000 if you have mA instead of A. For instance, given a current of 120 mA, you would divide 120 by 1,000 to get 0.120 A. (If you already have the current in amperes, you can skip this step.)

4. Multiply the current by the resistance given to you in the problem to find the voltage. In this example, given a current of 0.120 A and a resistance of 100 ohms, you would get a final answer of 12 volts.

References

• "College Physics"; Raymond Serway and Jerry Faughn; 2000
• NASA Glenn Research Center: Ohm's Law