# How to Find Equivalent Resistance

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• 0:00 What Is Equivalent Resistance?
• 0:47 Calculating Equivalent…
• 1:32 Example Equations
• 2:39 Lesson Summary
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Lesson Transcript
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 equivalent resistance and use equations to find the equivalent resistance of a basic series or parallel circuit.

## What Is Equivalent Resistance?

Before we address what equivalent resistance is, we should first define resistance itself. Resistance is a measure of how much a component or material resists the flow of electricity through it, measured in ohms. It's inversely proportional to current: more resistance means less current will flow; less resistance means more current will flow.

Equivalent resistance may sound like a complex term, but it's really not. It's just a fancy way of saying total resistance. When you have a circuit with more than one component in it, we need to have a way of calculating the total effective resistance of the whole circuit or of one part of the circuit.

## Calculating Equivalent Resistance

The way we calculate equivalent resistance depends on whether we're working with a series or a parallel circuit. A series circuit is one where all the components are connected in a single, continuous loop. A parallel circuit is one where the components are connected in separate branches. The method we use to calculate equivalent resistance is different for each type of circuit.

For a series circuit, we simply add up the resistances of each component. However, in a parallel circuit, the reciprocal of the total resistance is equal to the sums of the reciprocals of the resistances of each branch.

Let's go through an example of each type.

## Example Equations

Let's say you have a series circuit containing a resistor, a bulb, and a battery. The resistor has a resistance of 6 ohms, and the bulb has a resistance of 3 ohms. What is the equivalent resistance of the circuit?

Simple: 6 + 3 = 9 ohms

But now if we connect those two components in a parallel circuit, our calculations get a little more complex. We can say that the reciprocal of the total resistance equals one over six, plus one over three.

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