What is a Series Circuit?

series circuitsIn the last post, about electricity, or first post in the fundamentals of electricity, we laid the foundation. We started with the basics. Already we are jumping into some heady stuff – series circuits.

There are different kinds of electrical circuits – series, parallel, and series-parallel. Later we’ll cover the last two, or at least parallel (I am just now learning about series-parallel.) The two circuits you must have a handle on are series and parallel.

In each type of circuit there are rules that dictate the operation. For you to be able to later troubleshoot an electrical issue, or perform a calculation say trying to figure out total current, you need to understand the differences between the circuits.

You ready?

What is a Series Circuit?

A series circuit is a circuit in which devices are connected so that current can only flow through one path, things are connected in series.

The direction of current flow is the same direction that the electrons we talked about previously are moving. Check out this picture of a simple series circuit:

what is a series circuit

A series circuit – a battery at the bottom provides the voltage, 3 lamps connected in series.

Can you see how the current coming off of the negative terminal of the battery must hit every lamp? And how the current must pass through lamp 1 to get to lamp 2 and so on?

Quick question to make you think:

  • What do you think would happen to lamp 2 if lamp 1 was burned out?

It wouldn’t light right? Current would never make it through lamp 1 since it is burned out. No complete path for the electrons to flow. Sometimes you may not want to connect electrical devices in series for this reason.

Voltage in a Series Circuit

The total voltage (VT) in any series circuit is distributed across all of the devices connected to the circuit in a series of voltage drops. Check out the drawing below.

series circuits

Equal resistors in series.

The three squiggly lines at the top of the drawing are resistors. They can impede the flow of current. The filament of a light bulb offers some resistance to the flow, different metals have different resistances. Radio Shack makes them so that you can lower the value.

Anyways, all three resistors in the drawing are of equal value, 4 ohms, and they are connected in series. Current must pass through one to get to the next and so on.

The voltage through each device is equal to 1/3 of the total voltage. As you can see, the total voltage here is 6 volts and each device gets 2 volts.

The Ohm’s Law equation is V = I x R.

Or voltage (also represented by E) equals current (I) times resistance (R).

The total resistance (RT) of the series circuit above is 12 ohms, 4 + 4 + 4.

Our total voltage (VT) is 6 volts.

Plugging these numbers into the equation, we get:

  • 6 volts = 12 ohms x total current

Solving for total current (IT), we get .5 amps. Easy right?

Now let’s look at this drawing of another series circuit. In this one, the voltage across each resistor is proportional to the resistance of the device:

series circuit and ohm's law

Resistors with unequal values.

In both drawings you can see that if you add up all of the individual voltages across each device, they add up to the total applied voltage. 6 up above and 12 volts here in the second one.

This brings us to a rule for series circuits:

  • The sum of the voltage drops across individual resistors in a series circuit is equal to the total applied voltage.

An equation for this would like like this:

  • VT = V1 + V2 + V3 + …

For this drawing, what would our current be?

  • V = I x R
  • 12 volts = I x 16 ohms
  • I = 12/16 or .75 amps. Nice.

Current in a Series Circuit

We only have one path for current in a series circuit right?

Therefore the current passing through all of the devices in a series circuit is the same. An equation for this would like like this:

  • IT = I1 = I2 = I3 = …

The total current is equal to the individual currents flowing through each device.

Resistance in a Series Circuit

what is a series circuit

You’ve seen these before right? A resistor.

In a series circuit, the total resistance is equal to the sum of the resistances of all devices and parts of the circuit.

In the very top drawing of a series circuit here, the total resistance is the resistance from the negative terminal of the battery to the positive terminal of the battery with the voltage source disconnected.

The equation for total resistance would look like this:

  • RT = R1 + R2 + R3 = …

Short Circuits

If we come across a path in a circuit that shows very little resistance, like outside the norm, that’s called a short circuit.

Let’s take a regular old electrical cord. Underneath the orange insulation the wires twist around each other for the whole length. If the inner insulation becomes bare and two wires touch each other, a path of nearly zero resistance is created.

Why should the current work hard and push against all the resistance when it can go here? It’s like opening another valve on your water line. It follows the path of least resistance.

When a short circuit happens, there is a huge amount of current leading up to that point of contact. Stuff, like wires or devices, can overheat and melt.

Open Circuits

An open circuit happens when some part of the circuit is open, like a switch, or expended, like a fuse. When this happens there is no current anywhere in the circuit because it can’t flow.


Pretty good yeah? Covered quite a bit in this post on series circuits.

Now you should be able to define a series circuit and explain the basic relationships of voltage, current, and resistance in them. You should now also be able to apply Ohm’s Law and calculate any missing information like we did with the current above.

Remember not to apply the stuff and rule you learned here to other types of circuits such as parallel. We’ll dive deeper into that later. For now, just apply this knowledge to series circuits. Cool?

If I’m missing something or if you have anything to add or would like to know, don’t hesitate to leave a comment or email a question.

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