Using Ohm's Law with Circuits

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what Ohm's Law is and when it can be used. You'll also find out how to use Ohm's Law to solve circuit problems. A short quiz will follow.

What is Ohm's Law?

Ohm's law says that the current flowing through a conductor is proportional to the voltage across it and inversely proportional to the resistance of that conductor. In other words, as resistance goes up, current goes down, and as voltage goes up, current also goes up.

Let's make sure we understand the meaning of the terms we just used. Current is the flow of electricity, or flow of electrons, around an electrical circuit as measured in amps. Resistance is the tendency of a component or material to hold back the flow of current, which is measured in ohms. And voltage is the difference in potential between two parts of a circuit as measured in volts.

If you think of a circuit as like water flowing around a water slide, the current is the rate of water flow, and the voltage is how high the slope of the water slide is.

How to Use Ohm's Law

As an equation, Ohm's law can be written as I = V/R. Doing this allows you to calculate the three quantities about a particular circuit. If you know current and resistance, for example, you can figure out the voltage.

You can use Ohm's law for an individual component inside a circuit: the current through a bulb, voltage across the bulb, and resistance of the bulb. Or you can use Ohm's law for the whole circuit, using the total current, voltage of the battery (total voltage), and total resistance. You can even do it for an individual branch in a series circuit. It still works.

Ohms Law
Ohms Law

However, for Ohm's law to work, the components in the circuit have to be OHMIC. Not all electrical components follow Ohm's law - not all are ohmic - but most are.

Example

Let's say you have a parallel circuit containing a 12-volt battery and two bulbs in separate branches: one with a resistance of 4 ohms, and the other with a resistance of 3 ohms. How do you think we'd go about calculating the current going through the 3-ohm resistor?

To solve the problem, we'll need to use Ohm's law for the 3-ohm resistor. Remember, current equals voltage divided by resistance, or I = V/R.

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