A series circuit is an electrical circuit in which each device is linked to another device. If devices have positive and negative terminals, the positive terminal of each device is connected to the negative terminal of the next in the chain.

A typical high school series circuit might be as follows:

  • Connect positive terminal of battery to one terminal of light bulb.
  • Connect other terminal of light bulb to one terminal of resistor 1.
  • Connect other terminal of resistor 1 to one terminal of resistor 2.
  • Connect other terminal of resistor 2 to negative terminal of battery.

In a series circuit, current flows through each device in turn (in this circuit, current would flow from the positive terminal of the battery through the light bulb, then through r1, then through r2*) and voltage is shared between devices according to their resistance. Because of this voltage-sharing, adding another device to the circuit without changing the supply voltage means that the same voltage must be shared amongst more devices.

Taking Christmas tree lights as an example, this means that:

  • As bulbs are added to the chain, each individual bulb gets dimmer.
  • If one bulb burns out, there is no path for current to flow through the circuit, so every bulb goes out.

The resistance of a series circuit is simply the sum of each individual resistance in the circuit. For instance, if a 30-ohm bulb and two 100 ohm resistors are connected in series, the total resistance of the circuit will be 230 ohms.

* This is what you are taught in junior school. In senior Physics, they then tell you that they were lying, and in fact current - in the form of electrons - actually flows from the negative terminal to the positive terminal, since electrons have a negative charge...

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