Volume B Chapter 25Transmission of Heat

Oh no! You will spoil the flask if you put cold water in it.

After this, the flask cannot be used to keep water hot anymore.

Myth or Truth?

Why are the handles of saucepans usually made of plastic?

Why is food packed in styrofoam

boxes?

Cooling fins at the back of refrigerators.

Why are they always painted black?

What we will learn…

•Explain what is meant by Explain what is meant by conduction, convection & radiationconduction, convection & radiation

•The volume & density of a substance The volume & density of a substance will change during thermal will change during thermal expansionexpansion

•Different materials have different Different materials have different rates of heat flowrates of heat flow

Heat is a form of energy that is transmitted from a region of higher to lower temperature.

IN 3 WAYS…

Transfer of Thermal EnergyTransfer of Thermal Energy

T H E M E T H R E E : T H E R M A L P H Y S I C S

C h a p t e r 7

They are three processes of heat transfer shown in the picture.

1 represents

2 represents

3 represents

conduction.

convection.

radiation.

Conduction is transmission of heat through a material from the hot regions to the cold regions without any visible

movement of the material.

•Solids are better conductors of heat than liquids and gases as solid particles are closer together.

•Thus, kinetic energy is transferred more quickly in solids.

See movie

•When one end of a rod is heated, particles there gain energy and vibrate faster.

•These particles collide with less energetic neighbors.

•Some of their energy is transferred to neighboring particles which thus gains kinetic energy.

•This goes on until the cold end reaches same temperature as hot end

See movie

Observation:

Observe the lengths of wax that have melted on the different rods in a given interval of time.

WoodIron

AluminiumCoppe

r

Process:•When a metal is heated, free

electrons gain energy and move faster.

•They move in the spaces between atoms before colliding with some atoms and transferring energy to them.

Metals conduct heat much faster then non-metals as they have free electrons.

•Materials conduct heat at different rates

•Good conductors of heat allow heat to flow through rapidly

•Bad conductors or good insulators of heat allow heat to flow through slowly

Good conductors

Poor conductors (insulators)

SilverCopper

AluminumBrassIronLead

Stainless Steel

VacuumAir

StyrofoamBody fat

CorkWaterGlass

1. Uses of Good Conductors

•Used where heat has to be quickly transmitted

E.g. Pans, kettles, cooking utensils

•Usually made of metals like aluminium, copper, steel

2. Uses of Poor Conductors (Insulators)• Use where unwanted heat has to be

kept away or to prevent heat loss. E.g. vacuum flask, handles of saucepan• Made of Cloth, plastics, wood etc

insulator

• Trapped air between 2 layers of glass is good insulator, reduces conduction of heat

.

Brass Wood Aluminium Air Foam Copper

Iron Water Cotton Plastic Mercury Gold

Try This: Fill in the table appropriately

Good Conductors Good Insulators

.

Brass Wood Aluminium Air Foam Copper

Iron Water Cotton Plastic Mercury Gold

ANSWER: Fill in the table appropriately

Good Conductors Good Insulators (or Bad Conductors)

Brass Aluminium Wood Air

Copper Iron Foam Water

Mercury Gold Cotton Plastic

Convection is the transmission of heat through a fluid (liquid or gas) from the hot to the cold region by the circulating movement of the fluid.

cool (higher density)

hot(lower density)

liquid or gas

occurs in fluids (liquids and gases), but not in solids

When one part of fluid is heated, it expands, becomes less dense and rises

The surrounding cooler and denser fluid sinks

This produces a circulating movement called convection current.

See movie

•Add potassium permanganate (purple) crystals into a flask of water

•Heat flask gently just below the crystals

•Purple-streaked water rises upwards and fan outwards.

•Water becomes evenly purple in colour after a while.

7.2 Convection

Application 2: Car Radiator

• Car engines are kept cool by a water jacket

• When water in the water jacket gets heated, it flows into copper tubes which include many cooling fans.

• A fan causes air to flow past the tubes and cool water in them.

• Cooled water flows down and back into engine through a hose at the bottom.

