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Question Video: LED Forward and Reverse Bias Physics

The diagram shows 3 LEDs connected in parallel with a cell and a resistor. Which of the LEDs will emit light in this circuit?


Video Transcript

The diagram shows three LEDs connected in parallel with a cell and a resistor. Which of the LEDs will emit light in this circuit?

Okay, taking a look at our diagram we see our power cell, our three LEDs marked out A, B, and C, and then also the resistor that’s in the circuit. We want to know which of these LEDs will emit light. And another way of saying this is which of the LEDs will have current running through them.

To figure this out, It’s helpful to realize that an LED, a light-emitting diode, is indeed a diode. A diode, and this is the circuit symbol for a diode, works as a current valve. It allows flow of current in one direction but prevents it in the opposite direction. And looking at the symbol for a diode, that direction of allowed current flow is indicated by the way this arrowhead points. For a diode set up this way, current could flow left to right, but it will be blocked from flowing right to left. A diode, then, is like a one-way switch for electric current.

A light-emitting diode, an LED, works the same way. The main difference being that when an LED has current flowing through it in the allowed direction, it lights up. It gives off light. So, to know which of the LEDs in our circuit will emit light, we’ll want to follow the current as it moves through the circuit and see which LEDs it moves through in the allowed direction. Taking a look at our cell, we see that the direction of conventional current flow, the flow of positive charge, would move out from the positive terminal and go counterclockwise like this. So, we follow along with the current as it moves in this direction, and then it gets to this junction point.

At this point, the current, theoretically, could go up through LED A, and/or it could pass along to the right. But, take a look at the way that LED A is oriented. The arrowhead points downward, meaning that current can’t flow in the opposite direction, upward. That means that current flow through this branch of our circuit is disallowed. It’s blocked by the orientation of the LED. This means that 100 percent of the current from our cell will continue on, pass this branch, until it gets to this junction point.

Once again, our current, theoretically, could travel up through branch B or it could travel along to branch C, or it could travel through both branches. But if we look at the orientation of the LED in branch C, we see that, like the LED in branch A, this one is pointed opposite the direction of the current flow. In other words, it serves as a block or a stop for current trying to flow in this direction through it. So, current can’t flow through this LED, and therefore it can’t flow through branch C. There’s no current that travels along this part of the circuit.

Taking a look at LED B, we see that this LED is oriented so that current could travel from bottom to top up through it. That’s the allowed direction of current flow. Since branch B is the only of the three branches that allows current to flow through it based on the LED orientation, all of the current in the circuit flows up through LED B. Since current is flowing through the LED, it lights up; it emits light. And then our current, once it’s made it through LED B, travels along back through the resistor and then back to the negative terminal of the cell.

So, based on the orientations of our three LEDs, effectively we just have a circuit that includes a cell, LED B, and a resistor. Current flow through the other LEDs is not allowed. This gives us the answer to our question. It’s LED B of the three LEDs that will emit light in this circuit.

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