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Unit 5.0 - THE NATURE OF HEAT

Heat is a form of energy, in the form of infrared radiation.

Heat from the sun travels through space at the speed of

300,000,000 m/s. Upon arriving on earth, much of the radiant

heat is absorbed by different kinds of matter and is converted

into heat that we can feel (sensible heat).

HEAT AND THE MOTION OF MOLECULES

According to the kinetic theory, heat energy acquired by a body

is transformed into increased kinetic energy of the molecules.

We observe this increased kinetic energy whenever a solid, a

liquid, or a gas expands on heating. A further increase in

kinetic energy will eventually cause the particles of a solid or

liquid to become a gas.

When an ice cube (a solid) is heated, it melts and becomes

liquid water. When the water is heated, it vaporizes and

becomes gaseous water.

When an object is heated it grows bigger. We say it expands.

When an object cools down, it gets smaller. We say it

contracts.

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EXPANSION OF SOLIDS

The metal ball shown in the diagram

will just slip through the metal ring

when they are both cold. When the

ball is heated the ball will no longer

pass through the ring.

A bi-metallic strip is made of two different metal

strips, often brass and

iron. On heating the bi-

metallic strip bends

because brass expands

more than iron. This

shows that some

materials expand more than others.

The increase in size of the ball or in the length of the strip is not due to an increase in the size of the particles, but rather to

an increase in the average distance between the particles.

When an object is heated, its particles vibrate faster, collide

more violently, and consequently move farther apart, thereby increasing the volume of the object. This increase in volume is

called expansion.

When the object is cooled, the opposite change occurs and the volume of the object decreases. This decrease in volume is

called contraction. Different solids and liquids expand at

different rates when heated.

brass

iron

brass

iron

brass

iron

brass

iron

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EXPANSION OF LIQUIDS

When a fluid (liquid or gas) is heated, it

expands.

Liquids expand more than solids.

Gases expand more than liquids.

Liquids, like solids, expand when heated. When water is heated, it

expands. When the same water is

cooled to its original temperature, the

water contracts to its original volume. At lower temperatures,

however, the behaviour of water is an exception to this rule.

When water is cooled below 4° C, the water expands-unlike

other liquids. Since the volume of ice is greater than the

volume of water from which the ice is formed, the density of

ice is less than the density of water. This is why ice floats on

water.

EXPANSION OF GASES

Gases confined in an elastic container

expand when they are heated and

contract when they are cooled. Gases,

generally expand at the same rate when

heated to the same temperature, at a

given pressure.

When a gas is heated, the atoms start moving more and so

they take up more space. The substance expands.

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TEMPERATURE

Heat and temperature are two terms that are often confused. We know that the temperature of a small sample of molten iron

is considerably higher that the temperature of the water in the

ocean. However, the total heat in a sample of molten iron is

much less than the total heat of the water in the ocean.

Heat is related to the motions of particles in matter. Heat

depends on the total kinetic energy of the particles in a body.

Because the water in the ocean is colder than the sample of

molten iron, the velocity of the particles in the water is less

than the velocity of the particles in the molten iron. However,

the much larger quantity of water compensates for the smaller

velocity of the particles and thus the particles of water in the

ocean possess greater kinetic energy. This means that there is

more heat in the water in the ocean than in a small sample of

molten iron.

Temperature depends on the average kinetic energy of the

particles, that is, the kinetic energy per particle. So the large mass of ocean water has a smaller average kinetic energy per

particle and consequently has a lower temperature than a small

sample of molten iron.

MEASURING TEMPERATURE

Instruments designed to measure temperature are

called thermometers. Most thermometers are based on the

principle that matter, on heating, expands and, on cooling,

contracts. In general, matter expands and contacts regularly.

This means that the amount of expansion or contraction in

length is generally equal for the same increase or decrease in

temperature.

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LIQUID THERMOMETERS

Thermometers containing liquids such as mercury and alcohol

are useful and accurate because these liquids usually expand

and contract uniformly (regularly).

