6. Vectors.

Mass, temperature, energy, entropy, pressure, volume, and speed among many other quantities can be represented by a scalar, which is a simple number expressing the size or amount of something. Velocity, force, electric and magnetic fields, and acceleration each have magnitude and direction and therefore require two quantities to describe them, and they are called vectors. At school, you will probably have met a vector as an arrow drawn on a graph and which, therefore, has magnitude (length) and direction (angle). In biology, the idea is a related one, a vector being ‘something that carries’ and an infection is described as having a vector that causes its transmission from one creature to another.