What mistakes do students make with S.I. units?

Confusing nano with micro. Nano is 10⁻⁹ while micro is 10⁻⁶, so a nanometre is one thousandth of a micrometre. Multiplying by 10⁹ when converting from nanometres to metres. Multiply by 10⁻⁹ to go from nm to m, and multiply by 10⁹ to go from m to nm. Treating the prefix as a separate unit, such as writing 'n m' instead of 'nm'. The prefix and unit symbol form one symbol, for example nm or ns.

What are the exam tips for S.I. units in Physics OCR A-Level?

Replace the prefix with its power of ten before doing any other arithmetic. Check the direction of the conversion by deciding whether the numerical value should increase or decrease. Keep the prefix symbol attached to the unit symbol, for example write nm rather than n m.

OCR · A-Level · Physics

Course version: H556 · Version from Sept 2015

S.I. units: (p), nano

The prefix nano, symbol n, means 10⁻⁹ of the base unit.

Checked against the OCR H556 specification (opens in a new tab) on 10 November 2026.

Point 10 of 16

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

State that nano (n) represents a factor of 10⁻⁹.
Convert a value given with the nano prefix into the base unit by multiplying by 10⁻⁹.
Convert a base-unit value into nano units by multiplying by 10⁹.
Use the correct symbol n and distinguish it from p and μ.

Common mistakes and how to put them right

Confusing nano with micro.
Put it right: Nano is 10⁻⁹ while micro is 10⁻⁶, so a nanometre is one thousandth of a micrometre.
Multiplying by 10⁹ when converting from nanometres to metres.
Put it right: Multiply by 10⁻⁹ to go from nm to m, and multiply by 10⁹ to go from m to nm.
Treating the prefix as a separate unit, such as writing 'n m' instead of 'nm'.
Put it right: The prefix and unit symbol form one symbol, for example nm or ns.

Exam tips

Replace the prefix with its power of ten before doing any other arithmetic.
Check the direction of the conversion by deciding whether the numerical value should increase or decrease.
Keep the prefix symbol attached to the unit symbol, for example write nm rather than n m.

Try the lesson

The start of the S.I. units: (p), nano lesson, made from checked teaching notes for this specification point.

Listen

The prefix nano, symbol n, means 10⁻⁹ of the base unit. It is used for small quantities such as a nanometre (1 nm = 1 × 10⁻⁹ m) or a nanosecond (1 ns = 1 × 10⁻⁹ s). To convert a value in nanometres to metres, multiply by 10⁻⁹; to convert metres to nanometres, multiply by 10⁹. For example, 450 nm = 450 × 10⁻⁹ m = 4.5 × 10⁻⁷ m. The symbol n must not be confused with the prefix p for pico or μ for micro. In calculations, replace nano with 10⁻⁹ before substituting into an equation. A strong answer earns marks for these points. State that nano (n) represents a factor of 10⁻⁹. Convert a value given with the nano prefix into the base unit by multiplying by 10⁻⁹. Convert a base-unit value into nano units by multiplying by 10⁹. Use the correct symbol n and distinguish it from p and μ. A common slip: Confusing nano with micro. Instead, nano is 10⁻⁹ while micro is 10⁻⁶, so a nanometre is one thousandth of a micrometre. Exam tip: Replace the prefix with its power of ten before doing any other arithmetic.

See it

A flowchart with two conversion pairs. The first pair links a value in nanometres (nm) to a value in metres (m): multiplying by 10⁻⁹ goes from nm to m, and multiplying by 10⁹ goes back from m to nm. The second pair links a value in nanoseconds (ns) to a value in seconds (s): multiplying by 10⁻⁹ goes from ns to s, and multiplying by 10⁹ goes back from s to ns. A separate node stating that nano (n) equals 10⁻⁹ is connected by dotted lines to both the nanometres node and the nanoseconds node. — Shows how to convert between nanometres and metres, and between nanoseconds and seconds, using the factor 10⁻⁹ for nano.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 2 boxes numbered instead of named. A flowchart with two conversion pairs. The first pair links a value in nanometres (nm) to a value in metres (m): multiplying by 10⁻⁹ goes from nm to m, and multiplying by 10⁹ goes back from m to nm. The second pair links a value in nanoseconds (ns) to a value in seconds (s): multiplying by 10⁻⁹ goes from ns to s, and multiplying by 10⁹ goes back from s to ns. A separate node stating that nano (n) equals 10⁻⁹ is connected by dotted lines to both the nanometres node and the nanoseconds node.

1Box 1
2Box 2

0 of 2 labelled

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