Microwaves and Radio Waves
Use pages 138/9 and 146 to help you answer these questions.
Everyone must answer the seven non-* questions. Attempt the *’s (and **’s) for a challenge!
Microwave Ovens
1. What part of the food absorbs the microwaves? water
2. How is the microwave oven designed to protect the user? metal walls reflect microwaves
3. What happens to the microwave energy when it is absorbed by the food? changed to heat in food
4. * Why can you not use metal objects such as a fork in the microwave? reflects microwaves
5. ** The grille on the door has holes so you can see through. If the holes were larger the oven wouldn’t work. Can you suggest why this might be? microwaves would pass through
Satellite TV
1. What is the back (curved) part of your Sky TV aerial made of? Explain why. metal - reflects
2. * Apart from the safety aspect, what would be the problem with using shorter microwaves for sending signals like these? absorbed by water in atmosphere
3. * The microwaves used to send satellite TV signals have a frequency of 12,000 MHz. What is their wavelength? (Hint: first remember what their speed is) 300/12000 = 0.025m = 2.5cm
Mobile Phones
1. * Mobile phones in the UK mostly use a frequency of around 1800 MHz. What is their wavelength? (Hint: first remember what their speed is) 300/1800 = 0.16m = 16cm
2. * Bluetooth signals use the same frequency as microwave ovens. Why are you not harmed by using Bluetooth on your phone? (Hint: Bluetooth only works over a short distance) low power
3. ** Use your answers to suggest what effect using your mobile for a long period of time might have on your body. medium power so might heat up and damage cells over a long period of use
Radio Waves
1. What materials can radio waves pass through without being absorbed? dry, non-metals
2. What materials are good absorbers of radio waves? water, metal
3. What happens when radio waves hit an aerial? electrical waves created (alternating current)
4. ** Jodrell Bank receives radio waves from space. Why is it so much larger than your satellite TV dish at home [two reasons]? can collect more waves; waves are longer so bigger dish needed
5. ** All materials contain electrons. Can you suggest why aerials are made of metal? metals contain free electrons which can move to create the alternating current