Thermal radiation

What do you think the definitions of ‘radiate’ and radiation’ are? Use hand signals & key words to explain it to your talking partner (as if they were a puppy!)

7.9 Recognize that thermal energy transfer by radiation does not require a medium. All objects give out and take in thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits. Infrared radiation is a type of electromagnetic radiation that involves waves. In contrast to conduction and convection, no particles are involved, so radiation can even work through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth. Some surfaces are better than others at emitting, reflecting and absorbing infrared radiation.

What are these pictures of? Why do they look different? HOW CAN YOU TELL IF SOMETHING IS HOT? Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. An example of thermal radiation is the infrared radiation emitted by hot objects. A person near a fire will feel the radiated heat of the fire, even if the surrounding air is very cold. You cannot see the thermal radiation but you can see other energies (eg light). Explain this in a diagram! Title: Thermal Radiation

Basics of thermal radiation Thermal radiation is emitted as electromagnetic (infra-red) waves Thermal radiation waves are similar to visible light waves Infra-red thermal radiation is emitted by all matter The hotter an object is the more infra-red radiation it emits

? How does the sun warm you on a hot day? infrared radiation Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth? ? infrared radiation There are no particles between the Sun and the Earth so the heat cannot travel by conduction or by convection. The heat travels to Earth by infrared waves. They are similar to light waves and are able to travel through empty space.

INFRARED WAVES Heat can move by travelling as infrared waves. These are electromagnetic waves, like light waves, but with a longer wavelength. This means that infrared waves act like light waves: They can travel through a vacuum. They travel at the same speed as light – 300,000,000 m/s. They can be reflected and absorbed. Infrared waves heat objects that absorb them and so can be called thermal radiation.

ABSORBING THERMAL RADIATION Infrared waves will warm up any matter that absorbs (take in) them. Some surfaces are better at absorbing thermal radiation than others – good emitters are also good absorbers. best emitter worst emitter white silver matt black best absorber worst absorber Matt black surfaces are the best absorbers of radiation. Shiny surfaces are the worst emitters because they reflect most of the radiation away. QUESTION: Why are solar panels that are used for heating water covered in a black outer layer?

EMITING THERMAL RADIATION All objects emit (give out) some thermal radiation. Some surfaces are better at emitting thermal radiation than others. best emitter worst emitter white silver matt black Matt black surfaces are the best emitters of radiation. Shiny surfaces are the worst emitters of radiation. QUESTION: Which type of kettle will cool down faster: a black kettle or a shiny metallic kettle?

Why are shiny foil blankets wrapped around marathon runners at the end of a race? The shiny metal reflects the heat radiation from the runner back in to their body, which stops the runner from getting cold.

Why are houses painted white in hot countries? White reflects heat radiation and so keeps the house cooler.

How can we relate our new understanding to the greenhouse effect?

Summary notes Thermal or heat radiation is the transfer of energy by infra-red waves. These waves are part of the electromagnetic spectrum

Summary notes All objects emit (give off) thermal radiation. The hotter the object the more thermal radiation it emits. Heat radiation can travel through a vacuum like space. This is how we get heat from the sun. Thermal radiation is energy transfer by electromagnetic radiation. The transfer of heat energy by infra-red radiation does NOT involve particles.

Summary notes All objects emit (give off) thermal radiation. The hotter the object the more thermal radiation it emits. Heat radiation can travel through a vacuum like space. This is how we get heat from the sun. Thermal radiation is energy transfer by electromagnetic radiation. The transfer of heat energy by infra-red radiation does NOT involve particles.

Check your understanding Which part of the electromagnetic spectrum is concerned with heat energy? How does heat from the Sun reach the Earth? Do all objects give out the same amount of heat radiation? Explain your answer CLICK AGAIN FOR THE ANSWERS ANSWERS: Infra-red By radiation through space. No, the hotter the object the more heat it radiates

STARTER The heat transfer mechanism in solids. Absorber Electrons in a metal that are free to move throughout the metal. A material that gives out thermal radiation A material that takes in thermal radiation. The heat transfer mechanism in fluids, which occurs because hot fluids are less dense than cold fluids. A material that lets heat flow through it. Absorber Conductor Conduction Convection Emitter Free Electrons

7.10 Thermal Radiation: Identify infra-red radiation (from all objects) as part of the electromagnetic spectrum) If two objects made from the same material have identical volumes, a thin, flat object will radiate heat energy faster than a fat object. This is one reason why domestic radiators are thin and flat. Radiators are often painted with white gloss paint. They would be better at radiating heat if they were painted with black matt paint, but in fact, despite their name, radiators transfer most of their heat to a room by convection. Light from the sun reaching the earth! color finish ability to emit thermal radiation ability to absorb thermal radiation Dark dull or matt good good light shiny poor poor