Heat

Heat as a Form of Energy

• Heat as a form of energy . Heat can make things hot and we can use heat to do work.

• We feel hot when the Sun shines. This shows that the Sun is a source of heat energy and gives out heat. Most of the heat on the Earth comes from the Sun.

Other Sources of Heat

• Apart from the Sun, there are many other sources of heat. Heat can be produced in many ways.

• Here are some activities to show possible heat sources.

Heat sources Activities

•Rubbing or friction- rubbing two objects against each other can produce heat.

• Rub your hands together for some time and then hold

them to your cheeks. Your cheeks will feel warm,

showing that heat can be produced by friction.

Rubbing hands can produce heat

• Burning- when an object burns, it

produce a flame that gives out heat.

• Burn a candle or a piece of paper and put your hands above

it. Your hands will feel hot, showing that burning substances

can produce heat.

Burning substances can produce heat

• Electricity – when electric current flows

through a wire, heat is produced.

• Switch on the electric bulb for some time and place your hands

near the bulb. Your hands can feel the heat coming out from

the bulb.

Electricity can produce heat

• Heat can also be produced by:

a. Bending metal- bending a coat hanger or a wire back and forth several times can produce heat in the object.

b. Chemical reaction – after mixing solid sodium hydroxide with water in a test tube, the test tube will get warm.

c. Collision – when an iron nail is hit by a hammer several times, the hammer and the nail will get hot.

Bending Metal Chemical Reaction

Collision

Use of Heat in our Daily Life

• Heat is a useful form of energy. We use heat to:

a. Cooking foodb. Drying clothesc. Boiling waterd. Producing steam to generate electricitye. Food drying for reservationf. Providing warmth

Heat and Temperature

• Heat is a form of energy. An object becomes hot when it absorbs heat. Heat is measured in Joules (J).

• Temperature measures how hot or cold an object is. The unit of temperature is degree Celsius ( C) or Kelvin (K). Temperature can be measured with a thermometer.

• Heat and temperature are different.

• The table below shows the differences between heat and temperature.

Heat Temperature

• A form of energy • The degree of hotness or coldness of a body

• Heat can do work • Temperature cannot do work

• Measured in Joule (J) • Measured in degree Celsius (C) or Kelvin (K)

•Transfer from a hot area to a cold area.

Increases when heated and decreases when cooled

Differences between heat and temperature

Common mistakes

• Objects with the same temperature contain the same amount of heat. The higher the temperature of an object, the larger the amount of heat contained in it.

The truth

• The amount of heat contained in an object depends on

a. The type of the material that the object is made of.

b. The mass or the size of the objects, and

c. The temperature of the object.

Heat Flow

• Heat only travels from a hotter object (or place) to a cooler object (or place)

• Heat has several interesting ways of travelling:It travel by three ways:

a. By conduction through solidsb. By convection through liquids or gasesc. By radiation through vacuum

Conduction• The flow of heat energy through solids such as metals

is called conduction.

• Heat energy uses molecules to help it to get around.

• For example, when a pan is heated, the molecules at the bottom of the pan start to vibrate energetically. They collide with their neighboring molecules and cause them to vibrate faster. The passing of the heat energy from one molecule to the next continues. In this way, heat energy travels through the pan and then through the food in the pan.

Convection

• Heat can flow through a liquid or a gas because heated parts of the liquid gas move.

• Warm air rising above a heater is an example.

• The flow of heat that occurs when a warm liquid or gas moves is known as convection.

Radiation

• The process where heat energy travels through an empty space or a vacuum is known as radiation.

• An example of radiation is the transfer of heat from the Sun to the Earth through mostly empty space. Such a transfer cannot occur via convection or conduction, which requires the movement of material from one place to another, or the collisions of molecules within a material.

Heat Flow in Natural Phenomena

• Some natural phenomena occur as a result of heat flow.

• Warming of the earth by the Sun

a. The Sun gives solar energy every day. During the day, the Earth’s surface is warmed up and during the night, this energy is radiated back into space as radiant heat energy.

b. A fraction of the solar energy that reaches the Earth is absorbed, causing evaporation of water from the oceans, the lakes, the lands and plants. Convection current carries the water vapour up to the atmosphere to form clouds, which will form rains and thunderstorms

c. The warming of the Earth by the Sun can cause changes in the climatic conditions of the Earth and many natural phenomena such as land ad sea breezes, thunderstorms, hurricanes and so on.

