P.1 Book 1 Section 4.3 Radiation Frost Energy transfer by radiation Emission and absorption of radiation Check-point 4 Examples of heat transfer by radiation Check-point 5 4.3Radiation

P.2 Book 1 Section 4.3 Radiation Frost On cold winter days, frost may form even though temp is higher than freezing point. Frost forms only on poor conductors like grass and leaves, but not on good conductors like stone and cement.

P.3 Book 1 Section 4.3 Radiation Frost Do you know why frost is formed at a temp above the freezing point? Is it related to temp of the grass?

P.4 Book 1 Section 4.3 Radiation 1 Energy transfer by radiation There is a vacuum between the sun and the earth.  Conduction and convection cannot take place in a vacuum. Energy from the Sun is transferred to the Earth by radiation.

P.5 Book 1 Section 4.3 Radiation 1 Energy transfer by radiation All objects radiate energy. Hotter  more energy radiated One of the radiation components is infra-red radiation (IR radiation). A very hot object also radiates visible light. Filament bulb glows brighter as its temp .

P.6 Book 1 Section 4.3 Radiation 1 Energy transfer by radiation A hot object loses energy by radiation. E.g. Hot soup cools down by this way. If we are at a temp lower than an object, we feel warm when we absorb the radiation. Radiation is a process of heat transfer which does not require any medium or particles. It takes place in all directions.

P.7 Book 1 Section 4.3 Radiation 2 Emission and absorption of radiation When radiation falls on an object, it is partly reflected and partly absorbed. Different surfaces respond differently to radiation. Expt 4c Radiation Simulation 4.4 Radiation

P.8 Book 1 Section 4.3 Radiation Experiment 4c Radiation Absorption of radiation by different surfaces Which surface is better for absorbing radiation? silvery foil ~ 10 cm Blackened foil

P.9 Book 1 Section 4.3 Radiation Experiment 4c Radiation Emission of radiation from different surfaces painted dull black painted silvery 1.Pour the same amount of boiling water into each of the two flasks. 2.Take the two temp. 3.Repeat step 2 after 10 mins. Which flask loses heat faster?

P.10 Book 1 Section 4.3 Radiation Experiment 4c Radiation 4.5 Expt 4c - Radiation Video

P.11 Book 1 Section 4.3 Radiation 2 Emission and absorption of radiation Everything is both an absorber and a radiator. Expt 4c shows that: Dull black surfaces are good absorbers and good radiators. Shiny surfaces are poor absorbers and poor radiators.

P.12 Book 1 Section 4.3 Radiation 2 Emission and absorption of radiation During daytime, people in hot places wear light-coloured clothes people in cold places wear dark-coloured clothes Fuel storage tanks are painted a silvery colour Radiators in cars are painted black Other examples:

P.13 Book 1 Section 4.3 Radiation 2 Emission and absorption of radiation Example 4 White houses in Greek islands

P.14 Book 1 Section 4.3 Radiation Example 4 White houses in Greek islands Greece is a country which has very hot summers. Why do people there paint their houses white? Objects in white are poor absorbers of energy reflect most of the energy from the sun  People want to keep the insides cool.

P.15 Book 1 Section 4.3 Radiation 2 Emission and absorption of radiation Example 5 Colour of school uniform

P.16 Book 1 Section 4.3 Radiation Example 5 Colour of school uniform Why are school uniforms usually light-coloured in summer and dark-coloured in winter? Light-coloured clothes absorb less energy through radiation than dark-coloured clothes.  light-coloured school uniforms help students stay cool in summer  dark-coloured school uniforms keep students warm in winter

P.17 Book 1 Section 4.3 Radiation Check-point 4 – Q1 Dark-coloured objects are ___________ radiation absorbers. Therefore we feel ____________ if we wear dark-coloured clothes in the sun in winter. good warmer

P.18 Book 1 Section 4.3 Radiation Check-point 4 – Q2 By which of the following processes do spaceships in space lose heat? A Conduction B Convection C Radiation

P.19 Book 1 Section 4.3 Radiation 3 Examples of heat transfer by radiation a The vacuum flask and vacuum cooker A vacuum flask keeps food hot or cold by reducing heat exchange between food and environment through conduction, convection and radiation.

