Final answer:
In a series circuit, the voltage measured across an open is equal to the source voltage, as no current can flow through the open, leading to no voltage drop across any other components.
Step-by-step explanation:
In a series circuit, the voltage measured across an open will be the source voltage. An open in a circuit means there is a break or disconnection, which prevents current from flowing. Since current cannot flow through an open circuit, no voltage drop occurs across any components within the circuit. Therefore, when a measurement is made across the open, it will reflect the full voltage supplied by the source—there is nowhere else for the voltage to drop.
For example, if a series circuit consists of a voltage source and three external load resistors with an open after the last resistor, the voltage across the open would be equal to the voltage source's output, as no current is flowing through the resistors. This is supported by the conservation of energy principle within the circuit, and Ohm's law (V = IR), which indicates that the voltage drop across a resistor is proportional to the current through the resistor, multiplied by the resistor's resistance. With no current (I = 0), there is no IR drop, resulting in the full source voltage being present at the open.