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Science and Technology^169

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.

HOW DOES A VACUUM FLASK WORK?

A vacuum flask is designed to keep hot food

hot, or cold food cold. Double walls prevent

heat flow in or out by conduction. The walls

are coated in shiny silver 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.

The 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

-273.15°C (-459.67°F).

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.

WHERE IS THE HOTTEST PLACE IN THE UNIVERSE?

The centre of the Sun is 14 million°C (25 million°F).

However, in 2010, a laboratory in New York, USA,

generated temperatures 250,000 times hotter than

the Sun. But even these temperatures are small in

comparison to the Big Bang – this was more than

10 billion billion billon°C!

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 flowing

away from their bodies.

TEMPERATURE SCALE 3

Three different temperature

scales are commonly used. On

the Celsius scale the melting

temperature of ice is 0°C.

On the Fahrenheit scale

the temperature of melting

ice is 32°F. On the Kelvin scale

ice melts at 273.15 K.

TEMPERATURE

14 million°C,

25 million°F, 14 million K

Interior of the Sun

3027°C, 5480°F, 3300 K

Metals are welded

660°C, 1220°F, 933 K

Natural gas burns

184°C, 363.2°F, 457 K

Paper burns

100°C, 212°F, 373 K

Water boils

58°C, 136.4°F, 331 K

Highest temperature on Earth,

Death Valley, California, USA

37°C, 98.6°F, 310 K

Normal body temperature

1 GLASS THERMOMETER
This thermometer contains liquid
in a glass capsule. The liquid
expands when heated, and flows
up the scale. This gives the
temperature of the liquid.

4 VACUUM FLASK

The vacuum flask has

silvered double walls with a

vacuum in between. This prevents

heat transfer by conduction,

convection, and radiation.

-89°C, -128.2°F, 184 K

Lowest temperature on Earth,

Antarctica

THERMOMETERS

THERMAL INSULATORS

-273.15°C, -459.67°F, 0 K

Absolute zero

Double

walls with

vacuum

between

Flask

Stopper

heat