What will a voltmeter read if properly connected across a closed switch in a circuit with electrical power on?

What will a voltmeter read if properly connected across a closed switch in a circuit with electrical power on?

• A. Zero voltage
• B. Voltage across supply
• C. Double the supply voltage
• D. Infinite voltage

Answer: (A)

The key idea is that a closed switch acts like a very small resistance path. When the switch is closed, the two points it connects are effectively tied together, so the voltage difference between them is essentially zero. A voltmeter connected across that closed switch will read about zero volts. In a real switch there’s a tiny contact resistance, so you might see a small amount of voltage, but it’s still negligible. The supply voltage appears across the load or other parts of the circuit, not across the closed switch.

The key idea is that a closed switch acts like a very small resistance path. When the switch is closed, the two points it connects are effectively tied together, so the voltage difference between them is essentially zero. A voltmeter connected across that closed switch will read about zero volts. In a real switch there’s a tiny contact resistance, so you might see a small amount of voltage, but it’s still negligible. The supply voltage appears across the load or other parts of the circuit, not across the closed switch.