Video Transcript
The diagram shows a circuit containing several diodes and bulbs. All of the bulbs are connected in parallel with the cell. Which of the bulbs, if any, are lit?
In this circuit, we see the cell as well as the bulbs, bulb one, two, and three, connected with the cell in parallel. We can also see that the circuit includes one, two, three, four, five diodes. A diode is a circuit component designed to control the flow of charge. When a diode is in a circuit, it allows charge to flow one way in the direction of what we could think of as the arrowhead of the diode. But at the same time, charge cannot flow in the opposite direction through a diode.
We can think of a diode then as a valve or a one-way switch in a circuit. All this comes into play in our circuit here because we can see that the bulbs will only be lit up if current exists in them. That can only happen if the various diodes in the circuit allow the flow of charge in these directions. Looking at our cell, we see that the positive terminal is at the top and the negative terminal is at the bottom.
Conventional current travels from positive to negative. So we can imagine the flow of charge from our cell as moving in this direction. We can think of that charge eventually reaching this first junction point. If charge is permitted to travel in this branch, then we can see it will pass through bulb one and light up that bulb. And indeed, based on the orientation of this diode, which allows charge flow from top to bottom but blocks it from bottom to top, electric charge can travel in this direction through this branch of the circuit. That charge passes through bulb one, causes it to light up, and completes the circuit like this. So, bulb one in our circuit does light up.
Let’s now follow the potential flow of charge to other branches of our parallel circuit. At this junction, we want to know if charge can flow down in this direction and then pass on through bulb two. Based on the orientation of the diode in this branch, we see that charge flow in that direction is not allowed. In the circuit symbol for a diode, we can almost think of this line here as acting as a wall to block current coming against that direction. Due to this, no current exists in the second branch of our circuit, and therefore bulb two does not light up.
Now, let’s consider the branch including bulb three. Charge, we see, can flow in this direction through our diode here. But then, as we imagine that charge approaching bulb three, we see it encounters this diode. Just like the diode in the previous branch, this one is oriented to block the flow of charge from top to bottom.
When a diode points in this direction, opposing the direction of conventional current, we say that it is reverse-biased. This reverse-biased diode prevents the flow of charge through this third branch and therefore through bulb three. Bulb three, like bulb two, does not light up. In answer to our question then, bulb one in this circuit does light up, but it’s the only bulb that’s lit.
