M Manser

1
GCSE PHYSICS MODULE P1ENERGY FOR THE
HOMECooking with waves

• 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.

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
A bit more about the EM spectrum
Some waves from the electromagnetic spectrum are
visible. This is the light that we use to see.

• 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!

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.

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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.

6
Review thermal energy transfer by radiationCopy
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.

radiation
Radiation
heat or thermal
hotter
cooler
infrared
waves
dull
black
radiation
shiny
Shiny/ silvery
radiation/ waves
silver
black
7
Cooking with microwaves

• Use the websites below to find and record the
  answer to the following questions.
• What group of waves do microwaves belong to,
  within the electromagnetic spectrum?
• What type of molecule absorbs the energy from
  microwaves very well?
• What happens when these molecules absorb energy
  from the microwaves?
• Human cells are mainly what compound?
• Explain the effect microwaves would have if
  absorbed by human tissues.
• http//www.fda.gov/cdrh/consumer/microwave.html10
  
• http//www.geocities.com/yummyphysics/microwave.ht
  ml
• http//www.arpansa.gov.au/is_mwave.htm

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• Use this java applet on the internet to see a
  simulation of how microwaves affect water
  molecules.
• The link will take you to a page with quite a few
  simulations. On the left, scroll down to the
  Light and radiation section. Open it then click
  on the one titled microwave. You may need to
  wait a while for it to open. Once it does, use
  the microwave on/ off button to start the
  simulation. You can change the properties of the
  microwaves by using the frequency or amplitude
  cursors.
• Use the simulation to answer the following
  questions.
• Describe how the behaviour of the water
  molecules change once the microwaves are switched
  on. How is their KE affected?
• What is the relationship between the temperature
  of the water and the KE of the molecules?
• How is the energy carried by the microwaves
  affected by (a) increasing the frequency, and (b)
  decreasing the amplitude? Explain why you think
  so.

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What happens to 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.
Microwaves penetrate up to 1cm (depending on the
density) into the food.
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What happens to the food
The thermal energy is then transferred by
conduction (or convection of you are heating a
liquid) to the rest of the food.