P1e(i) Cooking and Communicating Using Waves You will learn about: How Infrared is given off How Microwaves work food How wavelength affects energy transference

Microwaves Microwaves can be very dangerous. If anything metallic is worked in a microwave it causes sparks which could be very dangerous. Similarly, light bulb gases explode if worked too long. CAUTION: Never put anything metallic in a microwave Microwaves are absorbed by water or fat molecules found in food. The molecules are excited and start to vibrate. This causes heat to be transferred from the kinetic energy. By conduction this heat energy is transferred to surrounding molecules of the food. If the food is in liquid form then the heat will be transferred by convection. If the food item does not contain water or fat then the food will not be worked i.e. it will not be heated. Microwaves can penetrate about 1cm into the outer parts of food. Microwaves easily pass through plastic or glass but any shiny surface will cause them to be reflected – just like all waves in the Electromagnetic Spectrum. The microwave door contains a special film that reflects the microwaves into the oven. The microwave door also has a mesh layer with gaps smaller than the microwave wavelength. This is another precaution stopping the microwaves escaping.

Infrared Cooking A toaster or grill uses infrared radiation to warm food. The infrared radiation is absorbed by the surface of the food. Not very much of the heat reaches the centre of the food. So even though a steak may appear to be cooked on the outside it may still be rare in the middle. For bread, the water molecules on the surface of the bread are evaporated. The bread is dehydrated and turns crispy. This is why a toaster is good at toasting bread. REMEMBER: All warm and hot objects emit infrared radiation. The hotter the object the more infrared radiation emitted. Infrared waves have a shorter wavelength than microwaves. This means infrared waves have a higher frequency and therefore higher energy. However, infrared waves cannot penetrate far into the food so even though they have more energy they cook food much slower than microwaves.

Re-cap of Similarities and Differences Infrared Radiation SIMILARITIES Heated molecules transfer thermal energy into kinetic energy Heat transfers via conduction or convection DIFFERENCES Does not penetrate far into the food Heats only the surface of the food Higher frequency thus higher energy wave Microwave Radiation SIMILARITIES Heated molecules transfer thermal energy into kinetic energy Transfers via conduction or convection DIFFERENCES Penetrates about 1cm into the food Works only water or fat molecules Lower frequency thus lower energy wave

Questions 1.Name three objects that emit infrared radiation. 2.Which colour in the Electromagnetic Spectrum is next to Infrared? 3.Which wave has a longer wavelength than microwaves? 4.You cannot make good toast in a microwave. Explain why? 5.A chef chooses a microwave oven over an infrared oven. Name two reasons he may do this. 6.Describe two ways a microwave retains the microwaves. 7.Billy is working hard for his next Physics test sitting near a radiator. Compare the infrared radiation of the radiator, Billy and the past exam papers he is holding.

Questions 1.Name three objects that emit infrared radiation. Anything warm or hot: a person, an animal, an engine, a fire etc 2.Which colour in the Electromagnetic Spectrum is next to Infrared? Red 3.Which wave has a longer wavelength than microwaves? Radiowaves 4.You cannot make good toast in a microwave. Explain why? A microwave does not evaporate the water molecules from the surface of bread like an infrared oven does. 5.A chef chooses a microwave oven over an infrared oven. Name two reasons he may do this. Faster cooking times because microwaves penetrate the food working it. Better even cooking of meats – no rawness in the inside. 6.Describe two ways a microwave retains the microwaves. Door contains a shiny film to reflect the microwaves back in. Door contains a wire mesh with gaps that are smaller than the microwave wavelength. 7.Billy is working hard for his next Physics test sitting near a radiator. Compare the infrared radiation of the radiator, Billy and the past exam papers he is holding. Radiator emits more infrared radiation, Billy a little less and the past exam papers even less. The past exam papers will only show heat where Billy has been holding it.