### Video Transcript

The diagram shows a NOT gate
connected to an electric circuit in which there is a current present. The input of the NOT gate has a
value of one if a current is present and a value of zero if there is no current
present. What is the value of the input to
this NOT gate?

So, this question is asking us to
work out the input to this NOT gate. Let’s start by recalling that a NOT
gate is a type of binary logic gate. A binary logic gate is one whose
inputs and output can each take one of two values: zero or one. The function of a NOT gate is to
output whatever value is not input into it. So, if the input is a zero, then
the output will be a one. If the input is a one, then the
output will be a zero.

Here, we have a NOT gate where the
input and the output are connected to an external electric circuit. This circuit is represented by
these dashed lines in the diagram. When a logic gate is connected to a
circuit in this way, the presence or absence of a current in the input or output
corresponds to the binary values. So, if there is no current, this
corresponds to a value of zero. And if there is a current, this
corresponds to a value of one.

In this question, we need to work
out whether the value of the input to the NOT gate is a zero or a one. In other words, we need to work out
if there is a current at this point in the circuit. Now, we don’t know much about this
circuit, but we do know that there is a current present. Let’s draw an arrow to represent
this current, which we’ll call 𝐼.

We can see that this current
reaches this open switch. But because the switch is open, the
circuit is broken and current can’t travel any further around the circuit. This means that there is no current
in this part of the circuit right here. In other words, there is no current
in the input to the NOT gate. Now, we already know that if
there’s no current, this corresponds to a binary value of zero. So, because the switch is open, we
know that the input to the NOT gate is zero. And this is our final answer.