Physics 9702/12 — October/November 2013

Background Concept

The SI system defines seven base quantities, each with a base unit:

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

All other quantities (e.g. charge, potential difference) are derived quantities with derived units that can be written in terms of base units.

Understanding the Question

We must choose the row where:

1. the quantity listed is one of the SI base quantities, and
2. the unit listed is the correct SI base unit for that quantity.

Approach

Check each option against the memorised SI base quantities/units list, and reject any option that:

• uses a derived quantity (like charge or potential difference), or
• uses a non-base unit for a base quantity (e.g. temperature in degrees Celsius instead of kelvin).

Step-by-Step Reasoning

• A (charge, coulomb): charge is not a base quantity. It is derived from current and time:

So coulomb is derived: $1\ \text{C} = 1\ \text{A s}$. Not correct.

• B (current, ampere): electric current is a base quantity and its base unit is the ampere (A). Correct.

• C (potential difference, volt): potential difference is derived (energy per unit charge), and volt is a derived unit:

So not a base quantity/unit pair.

• D (temperature, degree Celsius): temperature is a base quantity, but the SI base unit is the kelvin (K), not $^\circ\text{C}$. So incorrect.

Therefore, the only correct row is B.

Key Takeaways

• Know the seven SI base quantities and their base units.
• Charge (C) and potential difference (V) are derived, not base.
• Temperature’s SI base unit is kelvin (K), not degrees Celsius.

Common Mistakes

• Thinking coulomb is a base unit because it is widely used.
• Choosing temperature with $^\circ\text{C}$ instead of K.
• Confusing “common unit” with “SI base unit”.

Things to Be Careful About

• The question asks for an SI base quantity with its correct unit: both must be base.
• Degrees Celsius is related to kelvin by an offset, but it is not the SI base unit.
• Derived units can often be expressed using base units (e.g. $\text{C} = \text{A s}$), which is a clue they are not base units.