P1F: Cooking And Communication Using Waves

P1F: Cooking And Communication Using Waves

1. Infrared radiation and microwaves can be used to cook food. Microwaves are also used to transmit information, such as mobile phone networks.

2. COOKING WITH INFRARED

   • All objects give out and take in infrared radiation. The hotter an object is, the more infrared radiation it emits.
     • Very hot objects may glow, such as the heating element in a toaster or an electric fire. Other hot objects may not glow, such as an iron.
       • Some surfaces are better than others at emitting and absorbing infrared radiation
         • Shiny surfaces reflect infrared radiation.
           • When an object absorbs infrared radiation, its temperature increases. Food, for instance, begins to cook when its surface absorbs infrared radiation.

3. COOKING WITH MICROWAVES

   • Microwave radiation has lower frequencies and longer wavelengths than visible light. Microwaves cannot be seen, but they can cause burns if they are absorbed by body tissues.
     • Microwave ovens rely on the ability of microwaves to penetrate about 1 cm into food. They are absorbed by water in the food, causing it to warm up.
       • Microwaves pass through plastics and glass, but are reflected by metals. The water in living cells can also absorb microwave radiation.
         • As a result, they can be killed or damaged by the heat released.
           • To protect us from the harmful effects of microwaves, microwave ovens have metal cases and the glass doors have a metal mesh.
             • There is a British Standard which sets a safe limit of 50 watts per square metre at 5 cm.
               • This means that measurements of microwave power taken at 5 cm from the microwave must be less than 50 watts per square metre.

4. COMMUNICATING WITH MICROWAVES

   • Mobile phones communicate with their base stations using low-intensity microwave radiation.
     • Microwaves can be used to transmit information over large distances, although some areas have poor signals.
       • The use of mobile phones means people are exposed to microwave radiation in two ways:
         • to the head from the aerial in the phone
         • to the body from transmitters in base station masts.
   • There are concerns about children using mobile phones, and about people using them very often.
     • This is because microwaves from the phone can penetrate body tissues to a depth of a few centimetres. They are absorbed, and give up their energy to body tissues.
       • This can cause a small amount of heating, about 0.1°C. This is much less than if you stand in sunshine
         • There are limits to the amount of microwave radiation that can be emitted by mobile phones. This Specific Absorption Rate - SAR - should not be more than 2 watts per kilogram.

5. TRANSFERRING ENERGY

   • Kinetic energy of particles is increased when the radiation is absorbed:
     • infrared radiation is absorbed by all particles on the surface
     • Microwave radiation is absorbed by water particles, both on the surface and up to about 1 cm deep into the food.
   • The kinetic energy is transferred to the centre of the food by conduction or convection.
   • The higher the frequency of an electromagnetic wave, the greater the energy it transfers for a given amplitude. Infrared radiation has a higher frequency than microwaves. It can transfer enough energy to break chemical bonds. Microwaves cannot.

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