A series circuit is the simplest type of electrical circuit. In a series circuit, there is only one path for current to flow. All of the current flows to each component in turn. It also means that all the electrons flow at the same rate throughout all parts of the circuit. Current is equal everywhere within a series circuit.

In a series circuit, if there is more than one resistance in the circuit, those resistances are connected one after the other; thus the resistances add up. The total resistance in a series circuit is the sum of all of the individual resistances. For example, imagine a circuit with three resistors in series, each having 4 ohms of resistance, powered by a 12-volt battery. Total resistance of the three 4-ohm resistors is 12 ohms, since resistance adds up in a series circuit. According to Ohm’s law, if 12 volts is applied to a circuit that has 12 ohms of resistance, the resulting current flow will be 1 amp.

However, the voltage at different points within the circuit changes, as the electromotive force, or pressure, drops from a potential difference of 12 volts as it leaves the battery to virtually no difference, no voltage at all, as it returns to the battery. At each point in the circuit where current flows through a resistance, a drop in voltage occurs, which is called a voltage drop. Voltage drops are good when they occur inside of an intended load. They are bad when they occur where they are not wanted.

In Figure 36-29, after the first resistor, voltage has dropped from 12 to 8 volts. After the second, it is down to 4 volts. After the third, it is 0 volts. The voltage drop across each resistor can be found by subtracting the voltage after a resistor from the voltage before it, or the difference can be measured. The voltmeter will read 4 volts in each case because that is the difference between the two points, the potential difference, or voltage.

Ohm’s law can be used in series circuits to calculate voltage, resistance, and current. Any one of these can be calculated as long as the values of the other two are known.

The series circuit laws listed here provide a summary of how electricity behaves in a series circuit, which is defined as a circuit with multiple loads but only one path for current to flow:

- Current flow stays the same in a series circuit. Current flow is the same in all parts of the circuit.
- Voltage drops as current goes through resistance(s) in series. The applied voltage is equal to the sum of the individual voltage drops (Kirchhoff’s voltage law).
- Resistance adds up in series. Total circuit resistance is equal to the sum of the individual resistances; for example, R
_{T}= R_{1}+ R_{2}+ R_{3}, and so on.