PHYS 2212 Module 6.2

Resistors in Series and Parallel

Learning Objectives

By the end of this section, you will be able to:

• Define the term equivalent resistance
• Calculate the equivalent resistance of resistors connected in series
• Calculate the equivalent resistance of resistors connected in parallel

Many real world electronic devices are just collections of wires, resistors, capacitors, and batteries, forming circuits that do something (flashlights, toasters, blow dryers, radios, amplifiers, …) It’s important to understand (and predict) the currents and voltages in such circuits.

Resistors in Series

Resistors are in series whenever the current must flow through devices sequentially. In this figure, the total amount of current  is the current that is pumped through the battery and it’s the same current that must flow through R₁, R₂, and R₃. So these three resistors are wired in series. The current that goes through one must go through the next one. What goes in must come out — which is conservation of charge. This is not an approximation of any kind, it’s exactly true.

Let’s use the voltage loop rule and add up all the changes in voltage, starting at the battery and going around the circuit in the same direction as the current:

If we replaced these three resistors with one equivalent resistor:

The voltage loop rule equation would come out to be:

so when you compare the two loop rule equations for the equivalent circuits, you get:

When resistors are wired in series in a circuit, the resistance adds to get a bigger resistance. So the current drawn will be less the more resistors you have in series.

Now we will look at a similar circuit but we will replace the resistors with light bulbs. Light bulbs are also resistors, but when you buy a light bulb you aren’t told what the resistance is. Instead the light bulb package has a power rating on it.

These power ratings are determined by assuming the light bulb is connected to a power source of 120 V (which is the average voltage of a typical household electrical outlet). We can use the power rating and this 120-V voltage to calculate the resistance of the light bulb:

So the light bulb with a power rating of 100 W has a resistance of 144 Ω. The 40 W light bulb will have a different resistance:

In the example, the voltage across the bulbs will not be 120 V, so the bulbs will not dissipate energy at the labeled power rating, but the resistance of the bulb can be calculated using the method above.

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Resistors in Parallel

Resistors are in parallel when each resistor is connected directly to the voltage source by wires having negligible resistance. Each resistor thus has the full voltage of the source applied to it. The amount of current that flows through one resistor is not necessarily the same as the current that flows through another resistor when they are wired in parallel. In this circuit, the total amount of current that is pumped through the battery is . Some of that total current will flow through R₁, some through R₂, and the rest through R₃. Since charge is conserved, what we can say is .

We can use Ohm’s law to find the amount of current that flows through each resistor in the parallel circuit. Because each resistor is directly connected to the power source, the voltage across each resistor will be equal to the voltage of the battery. Then we get

If we replaced these three resistors with one equivalent resistor, 

the current through this equivalent resistor would be

What we can do is relate the total current in each circuit to get

and since the emf is the same in each term, we can find a way to calculate the equivalent resistance when resistors are wired in parallel:

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More Resistor Circuits