THE FAHRENHEIT AND CELSIUS TEMPERATURE SCALES

Temperature markings on thermometers are indicated in

Fahrenheit degrees or Celsius degrees. Both Fahrenheit and

Celsius scales are calibrated by using the boiling and freezing

points of water. The temperature of our bodies is 37oC. This is

equivalent to 98.6o on the Fahrenheit scale.

THE KELVIN SCALE

When an object cools down, the molecules vibrate more slowly. At a certain temperature, the molecules loose all their energy

and stop vibrating. This is the lowest temperature we can reach

and is known as ABSOLUTE ZERO.

Absolute zero, -273°C, is also called 0 Kelvin

TRANSFER of HEAT

When objects are at different temperatures, heat is transferred

from the warmer object to the cooler object until both objects are at the same temperature. Heat transfer can occur through

one of three methods:

Conduction, Convection, or

Radiation.

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CONDUCTION

When one end of a metal rod is

held in a flame, the entire rod will

become hot enough to burn the

hand. The heat from the flame

reaches the hand by travelling

through the rod. Substances that

allow heat to travel through them are called conductors. In general

metals are good conductors.

However, some metals conduct

heat more readily than others.

This can be demonstrated by heating rods of aluminium, copper,

iron and glass into a brass sphere

or disk and then attaching a small

ball of wax to the end of each rod.

When the end of the rods is

heated, the wax at the tip of each

metal melts in the order in which

the different metals conduct heat. The wax at the tip of the copper

melts first and the wax at the tip of the glass melts last.

When one end of a rod is heated, the molecules in that end of the rod vibrate faster and strike other nearby molecules,

causing them to vibrate faster. In this manner, the increased

molecular motion is transferred from one end of the rod to the

other, permitting the heat to travel through the rod.

Substances that do not readily allow heat to pass through

them are called insulators. Gases and liquids are

generally poor conductors of heat because their molecules

are farther apart than are the molecules in solids. Therefore,

neighbouring molecules in a gas or in a liquid are less affected

by the increased motions of heated molecules, and

consequently heat is not conducted rapidly.

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Substances like wood or plastic are poor conductors of heat,

so they are used to make handles for metallic objects that are

to be heated. The clothing we wear is also a poor conductor of

heat, enabling us to retain body warmth. Porous material is

generally non-conducting because it contains layers of trapped

air which do not permit heat transfer.

CONVECTION

Although gases and

liquids are poor conductors

of heat, heat is transferred

through them by the

process of convection.

Convection is the transfer

of heat due to the motion

of the liquid or gas itself.

For example, when a

beaker of water is heated the water layer closest to the heat

source is warmed slowly by

conduction.

As the water becomes warmer, it

expands, becomes less dense, and rises. This brings heat to the upper

layer. At the same time, cooler

water from the upper portion of the

beaker moves down, takes the

place of the rising water, and

becomes heated itself.

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When warm enough, this water rises and carries heat upward.

As these processes continue, heat that enters the bottom of

the beaker is distributed throughout the beaker until all the

water is at the same temperature. The moving water in such a

case is said to have set up convection current.

Heat is also transferred through gases by convection. It is by

this means, in part, that a stove or a radiator heats a room.

Heat from the radiator warms the air above it, causing the air to expand, become less dense, and rise. The cooler air that

moves in to take the place of the warmed air is also soon

warmed. As this air rises, a convection current is established.

The formation of a convection current in air is demonstrated with a convection box apparatus. First the candle is lighted,

then smoking touch paper is placed over the chimney, opposite

the candle. The smoke can be seen to move down this

chimney, across the box, and out through the other chimney.

SEA BREEZE

During the day, the land

warms up more than the sea.

The warm air over the land

rises up. The hot air is

replaced by cooler and

denser air which comes from

above the sea. So during the day we feel a sea breeze - wind blowing from the sea onto the

land.

