Physics 9702/11 — October/November 2013

Background Concept

SI base quantities are a small set of fundamental physical quantities chosen so that other quantities can be expressed in terms of them. Each SI base quantity has an SI base unit.

Relevant base quantities and units here:

• electric current: ampere (A)
• thermodynamic temperature: kelvin (K)

Many common quantities are derived, meaning their units are combinations of base units. For example:

• charge $Q$ is derived because $Q = It$, so $1\ \text{C} = 1\ \text{A s}$.
• potential difference $V$ is derived because $V = \frac{W}{Q}$, so $1\ \text{V} = 1\ \text{J C}^{-1}$.

Understanding the Question

You are given four rows pairing a quantity with a unit. You must choose the row where the quantity is an SI base quantity and the stated unit is its correct SI base unit.

Approach

1. Check whether the quantity named is one of the SI base quantities.
2. If it is, check whether the unit given is the correct SI base unit.
3. Eliminate any options where either the quantity is derived (not base) or the unit is not the SI base unit.

Step-by-Step Reasoning

• A: charge with coulomb.

  • Charge is not a base quantity; it is derived from $Q = It$.
  • So A cannot be correct.

• B: current with ampere.

  • Electric current is an SI base quantity.
  • Its SI base unit is the ampere.
  • So B is correct.

• C: potential difference with volt.

  • Potential difference is not a base quantity; it is derived (e.g. $V = W/Q$).
  • So C is not correct.

• D: temperature with degree Celsius.

  • Temperature is a base quantity, but the SI base unit is kelvin (K), not degree Celsius.
  • So D is not correct.

Therefore the only correct row is B.

Key Takeaways

• Know the SI base quantities and base units.
• Recognise that coulomb and volt are derived units.
• Thermodynamic temperature uses kelvin as the SI base unit (not degree Celsius).

Common Mistakes

• Thinking that charge is a base quantity because it is commonly used; it is derived from $Q = It$.
• Treating degree Celsius as an SI base unit; the base unit is kelvin.
• Assuming any commonly used unit must be a base unit (e.g. volt is derived).

Things to Be Careful About

• “SI base quantity” is stricter than “an SI unit”: degree Celsius is accepted for use with SI, but it is not the SI base unit for temperature.
• Distinguish the quantity (base vs derived) and the unit (base vs derived): both must be correct for the option to be right.