•Fit two glass chimneys to the top of a wooden box with plane glass window

•Light a candle below one chimney

•Heat from this initiates a convection current

•Hot air flows out of this chimney

•Cold air flows in through other chimney

Sea and Land Breezes

1. Sea Breeze (Daytime)•In the day, land heats up faster than

the sea•Air above land expands and rises•Cool air above sea moves in towards

the land •This result in sea breeze

Sea and Land Breezes

2. Land Breeze (Night time)•At night, the land cools faster than the

sea•Warm air above sea rises •Cool air above land moves out towards

the sea •Result in land breeze

Hot Water System

•Boiler at bottom heats up water and causes the hot water to rise

•Hot water then moves to the hot water storage tank

•Cold water from the cold tank then moves down to replace the hot water

•A convection current is set up

Hot Water System

Air- Conditioners

•Cold air blown out by the air-conditioner sinks because it is denser than warm air.

•Warm air rises and is drawn into air-conditioner to be cooled.

•A convection current is set up

Air- Conditioners

Q1. Which of the following is a good conductor of heat?

A. Water

B. Oxygen

C. Wood

D. Aluminium

Q2. Why does heat conduction take place faster in metals?

A. The molecules are larger than non-metals and this reduces the amount of time taken for energy to transfer.

B. The presence of free electrons causes heat energy to transmit quickly.

C.The molecules are packed closer than non-metals and this reduces the time taken for energy to transfer.

D.The molecules gain energy faster in metals.

Q2. Which of these would you use for the frying pan and its handle?

A.Pan: Plastic Handle: Copper

B.Pan: Glass Handle: Glass

C.Pan: Copper Handle: Brass

D.Pan: Steel Handle: Wood

Q3. The purpose of using a double-glazed window is to

A. Save money because glass is expensive.

B. Provide a double protection against harmful projectiles.

C.Reduce heat conduction by trapping air inside.

D.Improve heat convection by trapping air inside.

Q4. We need to put on more clothes in winter because

A. The air is cold and we do not want heat to escape from our bodies.

B. The clothes is able to trap cold air.

C. The air is cold and we will lose water too quickly.

D.Winter clothes are nice and trendy.

Q1. Convection takes place because

A. Free electrons make the heat energy travel faster in the fluid.

B. Hot liquid and cold liquid separates themselves from each other.

C. Hot fluid rises and colder fluid sinks.

D. Hot fluid sinks and colder fluid rises.

Q2. The purple color stream is seen to travel upwards from the center and downwards from the sides because

A. The heat from the fire causes the crystal to react with the water.

B. it is the only possible way for the crystal to dissolve.

C. The water at the top starts to cool down and water from the base has to replace it.

D. The heating at the base stirs up a convection current.

Q3. Convection of heat cannot take place in

A. Alcohol

B. Bricks

C. Nitrogen

D. Paraffin

Q4. We place air conditioners near the ceiling because

A. air blown from the top will fall to the ground due to gravity.

B. cold air can be found near the ceiling.

C. hot air rises to the ceiling and cooling it will create a convection current.

D. cold air rises to the ceiling and cooling it will create a convection current.

Q5. Sea breeze occurs

A. In the daytime.

B. in the night time.

C. only at midnight.

D. only when it rains.

Radiation is the transmission of heat from one place to another by means of infra-red radiation which can travel through vacuum or air

Rate of Emitting (giving out) and Absorbing Heat by Radiation is affected by: (a) surface temperature

(b) colour and texture of the surface

(c) surface area

•Dull black surfaces are good emitters and absorbers of heat

•Shiny white surfaces are good reflectors (bad emitter and absorber) of heat

Why does the cork on the dull black surface fall off first?

Ans: Dull black surfaces are good absorbers of heat, thus it melts the wax faster

Good EmittersGood Emitters Poor EmittersPoor EmittersCooling fins at the back of a refrigerator painted dull black

A shiny metal teapot and aluminum foil keep contents warm

Good AbsorbersGood Absorbers Poor AbsorbersPoor AbsorbersSolar heating panels are painted in dull black paintto absorb heat from sun

Houses in hot countries are painted in white or light-coloured paint, to keep them cooler.

Light colours are chosen for clothes and cars in hot weather

Vacuum Flask

Purpose: • keeps hot liquids hot

and cold liquids cold.

Function: • Reduces transfer

of heat by conduction, convection and radiation

Structure Reduces heat loss/gained by:

vacuum in betweenthe double glass walls

conduction or convection

Silvered walls•Outer silvered surface•Inner silvered surface

Radiation•reflects radiation in external surroundings away from flask•reflects radiation from hot fluids back into flask

Foam plastic support

conduction

Plastic cap convection

1. State the parts of a vacuum flask and explain what their functions are.

Structure Purpose (reduces heat loss/gained by)

They are three processes of heat transfer shown in the picture.

1 represents

2 represents

3 represents

conduction.

convection.

radiation.