2022 A/L Physics Past Paper · Question 16 (English Medium)

Answer: (5) Flip-flop

What the question asks

We want: the one device that can store one bit, a single $0$ or $1$.

What each word means:

bit: one binary digit, either $0$ or $1$.

logic gate: a circuit whose output is decided by the inputs at that moment.

flip-flop: a circuit made of gates that remembers its last output.

Relevant theory

In the logic gate circuits we have discussed so far, the output at an instance is decided by the inputs present at that particular instance.

Although a memory cannot be there in a single logic gate, a particular combination of logic gates known as a flip-flop can store a binary digit 0 or 1, also stored digit can be read when required.

Reading the options

For example, think of a door bell and a light switch.

A door bell rings only while you press the button. Let go, and it is silent again. It keeps nothing.

A light switch is different. Flip it on, let go, and the light stays on. Flip it off, and it stays off. The switch remembers its last setting.

A logic gate is like the door bell. A flip-flop is like the light switch.

The book says the same: one gate on its own cannot hold a memory. A flip-flop built from gates can store $0$ or $1$ and be read when needed.

Why the other options fail

An AND gate decides its output from its present inputs only. It cannot hold a bit.

A single NOR gate decides its output from its present inputs only. A flip-flop needs gates combined.

An XOR gate decides its output from its present inputs only. It cannot hold a bit.

An OR gate decides its output from its present inputs only. It cannot hold a bit.

Conclusion

A flip-flop is the one that can store a binary digit $0$ or $1$ and be read when needed.

The answer is (5).

A single logic gate has no memory. Its output follows its inputs.

To store one bit, use a flip-flop.

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