LAND BREEZE

During the night the sea has

more heat to lose and cools

more slowly. The air above

the sea is now warmer than

that above the land. Thus

hot air above the sea rises upwards, and is replaced by cooler air from above the land. During the night we feel a land breeze

- wind blowing from the land onto the sea

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RADIATION

We know that light energy and

heat energy travel from the sun

to the earth through space, which is an almost perfect

vacuum. These forms of energy,

are transferred from the sun to

the earth by radiation, that is, by

means of rays, or waves. You

can understand this method of

heat transfer by standing a short

distance from an open fire. Since

no source of heat is being touched, you cannot receive

heat by conduction. Since warm air rises vertically from the heat source, the heat cannot reach you by convection.

The heat that is transferred to you from the fire by

radiation.

The heat radiated by

one body ( the sun, for

example) is most rapidly

absorbed by other

bodies that are black in

color and rough in texture.

In warm climates, white clothing which reflects the radiant

heat of the sun is cooler than dark clothing which quickly

absorbs the radiant heat.

Similarly, bodies that are rough and dark tend to

radiate heat better than shiny smooth bodies. This is

why steam radiators are often dark and have a roughened

surface. It is for the same reason that coal burning stoves are black.

Bodies that are shiny and smooth do not absorb

heat readily. Instead, these bodies reflect heat.

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Thus, aluminium used for roofing keeps homes cool in the

summer and warm in the winter.

This principle is utilized in the

thermos (vacuum) bottle, which is so constructed as to permit

liquids to retain their

temperatures for a long time. A

thermos bottle is double walled,

with a partial vacuum between

the walls to prevent heat

transfer by conduction or

convection. A cork stopper also

prevents heat transfer by

conduction. The inner glass walls are silvered to reflect

radiant heat back into the

liquid, thereby minimizing heat loss by radiation. Thus, a

hot liquid remains hot because heat is lost very slowly. A

cold liquid remains cold in thermos bottles because

outside heat enters very slowly by conduction, convection,

or radiation.

THE GREENHOUSE EFFECT. The inside of a

greenhouse is warmer

than the outside. This is

due to the fact that the

rays coming from the sun

have a short wavelength

which can get through the

glass. These rays are

absorbed by the plants

which get warmer. The plant does not get very

hot, and the radiation it

emits is of a longer

wavelength which cannot pass through the glass. Thus

energy is radiated in, but cannot radiate out again.

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Evaporation:

When we put some deodorant or

some perfume on our hands, it feels

cold. This is because the liquid uses heat from our bodies to change into

a gas. The deodorant/perfume has

evaporated. Also when we run our

body sweats to keep us cool.

Similarly, when we wash our

clothes, we hang them outside to

dry up. The sun warms the clothes

up and the water molecules get

enough energy to escape into the air. In this case the water

evaporates and becomes a gas

called water vapour.

MEASURING HEAT

We measure the quantity of heat by a unit called the

Joule. The Joule is the amount of heat needed to raise

the temperature of 1 kilogram of water 1 degree Celsius.

CALORIES AND FOOD

Your body requires energy in order to perform its daily

tasks. Most of this energy comes from energy-rich foods

such as carbohydrates and fats. This energy is released

when the body utilizes these foods. Using special calo-

rimeters, scientists have measured the energy content, or

the number of calories present, in fixed quantities of

certain foods. For example, a slice of white bread contains about 60 000 calories; a typical chocolate bar may contain

about 300 000 calories.

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Specific Heat Capacity

Different materials have different specific heat capacities.

We can find the specific heat capacity of a substance

using:

Q = mc

Q = heat energy received/given out [ in Joules ]

m = mass of substance [ in kg ]

c = specific heat capacity [ in J/kgoC or J/kgK

]

= change in temperature [ in oC ]

The specific heat capacity of a

substance is the amount of

energy (in Joules) that is

needed to raise the

temperature of 1kg of a

substance by 1oC .

Different substances require

different amounts of heat to cause the same temperature

change in the same mass. The

‘thirst’ of a substance for heat

is measured by its specific heat capacity (symbol c).

The ability of water to stabilize temperature depends on

its relatively high specific heat. The specific heat of water

is 4200 J/kg C .Compared with most other substances;

water has an unusually high specific heat.

Because of its high specific heat, the water that covers

most of planet Earth keeps temperature fluctuations

within limits that permit life. Also, because organisms are made primarily of water, they are more able to resist

changes in their own temperatures.