1. By the end of this presentation, you should be able to: Explain how microwaves and infrared waves transfer energy to materials. Describe how the energy associated with microwaves and infrared depend on their frequency and relate this to their potential danger. GCSE PHYSICS: MODULE P1 ENERGY FOR THE HOME Cooking with waves

2. What are microwaves and infrared waves? They are both electromagnetic waves that are invisible to the human eye. Click here to see more about the properties and dangers of the waves of the electromagnetic spectrum.

3. Essentially they are made of tiny packets of energy which vibrate up and down as they travel. Click here for an animation of this These waves travel at the speed of light, 300 000km per second! Some waves from the electromagnetic spectrum are visible. This is the light that we use to see. A bit more about the EM spectrum

4. More about infrared waves You cannot detect these waves with your eye, but you can with your skin! The energy from IR waves are absorbed by molecules on the surface of your skin, and it increases their kinetic energy. Your skin then feels hotter as the temperature of that area starts to rise.

5. Hot Objects All warm objects will emit IR waves. A hot object with a black, dull surface will emit more radiation than a white or silvery object at the same temperature. Which of these objects do you think will emit the most, and which will emit the least IR radiation? The hotter the body, the more IR is emitted. The hot wires in a toaster will emit the most, and the human body, which is at the lowest temperature of the three will emit the least IR waves. The hot wires in a toaster The hot plate of an iron A marathon runner

6. Review thermal energy transfer by radiation Copy the paragraphs and fill in the blank spaces. Then click to see the answers Cool objects can be heated by conduction, convection or __________. __________ involves the transfer of ____________ energy from a _______ region to a ________ region by ________ ______. An object that has a _____, ______ surface will absorb __________ better than one which has a ________ surface. ____________ surfaces will reflect the _____________. A cold drink in a ______ cup will then stay cold longer than if it was in a ______ cup. radiationRadiation heat or thermalhotter coolerinfraredwaves dullblack radiationshiny Shiny/ silvery radiation/ waves silver black

7. Cooking with microwaves Phet Simulation

8. What happens to the food Microwaves penetrate up to 1cm (depending on the density) into the food. Microwaves can pass through glass and plastic since these particles do not absorb its energy. They are reflected by shiny surfaces though, so no metallic containers please.

9. The thermal energy is then transferred by conduction (or convection of you are heating a liquid) to the rest of the food.