Question

Look at the simple circuit illustrated in the figure above. What will happen when only switch S1 is closed?

A. No current will flow in the circuit.
B. Nothing will happen; the light bulb won't light up.
C. The light bulb will light up.
D. The resistance of the circuit will increase.

Answer 1

If only S1 is closed, due to the diode across S3 is pointing to the right, the current cannot be flowing to the left.

So nothing will happen, the bulb is not on.

Answer 2

Answer: A. No current will flow in the circuit.

Step-by-step explanation:

The Figure 1 illustrates the equivalent circuit at only switch S1 closed. That is a series circuit with two diodes, a bulb, and a battery.

Suppose that there is a current flowing in the circuit. A current flowing in the circuit implies that D1 is on. If D1 is on, there should be 0.7 V between anode and cathode (see Figure 2 left). However, there is not possible a low voltage on the top and a high voltage at the bottom of D1 (see Figure 2 right). Then, it is not viable 0.7 V between anode and cathode of diode D1. Therefore, D1 is off.

Now, we already know that this is a series circuit with the diode D1 off, so we can conclude that no current will flow in the circuit.

Correct answer is A. No current will flow in the circuit.

There isn't current flowing in the circuit next the light bulb won't light up. But other things happen, for example, the high voltage goes to the light bulb at only switch S1 closed. we can say that option B. is partially wrong and options C. is wrong

Finally, there are no resistances in the circuit. So, option D. is not right