Explain that a scalar quantity has magnitude (size) but no specific…

The idea

A scalar quantity is fully described by its magnitude alone: the size of the quantity, given with a unit. No direction is needed or given, because a scalar carries no directional information. Distance is a scalar: saying a runner covered 400 m tells you how far, not which way. Speed, mass, time, energy and temperature are also scalars. Contrast this with a vector such as displacement or velocity, which needs a direction as well as a size. In a multiple-choice question, test each option by asking whether the quantity would still be complete without any direction; if it would, it is a scalar. Watch for quantities that sound directional but are quoted as sizes, and for options that add a direction to a scalar, which makes the description wrong.

What earns the marks

• Selects the option that identifies a scalar as having magnitude but no specific direction.
• Rejects an option that claims a scalar needs a direction to be fully described.
• Recognises distance, speed, mass, time and energy as examples of scalar quantities.
• Explains that a scalar is complete once its size and unit are stated.

Common mistakes and how to put them right

• Treats a scalar as needing a direction as well as a size.

  Put it right by stating that a scalar has magnitude only.

• Confuses speed with velocity when choosing a scalar.

  Put it right: Note that speed is a scalar while velocity is a vector.

• Adds a compass direction to a distance value.

  Put it right: Keep distance as a magnitude-only scalar.

Exam tips

• Read each option and ask whether a direction is required; if not, the quantity is a scalar.
• Check the unit is present, since magnitude means size with a unit.
• Eliminate options that attach a direction to a quantity described as a scalar.