That differences, for example between pressures or temperatures or…

The idea

A difference in a quantity between two places or states provides the push that makes things change. Where pressure differs, a force acts from high pressure towards low pressure: a balloon shrinks when the surrounding air pressure rises, and air rushes out of a punctured tyre. Where temperature differs, energy transfers by conduction, convection or radiation from hot to cold until temperatures equalise, so a hot drink cools in a cool room. Where electrical potential differs, charge flows: a cell maintains a potential difference across a circuit, driving current through components. In each case the size of the difference controls the rate of change, and when the difference is zero there is no net change. This idea links the pressure, heating and electricity topics and explains why a complete circuit, a temperature gradient or a pressure gradient is needed for change to occur.

What earns the marks

• States that change is driven by a difference in a quantity between two regions or states, not by an absolute value.
• Gives pressure as an example: a pressure difference produces a resultant force from higher to lower pressure, causing movement or deformation.
• Gives temperature as an example: a temperature difference causes energy transfer from hotter to cooler regions until thermal equilibrium.
• Gives electrical potential as an example: a potential difference across a circuit drives current; no potential difference means no net current.
• Explains that a larger difference generally produces a faster rate of change, and that zero difference means no net change.
• Applies the idea to an unfamiliar context, such as identifying the difference that drives a stated process.

Common mistakes and how to put them right

• Thinking that a high pressure alone causes flow.

  Put it right: Flow requires a difference between two pressures, so equal pressures give no net movement.

• Believing that a hot object transfers energy because it is hot.

  Put it right: Energy transfers only when there is a temperature difference, and transfer stops at equal temperatures.

• Assuming current flows from a cell because the cell has charge.

  Put it right: The cell maintains a potential difference, and that difference drives the current.

Exam tips

• Name the two places or states being compared and state which quantity differs between them.
• Use the phrase 'from higher to lower' when describing the direction of the resulting change.
• Link the size of the difference to the rate of change, and state that no difference means no net change.