P.20 Book 1 Section 4.3 Radiation a The vacuum flask and vacuum cooker plastic or cork stopper:  heat loss by conduction and convection silvery glass or stainless steel walls:  heat loss by radiation vacuum between the double walls:  heat loss by conduction and convection

P.21 Book 1 Section 4.3 Radiation a The vacuum flask and vacuum cooker A vacuum cooker works on the same principle. Food is first cooked using the inner pot, Insulated outer pot Conducting inner pot then put into the insulated outer pot and kept hot without heating.

P.22 Book 1 Section 4.3 Radiation a The vacuum flask and vacuum cooker outer vacuum insulated pot:  heat loss by conduction and convection inner and outer lids:  heat loss by conduction and convection silvery steel walls:  heat loss by radiation

P.23 Book 1 Section 4.3 Radiation 3 Examples of heat transfer by radiation b Greenhouse The radiations entering a greenhouse warms up the contents inside and the floor. A greenhouse has a glass roof and glass walls  allow radiations from the sun to pass through easily, except IR radiation.

P.24 Book 1 Section 4.3 Radiation b Greenhouse Warmed objects gives back IR radiation which cannot pass through the glass  energy is trapped inside Warm air is partly trapped inside  slows energy loss by convection So plants can grow even in cold weather.

P.25 Book 1 Section 4.3 Radiation 3 Examples of heat transfer by radiation Expt 4d Heating trapped air

P.26 Book 1 Section 4.3 Radiation Experiment 4d Heating trapped air 1.Set up the apparatus. 2.Take the readings of thermometers A and B. 3.Switch on the lamp and repeat Step 2 after 20 mins. 10 cm boiling tube thermometer 10 cm A B 100 W lamp 4.6 Expt 4d - Heating trapped air Video 4.Switch off the lamp and repeat Step 2 after 20 mins.

P.27 Book 1 Section 4.3 Radiation 3 Examples of heat transfer by radiation air inside the boiling tube warms up more quickly and cools down more slowly than the air outside. Example 6 Cooling system of a car Expt 4d shows that:

P.28 Book 1 Section 4.3 Radiation Example 6 Cooling system of a car A car engine is hot when it operates. A special kind of cooling system is designed to cool the engine. It uses water as a coolant. There is a water circulating system around the pipes in the car engine. As the water passes through the engine, it absorbs energy from the engine and becomes hot. Then the liquid leaves the engine and passes through the radiator (with fins), where the liquid loses energy and cools down. The cooled liquid then passes through the engine again. This cycle repeats to keep the engine from over- heating. A fan is used to draw air past he radiator.

P.29 Book 1 Section 4.3 Radiation Example 6 Cooling system of a car (a) Explain briefly why water is used as a coolant. Water has a very high specific heat capacity.  absorbs a large amount of energy without a big increase in temp.

P.30 Book 1 Section 4.3 Radiation Example 6 Cooling system of a car (b)Give two features of the cooling system that allow radiators to be cooled effectively. Explain the purpose of each feature briefly. A fan is installed.  enhances the flow of air that brings away the heat. The pipe of the radiator has fins.   the surface area of the pipe  conduction of heat is enhanced

P.31 Book 1 Section 4.3 Radiation Example 6 Cooling system of a car (c) What colour should be used to paint the radiator? Black. Black surfaces lose energy as radiation more readily.

P.32 Book 1 Section 4.3 Radiation Example 6 Cooling system of a car (d) The water is pumped to pass the through engine from A to B, instead of from B to A. Explain the advantage briefly. Water gets hot and rises as it passes through the engine. If water is pumped from B to A, convection current will be against the flow.  circulation less efficient.

P.33 Book 1 Section 4.3 Radiation Check-point 5 – Q1 Which part of a vacuum flask is designed to prevent heat loss by radiation? (1) Plastic stopper (2) Stainless steel wall (3) Vacuum layer between the double walls A (2) onlyB (1) and (3) only C (2) and (3) onlyD (1), (2) and (3)

P.34 Book 1 Section 4.3 Radiation Check-point 5 – Q2 The roof and walls of a greenhouse are made of glass because glass A blocks infra-red radiation. B blocks visible light. C is a good conductor. D is a good absorber of visible light.

P.35 Book 1 Section 4.3 Radiation The End