# Question Video: Identifying Circuit Symbols Physics

Which of the following symbols represents a NOT gate?

02:31

### Video Transcript

Which of the following symbols represents a NOT gate?

So in this question, we’ve been given four fairly similar-looking symbols. And we need to figure out which one of them is used to represent a NOT gate. We can actually work out the answer to this question by recalling how a NOT gate works. The NOT gate is a logic gate that gets its name from the fact that its output value is not the same as the input value. The inputs and outputs of logic gates can only take the values zero or one. This means that if the input value of a NOT gate is zero, then its output value must be one. Conversely, if the input value of a NOT gate is one, then the output value must be zero.

We can represent this information in a truth table, which shows us the outputs that are produced as a result of all of the possible inputs. This statement and our truth table both tell us something important about NOT gates, that is that they only have one input. And like all other logic gates, they also have one output. We can use this information to help us decide which of our answer options is correct. When we draw a logic gate symbol, we usually draw the inputs on the left and the outputs on the right. If we look at option A, we can see that it has one output, and it has two inputs represented by the two horizontal lines on the left. This means that the symbol shown in option A can’t possibly be a NOT gate because it has two inputs and a NOT gate only has one input.

If we look at the other available options, we can also see that options B and C have two inputs each, which means that neither B nor C can be the correct answer either. So by process of elimination, we’re just left with option D as it’s the only option we’ve been given that just has one input as well as one output. To help us recall the symbol that we use to represent a NOT gate, we should recall that the symbol we use actually represents the function of a NOT gate. This small circle in the symbol for a NOT gate is actually used in many different logic gate symbols, and it represents an inversion. In other words, it represents a zero being changed to a one or a one being changed to a zero.

The other part of the symbol for a NOT gate is just an arrow pointing from the input to the output. So the symbol for a NOT gate represents the fact that it takes a value from the input, passes it towards the output, and inverts it along the way. Meaning that the output value will be the opposite or inverse of the input value. Just as we’ve shown in our truth table for a NOT gate. So this helps us confirm that the correct symbol that represents a NOT gate is indeed option D.