Give the output of NOT(A AND B) when A = 1 and B = 1. [1 mark]

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What do the AND, OR and NOT logic gates do, and how do you build and read a truth table?

The logic operators AND, OR and NOT, their logic gates and truth tables, and combining them in simple logic circuits and Boolean expressions of up to three inputs.

An Eduqas GCSE Computer Science answer on the AND, OR and NOT logic gates and their truth tables, and how to build and evaluate simple logic circuits and Boolean expressions of up to three inputs.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The three logic gates
Truth tables for combined expressions
Reading and drawing logic circuits
Why logic gates matter
Try this

What this dot point is asking

Eduqas wants you to know the three logic operators AND, OR and NOT, their logic gates and truth tables, and to combine them in simple logic circuits and Boolean expressions of up to three inputs. Completing a truth table for a given expression is the recurring Component 1 question.

The three logic gates

The single-gate truth tables, with inputs $A$ and $B$ :

$A$	$B$	$A$ AND $B$	$A$ OR $B$	NOT $A$
0	0	0	0	1
0	1	0	1	1
1	0	0	1	0
1	1	1	1	0

Truth tables for combined expressions

Complete a truth table for Q = (A OR B) AND (NOT C)

Build the truth table for $Q = (A \text{ OR } B) \text{ AND } (\text{NOT } C)$ for inputs $A$ , $B$ , $C$ .

Step 1: List the input combinations

With three inputs there are $8$ rows: $A,B,C$ run through $000, 001, 010, 011, 100, 101, 110, 111$ .

Step 2: Work out the intermediate columns

Add a column for $A \text{ OR } B$ (which is $1$ whenever $A$ or $B$ is $1$ ) and a column for $\text{NOT } C$ (which is $1$ when $C = 0$ ).

Step 3: Combine for Q

$Q$ is $1$ only where both $A \text{ OR } B = 1$ and $\text{NOT } C = 1$ . So $Q = 1$ when ( $A$ or $B$ is $1$ ) and $C = 0$ : that is the rows $010, 100, 110$ . Every other row gives $Q = 0$ .

Reading and drawing logic circuits

A logic circuit is the gates wired together to carry out a Boolean expression. To find the output of a circuit for given inputs, work through it gate by gate, writing the value on each wire, until you reach the output. To draw a circuit from an expression, place a gate for each operator and feed the inputs in, handling brackets and NOTs first, exactly as you would when evaluating the expression.

Trace a logic circuit

A circuit computes $Q = (A \text{ AND } B) \text{ OR } (\text{NOT } A)$ . Find $Q$ when $A = 0$ and $B = 1$ .

Step 1: Work out each part

$A \text{ AND } B = 0 \text{ AND } 1 = 0$ (AND needs both to be $1$ ). $\text{NOT } A = \text{NOT } 0 = 1$ .

Step 2: Combine with the OR

$Q = 0 \text{ OR } 1$ . An OR gate outputs $1$ when at least one input is $1$ , so $Q = 1$ .

Step 3: Check another input

If instead $A = 1$ and $B = 0$ : $A \text{ AND } B = 0$ , $\text{NOT } A = 0$ , so $Q = 0 \text{ OR } 0 = 0$ . Tracing wire by wire like this gives the output for any input combination.

Why logic gates matter

Logic gates are the physical building blocks of a processor. The ALU described in the hardware topic carries out its arithmetic and comparisons using circuits built from AND, OR and NOT gates, and the same gates combine to store and move data. Understanding the three basic gates and how they combine in expressions is therefore the foundation for understanding how a computer makes decisions and performs calculations at the lowest level.

Try this

Q1. State when an AND gate outputs $1$ . [1 mark]

Cue. Only when both inputs are $1$ .

Q2. State how many rows a truth table has for an expression with three inputs. [1 mark]

Cue. $8$ rows ( $2^3$ ).

Q3. Give the output of $\text{NOT}(A \text{ AND } B)$ when $A = 1$ and $B = 1$ . [1 mark]

Cue. $A \text{ AND } B = 1$ , and NOT of that is $0$ .

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas Component 1, 20224 marksComplete a truth table for the Boolean expression Q = A AND (NOT B), for all combinations of A and B.

Show worked answer →

List all four input combinations of A and B (00, 01, 10, 11), work out NOT B, then AND it with A.

A=0, B=0: NOT B = 1, A AND 1 = 0. A=0, B=1: NOT B = 0, A AND 0 = 0. A=1, B=0: NOT B = 1, A AND 1 = 1. A=1, B=1: NOT B = 0, A AND 0 = 0.

So Q is 1 only when A=1 and B=0. Markers award marks for the NOT B column and the final Q column. A common error is forgetting to invert B first.

Eduqas Component 1, 20233 marksState the output of an AND gate, an OR gate and a NOT gate, describing when each gives an output of 1.

Show worked answer →

AND (1 mark): output is 1 only when both inputs are 1; otherwise 0.

OR (1 mark): output is 1 when at least one input is 1; only 0 when both inputs are 0.

NOT (1 mark): has one input and inverts it, so an input of 0 gives 1 and an input of 1 gives 0.

Markers reward the precise condition for a 1 output from each gate. Saying AND outputs 1 when "either" input is 1 confuses it with OR.

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Sources & how we know this

WJEC Eduqas GCSE Computer Science specification (from 2016) — Eduqas (2020)