Volume B Chapter 25 Transmission of Heat

Myth or Truth? 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 conduction, convection & radiation The volume & density of a substance will change during thermal expansion Different materials have different rates of heat flow

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

Heat Moves! IN 3 WAYS… CONDUCTION CONVECTION RADIATION

1 represents conduction. 2 represents convection. 3 represents They are three processes of heat transfer shown in the picture. 1 represents conduction. 2 represents convection. 3 represents radiation.

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

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

How Conduction Works in solids When one end of a rod is heated, particles there gain energy and vibrate faster. These particles collide with less energetic neighbors.

How Conduction Works in solids 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

Conduction of heat through solids Wood Iron Aluminium Copper Observation: Observe the lengths of wax that have melted on the different rods in a given interval of time.

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

Good & Bad Conductors of Heat 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

Poor conductors (insulators) Good & Bad Conductors of Heat Good conductors Poor conductors (insulators) Silver Copper Aluminum Brass Iron Lead Stainless Steel Vacuum Air Styrofoam Body fat Cork Water Glass

Application of Conduction 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

Application of Conduction 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

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

Good Conductors Good Insulators Try This: Fill in the table appropriately Brass Wood Aluminium Air Foam Copper Iron Water Cotton Plastic Mercury Gold Good Conductors Good Insulators .

Good Insulators (or Bad Conductors) ANSWER: Fill in the table appropriately Brass Wood Aluminium Air Foam Copper Iron Water Cotton Plastic Mercury Gold Good Conductors Good Insulators (or Bad Conductors) Brass Aluminium Wood Air Copper Iron Foam Water Mercury Gold Cotton Plastic .

CONVECTION 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

CONVECTION 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.

CONVECTION IN LIQUIDS See movie

CONVECTION IN LIQUIDS 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 Explanation: When a liquid is heated, it expands, becomes less dense and floats upwards. Cold and denser liquid moves down to replace the rising liquid masses. This cold liquid, in turn, gets heated up. The cycle repeats until the boiling point of the liquid is reached.

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.

CONVECTION IN GASES

CONVECTION IN GASES 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

Application of Convection Sea and Land Breezes

Application of Convection 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

Application of Convection Sea and Land Breezes

Application of Convection 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

Application of Convection Hot Water System

Application of Convection 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

Application of Convection Air- Conditioners

Application of Convection 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

QUIZ TIME!!

A. Water B. Oxygen C. Wood D. Aluminium 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. The molecules are packed closer than non-metals and this reduces the time taken for energy to transfer. The molecules gain energy faster in metals.

A.Pan: Plastic Handle: Copper Pan: Glass Handle: Glass Q2. Which of these would you use for the frying pan and its handle? A.Pan: Plastic Handle: Copper Pan: Glass Handle: Glass Pan: Copper Handle: Brass Pan: Steel Handle: Wood

Q3. The purpose of using a double-glazed window is to A. Save money because glass is expensive. Provide a double protection against harmful projectiles. Reduce heat conduction by trapping air inside. 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. The clothes is able to trap cold air. The air is cold and we will lose water too quickly. Winter clothes are nice and trendy.

QUIZ TIME!!

Q1. Convection takes place because A. Free electrons make the heat energy travel faster in the fluid. Hot liquid and cold liquid separates themselves from each other. Hot fluid rises and colder fluid sinks. Hot fluid sinks and colder fluid rises.

Q2. The purple color stream is seen to travel 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. it is the only possible way for the crystal to dissolve. The water at the top starts to cool down and water from the base has to replace it. The heating at the base stirs up a convection current.

A. Alcohol Bricks Nitrogen Paraffin Q3. Convection of heat cannot take place in A. Alcohol Bricks Nitrogen Paraffin

A. air blown from the top will fall to the ground due to gravity. Q4. We place air conditioners near the ceiling because A. air blown from the top will fall to the ground due to gravity. cold air can be found near the ceiling. hot air rises to the ceiling and cooling it will create a convection current. cold air rises to the ceiling and cooling it will create a convection current.

Q5. Sea breeze occurs A. In the daytime. in the night time. only at midnight. only when it rains.

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

Radiation 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

Radiation Dull black surfaces are good emitters and absorbers of heat Shiny white surfaces are good reflectors (bad emitter and absorber) of heat

Radiation 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

Application of Radiation Good Emitters Poor Emitters Cooling fins at the back of a refrigerator painted dull black A shiny metal teapot and aluminum foil keep contents warm

Application of Radiation Good Absorbers Poor Absorbers Solar heating panels are painted in dull black paint to 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

Application of Radiation Vacuum Flask Purpose: keeps hot liquids hot and cold liquids cold. Function: Reduces transfer of heat by conduction, convection and radiation

Application of Radiation

Reduces heat loss/gained by: Application of Radiation Structure Reduces heat loss/gained by: vacuum in between the 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

Purpose (reduces heat loss/gained by) LET'S RECAP State the parts of a vacuum flask and explain what their functions are. Structure Purpose (reduces heat loss/gained by)

1 represents conduction. 2 represents convection. 3 represents They are three processes of heat transfer shown in the picture. 1 represents conduction. 2 represents convection. 3 represents radiation.