Series circuits

​​In a nutshell

In a series circuit, the components are placed one after the other. The current is the same at all points around the circuit and the potential difference of the power supply is distributed across all of the components. If one component breaks then it breaks the whole circuit. The total resistance is the sum of all of the resistances of the individual components. 

Current in a series circuit

Current is the flow of charge. The charge carriers in electrical circuits are electrons. The electrons leave the power supply with electrical energy and transfer the electrical energy around the circuit.

In a series circuit the electrons only have one pathway to take. The electrons have to pass through each component to get back to the power supply. This also means that if a component or wire were to break then the electrons would lose their pathway and no current would flow. This means the whole circuit is broken. 

The number of electrons passing through any section of the circuit is constant. This means that the current at any point in a series circuit it the same. 

​$$I_1=I_2=I_3=I_4$$​​

Potential difference in a series circuit

When electrons leave the power supply, they are given electrical energy which must be used up around the circuit. The electrical energy is transferred into different types of energy depending on the type of component.

The higher the resistance of a component the more energy it will take for the electron to pass through it. Therefore electrical components with higher resistances will use up more of the electrical energy the electrons have. 

As energy has to be conserved, the electrical energy which is transferred to the electrons in the power supply will be equal to the electrical energy the electrons transfer to the circuit components. 

Example The circuit shown in the diagram has a power supply with a potential difference of $$6\ V$$​. Assuming all of the resistors have the same resistance, the potential difference will be divided across them all equally.

Resistance in a series circuit

As current only has one path to go in a series circuit, the current will have to travel through all of the resistors. This means that the total resistance of the series circuit is the sum of the individual resistances. 

​$$R_T=R_1+R_2+R_3+...$$​​

Example

In the circuit diagram, there are three identical resistors, each with a resistance of $$50 \, \Omega$$

First, write down the equation for the total resistance.

$$R_T=R_1+R_2+R_3+...$$​​

Next, substitute in the values.

$$R_T=50\,\Omega+50\,\Omega+50\,\Omega$$

$$R_T=150\,\Omega$$

The total resistance is $$\underline{R_T=150\,\Omega}$$​