In a series circuit, the resistors are added one after the other on the wire. The current flows through one resistor and then through the next and so on. If one of the resistors on the circuit stops working, then the whole circuit stops working. Take for example, a series of light bulbs on a circuit. If one light bulb in this circuit burns out (i.e. the filament breaks), the circuit is no longer closed, and there is no path through which the current can flow. Strings of holiday lights used to be made this way, because it is less expensive to make them that way. If one light went out, the whole strand went out. Unfortunately for the user, that means the burned out bulb has to be replaced before the circuit will work again and they were very difficult to locate.

The total resistance on a series circuit can be determined. When several resistors are linked together in a series circuit, the resistance (in ohms) of each resistor is added together with all of the other resistors. Together they equal the total resistance of the circuit. If each light provides 1 ohm of resistance and there are 25 lights in the string, the total resistance of the circuit would be 25 ohms.

When you add more resistance to the circuit by adding resistors, and the total voltage (determined by the power source) stays the same, then by Ohm’s Law, the current will decrease. So if you want to determine the amperage of a series circuit, you must first add up all the resistances in the circuit. The voltage will drop as it goes through each resistor—in other words, energy gets used by each resistor. The more resistors you have, the less voltage difference there will be across each one. All the individual voltage drops added together will add up to the total voltage of the circuit, which is determined by the power source.

As you consider series circuits, keep these characteristics in mind: