HEAT

Everything around us contains heat. Heat is a form of energy—the energy of the random jiggling motion of particles that make up all matter. TEMPERATURE is a measure of how hot an object is—it can be recorded on a THERMOMETER.

HOW DOES HEAT FLOW?

Heat always flows from a higher temperature to a lower temperature. If you stand next to a hot oven, the heat from it warms you up. Different materials let heat flow through them at different rates. Metals are the best conductors (they let heat flow easily). THERMAL INSULATORS don’t conduct heat well.

WHAT ARE THE THREE TYPES OF HEAT TRANSFER?

Conduction is the transfer of heat from one molecule to another. Energetic molecules pass on heat energy when they collide with less energetic molecules. Convection is the transfer of heat through a liquid or a gas as warm fluid rises and cool fluid sinks. These movements are convection currents. Radiation is the movement of heat by electromagnetic rays. All objects emit radiation, but the hotter they are, the more radiation they give off.

CAN WE SEE HEAT?

We cannot see heat, but we can see its effects. Convection currents rising from a hot asphalt road make the air above shimmer. A thermal-imaging camera detects radiation emitted by hot objects. Electronics convert invisible electromagnetic rays into an image on a television screen.

TEMPERATURE

Temperature tells us how hot an object is. It is a measure of how fast the particles in the object are moving. The hotter an object, the more vigorously its molecules move. Temperature is measured in degrees on a temperature scale.

WHAT IS ABSOLUTE ZERO?

If you could remove all the heat energy from an object, its molecules would be effectively stationary. The temperature at which molecules are no longer moving is called absolute zero. It is the lowest possible temperature. Absolute zero is equivalent to –459.67°F (–273.15°C).

WHERE IS THE HOTTEST PLACE IN THE UNIVERSE?

The center of the Sun is 25 million°F (14 million°C). Temperatures 30 times higher are created in laboratories to produce nuclear fusion. But even these temperatures are tiny compared to the Big Bang—this was more than 18 billion billion billon°F!

THERMOMETERS

The instrument for measuring temperature is called a thermometer. Most thermometers have a scale with two fixed points—the melting point of ice and the boiling point of water. All temperatures are measured against these points.

HOW DOES A THERMOSTAT WORK?

Thermostats control the temperatures of buildings and machines. A simple room thermostat has a two-metal strip as part of a switch that turns the heat on and off. As the temperature rises, the two metals in the strip expand by different amounts. The strip bends, breaking the circuit and turning off the heat supply. As the room cools, the strip straightens, completes the circuit again, and turns the heat back on.

THERMAL INSULATORS

Any material that resists the flow of heat is a thermal insulator. Many animals keep warm because fur is a good insulator—it stops heat from flowing away from their bodies.

HOW DOES A VACUUM BOTTLE WORK?

A vacuum bottle (Thermos™) is designed to keep hot food hot, or cold food cold. Double walls prevent heat flow in or out by conduction. The walls are silver-coated to reflect electromagnetic rays, reducing heat transfer by radiation. The space between the walls contains air at a low pressure (vacuum). This reduces heat transfer by convection.

VACUUM BOTTLE

The vacuum bottle has silvered double walls with a vacuum in between. This design prevents heat transfer by conduction, convection, and radiation.

FIND OUT MORE

Atoms
Circulation
Energy
Sun

Copyright © 2007 Dorling Kindersley