Microwaves and Microwave Oven

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Microwaves and Microwave Oven
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History

• The idea of microwave oven was first conceived by
  Percy L. Spencer in 1946 and finally patented in
  1950.
• Mechanismthe principle of frictional heat
  production using microwaves.
• The early ovens are huge--weighted over 300 kg
  and measured over 1.5 metre in height--were
  mainly used in large restaurants and food
  outlets.
• The first home version was produced by Raytheon
  Corporation in 1965.

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What are Microwaves?

• Microwaves are electromagnetic waves which
  consists of both electric and magnetic fields
  perpendicular to each other and propagates at the
  speed of light.
• Microwaves form parts of the electromagnetic
  spectrum with typical wavelengths from 1
  millimetre to 10 centimetres something in
  between light waves and radio waves.
• Microwaves are also used in telecommunications,
  e.g., radars, wireless computer networks and
  mobile phones.
• The entire universe is filled with microwave
  radiation left by the Big Bang explosion of the
  early Universe.

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The Microwave Oven
A domestic microwave oven

• The microwave generator is a magnetron.
• Magnetron converts electrical energy into
  microwave energy. Microwaves produced are
  transported to the cooking chamber by a
  wave-guide.

• The cooking chamber is designed to contain the
  microwave, so that microwave bounces off the
  walls like beams of light in a mirrored room
  around.
• Metal wiring in the glass window of the door
  keeps the microwaves from leaving the cooking
  chamber.

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Cooking with Microwaves

• Microwave cooking is a process of exciting the
  water molecules in food. Food substance that does
  not contain water, such as oil, usually does not
  get cooked in a microwave.
• The polarized water molecules in the food will
  get pulled back and forth at the rate of about
  2.5 billion times per second by the electric
  fields of the microwaves.
• This rapid back-and-forth motion between the
  water molecules creates friction, and hence heat.

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• Microwave can only penetrate about 3.5 to 5 cm
  into the food. The centre of the food is mainly
  cooked by heat conduction.
• The interference effect of the microwaves
  produces the hot and cold spots on the food,a
  turntable or a rotating source is often used to
  distribute evenly around the food,

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Cooking with Microwaves
Magnetron produces microwaves which pull the
water molecules back and forth at the rate of
about 2.5 billion times per second.
This rapid back-and-forth motion between the
water molecules creates friction, and hence heat.
And then the heat is transferred to the molecules
of food nearby.
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• The exact amount of microwave energy need to cook
  a piece of food depends on its properties.
• Food substance containing a substantial amount of
  water is a good absorber of microwave energy.
• The effectiveness of microwave cooking are
  determined by the shape and material composite of
  the container used. Round containers are
  generally more suitable.

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Metallic Objects

• Microwave radiation can pass through plastic and
  glass, but not metallic objects. This is why the
  glass window of the doors of a microwave is laced
  with metal wiring.
• Metals reflect microwaves. When microwaves are
  reflected from a metallic object, it produces a
  so-called arcing effect. Arcing results from the
  build-up of excessive charges in metallic
  materials.
• When two or more metallic objects, e.g., a
  metallic food container and the metallic interior
  of the wall, are placed close to each other
  causing the air between them to ionize, producing
  sparks.
• If the arcing object is further bombarded by
  microwaves, the temperature of the object rises
  sharply and overheats and eventually causing a
  fire in the oven.

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Superheating

• It is known to occur when a cup of water is
  heated in a microwave oven.
• The water is referred to as superheated when its
  temperature increases above its normal boiling
  point (the boiling point for water at standard
  atmospheric pressure is 100oC ).
• The superheated state is an unstable state.
• The presence of an external agent such as a spoon
  or even milk powder can cause the superheated
  water to boil vigorously into explosion.
• If one litre of water is superheated by only 1oC,
  it can produce about 3 litres of steam.

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Safety of Microwaves

• Metallic objects or boil water should not put in
  microwave oven.
• Other tests have revealed that nearly all
  microwave ovens in the market emit microwaves
  whilst in operation.
• Mobile phonesWill the heat produced by microwave
  radiation affect our body?
• Micro-waved foodSome tests have revealed that
  the molecular structures of nutrient in
  vegetables, such as carrots and broccoli, are
  being deformed at cellular level by high
  frequency microwaves.

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