•Land and Sea Breezes

a. The unequal heating or air over land and water will result in breezes near the shores.

b. During the day, both land and sea are heated by the Sun. However, the land gets heated up faster than the sea. The air above the land surface heats up, expands and rises. It is lighter than the surroundings air. To replace the rising air, cooler air is drawn in from the surface of the sea. This is the sea breeze. It can offer a pleasant cooling effect on a hot afternoon.

The phenomena of a sea breeze

c. At night, the land cools faster than the sea. When this happens, the air over the warmer surface of sea heats up and rises, pulling in air from the cooler land surface to replace it. This is the Land breeze.

The phenomena of land breeze

Keeping Building Cool• A building can be kept cool by having a good

ventilation system, so that air circulation in the building is ongoing. Hot air from the building flows out from the top and cool air can flow in from the bottom.

• Most of the traditional houses are built with ventilation holes at the base of the house and at the top near the roof. Hot air inside the house will rise and flow out through the ventilation holes at the roof. Cool air will enter through the openings at the base of the house to replace the hot air. This produces a natural convection current inside the house.

• In modern buildings, the ventilation system is made more effective by installing exhaust fans and extractor fans. Hot and humid air can be sucked out and replaced with fresh and cool air.

• Modern houses are equipped with fans and air conditioners to make the house cooler. Modern buildings use centralized air conditioning system to cool the whole buildings.

• Insulation can also help to keep a building cool. Buildings with loft insulation, insulating cavity walls or double gazed windows are usually much cooler than those which are built without insulating materials.

Heat conductors and heat insulators

• When material allow heat to pass through them rapidly, they are known as conductors. All metals are good conductors of heat compared with other materials.

• Other materials like- metals, liquids and gases that do not allow heat to pass through easily are known as bad conductors of heat or insulators.

Examples of heat conductors and insulators

Conductors Insulators

Mercury (liquid) Glass Silver Air Copper Water (liquid)Aluminum PlasticsZinc RubberIron WoodLead Materials containing trapped air

( wool, plastic foam, expanded polystyrene)

Uses of heat conductors

Materials/ devises Uses

Cooking utensils

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kettle

•Cooking utensils such as saucepans, pots and kettles are usually made of aluminum or stainless steel.

•They conduct heat quickly and easily to the food as thus save fuels.

Examples of the uses of heat conductors indaily life.

Electrical appliances

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iron

•Electrical irons and hot plates are made of iron or stainless steel that conducts heat well.

•Heat sinks that are used in computers, disk drives and televisions as cooling fins are made of aluminum

Others

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Soldering iron rods

•Radiator coils and cooling fins behind the refrigerator are made of copper.

•Soldering iron rods are made of iron with copper tips.

Uses of heat insulators

Materials/ devices Uses

Wood or plastics •Handles for cooking utensils, kettles, teapots, soldering iron rods and so on.•They protect our hands from the hot surfaces.

Cork, asbestos sheets, tiles To prevent tabletops from being damages by hot kitchenware or objects.

Sawdust To cover ice blocks to slow down the melting process.

Insulators are used in many applications where we want to minimize heat flow or heat loss.

Woolen blanket or cloth •Used to keep the body warm on cold days. The woolen blanket and the air layer trapped inside can prevent heat loss from the body.

Fiberglass, expanded polystyrene foam

•Used as insulators in the walls of ice boxes and refrigerators. Air trapped inside acts as insulator.

insulating cavity wall, double- glazed glass

Used in the buildings to prevent heat from entering by conduction during

Aluminum Plastics the day and prevent heat loss at night.

Benefits of heat flows

There are many uses of heat flow in our daily life:a. The flow of heat through

conduction is used for cooking and boiling. Conduction of heat is also applied in electric irons, ovens and toasters.

b. The flow of heat by convection and radiation is used to dry wet clothes, salted fish and others. Drying wet

clothes

c. Heat flow through radiation gives us hot water when we use a solar heater to absorb heat from the Sun.

d. Convection currents can help to improve air circulation and cool our houses and buildings. Fans and air conditioners help to cool the surrounding air through convection currents

Air conditioner.

e. Our life can be made healthier and more comfortable with a good ventilation system in our houses. Windows, opening and exhaust fans are things that can help to improve the ventilation of air in a house. Warm air inside the house can be drawn out and replaced with fresh, cool air from the outside.

Ventilation of buildings