Physics 9702/13 — May/June 2019

Background Concept

The SI system has seven base quantities with corresponding base units:

• length: metre ($\text{m}$)
• mass: kilogram ($\text{kg}$)
• time: second ($\text{s}$)
• electric current: ampere ($\text{A}$)
• thermodynamic temperature: kelvin ($\text{K}$)
• amount of substance: mole ($\text{mol}$)
• luminous intensity: candela ($\text{cd}$)

All other SI units are derived units, formed by combining base units (e.g. $\text{N} = \text{kg m s}^{-2}$, $\text{V} = \text{kg m}^2\text{s}^{-3}\text{A}^{-1}$).

Understanding the Question

You are asked to choose which option is an SI base unit.
The options include:

• one physical quantity name (not a unit),
• one mass unit that is not the SI base unit,
• one temperature unit,
• one electrical unit that is derived.

Approach

1. Recall the list of SI base units.
2. Check each option:
   • Is it actually a unit (not a quantity)?
   • If it is a unit, is it one of the seven base units, or is it derived?

Step-by-Step Reasoning

• A current: “current” is a physical quantity. Its SI base unit is the ampere ($\text{A}$), so this option is not a unit.
• B gram: the SI base unit for mass is kilogram ($\text{kg}$), not gram. Gram is related by a prefix ($1\ \text{g} = 10^{-3}\ \text{kg}$) but is not the base unit.
• C kelvin: kelvin ($\text{K}$) is the SI base unit for thermodynamic temperature. This matches the base-unit list.
• D volt: volt ($\text{V}$) is a derived unit of potential difference:

$$\text{V} = \frac{\text{J}}{\text{C}} = \text{kg m}^2\text{s}^{-3}\text{A}^{-1}$$

So the only SI base unit given is kelvin.

Key Takeaways

• Memorise the seven SI base units.
• Distinguish quantity (e.g. current) from unit (ampere).
• Recognise that many named units (newton, joule, volt) are derived, not base.

Common Mistakes

• Choosing current because it “sounds fundamental” (but it is not a unit).
• Choosing gram because it is commonly used (but the SI base unit is kilogram).
• Assuming any named unit like volt is a base unit (many named units are derived).

Things to Be Careful About

• The question asks for a base unit, not a base quantity.
• For mass, SI uniquely uses kilogram as the base unit (not gram), so prefixes do not make gram a base unit.