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Lava is hot, melted rock from inside the Earth.Lava is hot, melted rock from inside the Earth. Heat is a type of energy—the vibrations of atoms and molecules. The more an atom moves or vibrates, the more heat or thermal energy it has. In a solid, the atoms have fixed central positions but each atom vibrates slightly about its central position, like a ball tied to a nail by elastic. Heat the solid and the atoms vibrate more. When they have enough vibrations, the atoms break from their fixed positions (the “elastic” snaps), and they move about at random. The solid has melted into a liquid. Heat it more and the atoms fly further apart. The liquid becomes a gas.

The Earth, showing the amount of heat radiatedThe Earth, showing the amount of heat radiated

Using heat

Heat is a vital part of our lives. We need to keep our bodies comfortably warm with clothing, especially in cold conditions. If body temperature falls from its normal 37°C to below about 30°C (98.6–86°F), fatal hypothermia may set in. We cook our food with heat. Countless machines and industrial processes use heat, from making pottery to a steelworks or power station. Heat is also given off as a waste form of energy by many processes. In a power station most of the heat is used to generate electricity, but some is released as clouds of steam from cooling towers.


Flying at Mach 3 (three times the speed of sound), a super-fast jet aircraft's surface may reach temperatures of 300°C (nearly 600°F). The aircraft grows nearly 80 cm (30 inches) longer.

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