Physics 5054/12 — May/June 2011

Walkthrough

This question is testing whether you know the correct SI unit for each quantity. Look at each pair in turn.

• A says e.m.f. is measured in newtons. The word “force” in “electromotive force” can be misleading, but e.m.f. is a voltage, so its unit is the volt (V), not the newton (N).
• B says latent heat is measured in joules. Latent heat is the energy absorbed or released when a substance changes state without changing temperature. Because it is a form of energy, its unit is the joule (J). This is correct.
• C says pressure is measured in $\text{kg/m}^3$. That is actually the unit of density. Pressure is force per unit area, so its unit is the pascal (Pa), which is the same as $\text{N/m}^2$.
• D says weight is measured in kilograms. Kilograms measure mass, not weight. Weight is a force, so it is measured in newtons (N).

So the only correct quantity-unit pair is B.

Key Takeaways

• Every physical quantity has its own SI unit.
• Latent heat is a form of energy, so its unit is the joule (J).
• Weight is a force, measured in newtons (N), while mass is measured in kilograms (kg).
• Pressure is force per unit area, measured in pascals (Pa) or $\text{N/m}^2$.
• e.m.f. and potential difference are both measured in volts (V).

Common Mistakes

• Confusing mass and weight: weight is a force, so its unit is N, not kg.
• Confusing pressure and density: pressure is $\text{N/m}^2$ or Pa, while density is $\text{kg/m}^3$.
• Thinking e.m.f. is a force because of the word “force”: e.m.f. is measured in volts.
• Forgetting that latent heat is energy: it is not a rate or a force, so it must be measured in joules.

Things to Be Careful About

• Read the unit carefully: J is the symbol for joule, N for newton, Pa for pascal and V for volt.
• Latent heat is an amount of energy, so its unit is J. Specific latent heat is energy per unit mass, so its unit is J/kg.
• In a multiple-choice question like this, check every option, not just the one that looks familiar.