Video Transcript
The diagram shows a circuit
containing a cell, a resistor, and an LED. The LED does not turn on. Which of the following reasons
explains why?
Okay, before we get to those
reasons, let’s take a look at this diagram. We see in it that there’s a power
cell, a resistor, and this symbol here, which represents an LED, a light-emitting
diode. We’re told the led is not turning
on with the circuit set up as is. Let’s look now at some possible
reasons why.
Option A, an LED can only be used
in logic circuits. Option B, an LED cannot be placed
in series with a resistor. Option C, an LED can only be used
with alternating current power sources. A cell provides direct current, so
the LED will not work. D) an LED is a type of diode. Current only flows in one direction
through a diode, and in this circuit, the diode is connected in the wrong direction
to allow current through it. And finally, option E, the current
through the LED is too large. LEDs can only work with small
currents.
All right, of these five answer
options, we want to pick which one correctly explains why the LED is not lit up. Option A claims that LEDs can only
be used in logic circuits. Now, a logic circuit is one where
binary inputs, inputs that are either one or zero, are combined together to give a
single binary output, an answer or a result also of one or zero. It’s true that LEDs could be used
in logic circuits. Say an LED was placed so it would
light up when current flows through a certain part of the circuit, indicating on or
true. But this doesn’t mean that LEDs can
only be used in such circuits. It’s perfectly fine to put an LED
in a circuit that doesn’t involve input and output ones and zeros.
For example, if we designed a
circuit with the goal of creating light emitted by an LED, then that simple circuit
need not be a logic circuit, but it’s simply a practical circuit designed to give
off light. So while LEDs can be used in logic
circuits, they need not only be used that way. So option A isn’t our choice. Option B then says that an LED
cannot be placed in series with a resistor. Looking back at our diagram, we see
that this is indeed the case here. An LED is in series with a
resistor. But actually, an LED and a resistor
being part of the same circuit is more of a good sign than a bad sign. That’s because LEDs tend to draw
more current than they’re capable of handling. And so resistors do a good job of
limiting the current that an led is exposed to and thereby safeguarding the LED from
burnout. So light-emitting diodes cannot be
placed in series with a resistor. And often, that’s a helpful
approach.
Moving on to option C, this says
that an LED can only be used with alternating current power sources. This option points out that the
cell in our circuit provides a direct current. And that’s the reason the LED will
not work. As we analyze this answer option,
let’s take a closer look at the symbol for the light-emitting diode. If we were to forget for a moment
these two arrows here, as well as the circle that’s part of the LED symbol, then we
would have a symbol that looks like this. And this is the electrical circuit
symbol for a diode.
Now, a diode, and a light-emitting
diode is a type of diode, is designed so that it only allows conventional current
that is the flow of positive charge to pass through in one direction. And that direction, by the way, is
indicated by the way that this triangle points. In this case, current can move
through the diode, left to right. Current trying to flow the other
way will be blocked. Keeping that in mind, if we think
about an alternating current or AC power supply, this is a power source where the
current direction flips or reverses rapidly, often many times a second. Based on the way a diode works, it
would prevent current from flowing in a circuit with an AC power supply any time
that power supply try to send current in the direction opposite the way the diode
pointed.
Since current in an AC circuits
spends about half its time running one way and half its time running the opposite
way, we might expect a diode in an AC circuit to be rapidly turning on and off, on
and off over and over every time the current change direction. In order for a diode, and by
extension a light-emitting diode, to operate steadily, we would want the diode to be
in a circuit with a direct current supply, one that’s always pointing in the same
direction. And one that’s able to pass through
the diode. So it’s not true that LEDs can only
be used with AC power sources. So we cross off option C.
Option D then says that an LED is a
type of diode. Current only flows in one direction
through a diode. And in this circuit, the diode is
connected in the wrong direction to allow current through it. Well, we’ve seen that, indeed, this
description of a diode is correct. It only allows current to flow one
way through it. And looking again at the cell in
our circuit, we see that, based on its orientation, this cell would create
conventional current that flows in a clockwise direction. But then look at this. The triangle or the arrowhead on
our light-emitting diode points the opposite way. This means that the way this LED is
oriented in the circuit will prevent it from allowing any current to pass
through. As this answer option states, the
diode is connected in the wrong direction. If we were to flip it around to
reverse its polarity, it would allow current to flow through. But as is, current is blocked.
So it looks like option D will be
our answer. But let’s take a look at option E
just to see what it says. This option says the current
through the LED is too large. LEDs can only work with small
currents. But looking back at our circuit,
since the LED is not lit up, that means no current is flowing through it. And that means no current is
flowing in the circuit at all. So the current running through the
LED can possibly be too large when that current is zero.
And then further, this answer says
that LEDs can only work with small currents, but that’s not true either. While there is an upper limit to
the current that a given LED can sustain without breaking down, it is possible to
make LEDs which are capable of handling relatively large currents. So Option E is not an accurate
description of why the LED isn’t turning on. Our answer then is option D. An LED is a type of diode. Current only flows in one direction
through a diode, and in this circuit the diode is connected in the wrong direction
to allow current through it.
