(EAM) Series Circuits Lesson

Series Circuits

In an electrical circuit, several electrical devices such as light bulbs can be placed in a line or in series in the circuit between the positive and negative poles of the battery. This is called a series circuit. One problem with such an arrangement is if one light bulb burns out, then it acts like a switch and turns off the whole circuit.

Consider the series circuit sketched above. This circuit has a voltage drop for the entire circuit of 120 V and has three resistors connected in series. The current in this circuit is drawn in terms of electron flow. The electrons leave the potential difference (voltage) source at the negative terminal and flow through the three resistors, starting with R₃ . The voltage drops occur when the current passes through each of the resistors and the total voltage drop for the entire circuit is equal to the sum of the voltage drops through the three resistors. In other words, the voltage across the battery in the circuit is equal to the sum of voltages across the resistors in series:

$V_T=V_1 + V_2 + V_3$

The current through each of the resistors must be exactly the same because the current in a series circuit is the same everywhere. The current is moving in the entire circuit at the same time.

$I_T=I_1 + I_2 = I_3$

Since the current passes through each resistor, the total resistance in the circuit is equal to the sum of the resistors. In the circuit above, the total resistance is: 

$R_T = R_1 + R_2 + R_3=30\Omega+15\Omega+15\Omega=60\Omega$

Consider the diagrams below. On the left, there is a circuit with a single resistor and a battery. No matter where we measure the current, it is the same in a series circuit. On the right, we have added a second resistor in series to the circuit. The total resistance of the circuit has increased and you can see from the reading on the ammeter that the current in the circuit has decreased.

Series Circuit Examples Links to an external site.

Please watch the following video showcasing series circuits. Use this resource to answer the questions that follow.

1. How do the voltage drops across the two light bulbs in the video related to the total voltage drop for the entire circuit?
2. In the video, what was the assumed voltage drop for the connecting wires and the switch?
3. What was the current through the second light bulb as compared to the current through the first light bulb?

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