Video Transcript
The figure shows a circuit that has a diode, two lamps L one and L two, a battery,
and a switch S. If L one and L two are identical, which lamp, if any, is illuminated when S is
closed? (A) L one only, (B) L two only, (C) L one and L two, (D) none of the lamps.
In this question, we’re essentially being asked if the current will pass through
either of the lamps L one or L two when the switch S is closed. First, let’s remember a couple important things about the components in this
circuit.
A diode, which looks like an inverted triangle with a line underneath it, is a
component of an electrical circuit that allows a current to pass through it in one
direction but not in the opposite direction. For conventional current, from the positive terminal to the negative terminal, the
symbol points in the direction of the allowed current. This means the current in the circuit will be traveling roughly in a clockwise
direction.
If S is open, there will be current present along this branch, which we’ll call
branch two. But the diode won’t let current be there because it only allows current through in
the opposite direction. If S is closed though, there will be a current through branches one and two. But the diode does not allow the current in branch two through because the triangle
points in the opposite direction to the current.
Remember that the direction of current is from the positive terminal to the negative
terminal. This means there will only be a current along branch one. Using this knowledge, we can work out that only the L one lamp is turned on. So the correct answer is option (A).
