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Metal heated in a furnace shows that it is hot by glowing red and sending out sparks – but there is also some heat in ice and snow. Heat is the energy of movement, or kinetic energy, stored inside every object, hot and cold alike. Heat energy makes the particles (atoms and molecules) inside the object move about. TEMPERATURE is how hot or cold an object is, depending on its heat energy. Temperature is measured with a THERMOMETER.


When iron is heated in a furnace, it glows red-hot and then melts at a temperature of 1,535°C (2,795°F). At this temperature, its particles move about with lots of kinetic energy. At higher temperatures they move even faster, and the iron in the furnace starts bubbling.


Ice is cold, but it still contains some heat energy. An iceberg is made up of particles of water, held in a rigid crystal structure. They still vibrate slightly. If the iceberg cooled down so that its particles stopped moving altogether, it would be at the lowest possible temperature that can ever, in theory, be reached. This is absolute zero.


Temperature is a measure of how hot or cold something is. Things that have high temperature are hotter than things that have lower temperature, because they have more heat energy inside them. Any object can transfer heat energy to a colder object. As it does so, it cools down and its temperature falls. The colder object warms up and its temperature rises.


Temperature is measured in degrees Celsius or Fahrenheit (°C or °F) or on the absolute temperature scale, in units called Kelvins (K). The Celsius (also called Centigrade) scale runs from freezing point (0°C) to boiling point (100°C).


This is a device that measures how hot or cold something is on a temperature scale. When things get hotter, their heat energy makes them expand or get bigger. This is how a thermometer measures temperature. As the liquid inside expands, it creeps up a tube, which is marked with a scale and numbers that show the temperature.


The thermometer contains a small amount of liquid mercury in a glass bulb at the bottom. To take someone’s temperature, the glass bulb is placed inside their mouth. As the mercury is warmed by the person’s body, it expands up the tube, and climbs the temperature scale. A kink in the tube stops the mercury falling back too quickly, so the temperature can be read and recorded.


A thermostat switches an air-conditioning unit on and off to keep a room at a constant temperature. As the room heats up, the brass strip inside the thermostat expands more than the iron strip attached to it. The strip bends inwards, completes an electrical circuit, and switches on the air-conditioning unit.

Copyright © 2007 Dorling Kindersley

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