In a 12V series circuit with a single 20 Ohm bulb load, a Test Light connected to ground is used; with the switch closed and the bulb lit, what will the Test Light indicate?

In a 12V series circuit with a single 20 Ohm bulb load, a Test Light connected to ground is used; with the switch closed and the bulb lit, what will the Test Light indicate?

• A. Light before the load but not after the load
• B. Light after the load
• C. Light both before and after the load
• D. Light nowhere

Answer: (A)

A test light glows when there is a voltage difference between its probe and ground. In this simple 12V circuit, with the switch closed the hot side feeding the bulb is at +12V relative to ground, while the side of the bulb connected to ground sits at 0V. Placing the test light on the hot side creates a 12V difference to ground, so it lights. Placing it on the side after the load (the ground side) has essentially no voltage to ground, so it stays off.

A test light glows when there is a voltage difference between its probe and ground. In this simple 12V circuit, with the switch closed the hot side feeding the bulb is at +12V relative to ground, while the side of the bulb connected to ground sits at 0V. Placing the test light on the hot side creates a 12V difference to ground, so it lights. Placing it on the side after the load (the ground side) has essentially no voltage to ground, so it stays off.