Infra red radiation WALT Explain that the absorptionemission

Infra red radiation

WALT • Explain that the absorption/emission of radiation depends on the nature of the surface

WILF GW Know that the type of surface can affect the amount of radiation absorbed/emitted BI – Describe which surface is the best/worst at absorbing/emitting EW – Explain how some things are designed to absorb/emit radiation

DO NOW TASK (RP) True or false: 1. Metals are good conductors of thermal energy. This means they have a high thermal conductivity rating. True 2. True or false: Particles in a liquid have more energy than those in a gas False 3. What is the unit of weight a) Kilogram (Kg) b) Newton (N) c) Pound (Lb) 3. Why can you compress a gas but not a liquid? There is space between particles in a gas, they can be moved closer together. Particles in a liquid are touching and cannot be moved closer 16/01/2022 4. Name 3 ways of preventing heat loss in the home. Dbl glazing, cavity wall insulation, loft insulation, etc.

Starter In pairs write two sentences in the back of your book about the following • Conduction • Convection

Using a grill If hot air rises, and air is a bad conductor, how does the heat get from the grill down through the air to the toast? There is another way of transferring heat!

Pen to the back of the class. How could we get a pen to the back of the class? Passing it along – conduction Walking to the back – convection Throwing it – radiation Thrower – emitting Catcher - absorber

Infra red radiation All objects emit infra red radiation. The hotter the object the more it emits. Unlike convection and conduction IR radiation does not need particles.

Infra red detectors Just like our eyes can detect visible light we can use certain devices to detect Infra red radiation.

The Sun How do we know that heat from the Sun must travel to us by infra red radiation?

Practical • Place a black and silver test tube the same distance from a bunsen flame • Same amount of water in each test tube • Measure the temperature in each tube • Measure the temperature after 5 mins Test tube Black Silver Temp start Temp end Temp change

Conclusion Do all colours absorb infra red radiation the same amount? No. Colour has a large effect on the absorption of infra red radiaiton.

Good and bad absorbers of heat

Task Describe which colours absorb infra red radiation and which colours reflect radiation.

Leslie’s cube NOTE – all the surfaces are at the same temperature.

Emitting infra red radiation Black surfaces Matt surfaces Good emitters Light coloured (or shiny silver) Smooth surfaces Bad emitters

Infra red radiation key points • All objects emit Infra red radiation • The hotter the object the more infra red radiation it emits • Dull, matt black surfaces are the best absorbers of infra red (NOTE – they are also good emitters) • Shiny, smooth white (or silver) surfaces are the best at reflecting infra red. (NOTE – they are poor emitters) • Radiation does not use particles of matter so it can travel through a vacuum.

Practical infra red radiation

Why white in Spain?

Why black cooling fins?

Example question Light coloured shiny teapot Dark coloured dull teapot If both of these teapots are the same size containing the same amount of water in the same room, in which teapot will the tea cool down the quickest? Why? The dark, dull one. It is better at emitting thermal radiation.

Example The diagram shows four identical pieces of aluminium. Each had been painted with a different type of paint. A drop of water was placed on each and they were then heated by a radiant heater held about one metre above them. (i) Suggest in which order the pieces of aluminium would become dry. (1) (ii) Explain why you chose your order. (2)