The circuit photographed below includes three identical light bulbs and two identical batteries, wired as seen in the photograph. In this photograph the switch is OFF, that is, the circuit is open at that point. The intensity level of the two light bulbs is identical (well, almost in the photograph).

The question this week is: What will happen when the switch is turned ON, closing the circuit.

The upper light bulb will:

• (a) become brighter.
• (b) become dimmer.
• (c) stay the same.

The lower light bulb will:

• (a) become brighter.
• (b) become dimmer.
• (c) stay the same.

The center light bulb will:

• (a) go ON.
• (c) stay OFF.

After February 28th, 2014, click Read More for the answer.

For the first two questions, the answer is (c): the bulbs remain at the same brightness, as can be seen by comparing the two photographs below.

  

The center bulb will remain OFF. Because the two sides of the circuit connected by the middle wire are at the same potential, no current can flow in that wire, and the bulb will not be lit.

The fundamental feature for this circuit is that the potentials at the points on opposite sides of the switch are the same before the switch is closed. This can be seen by noting that the two batteries are identical and the two light bulbs are identical. Therefore, closing the switch does not do anything to the circuit.