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How does a microwave oven work?

Now, microwave oven is so popular that majority of houses have one. Many use it mainly to reheat food without loss of nutrient. The advantage is that the microwave heats up the food very fast.

Some observations made on microwave oven are,

  • It does not heat up the vessels directly
  • It is not able to cook food contained inside metal vessels
  • If the turn table does not work, cooking will not be even.

 

So, it is pertinent to ask how the mechanism of heating in a microwave oven is different from other heating devices?

Here the water molecules, which are polar in nature play the magic. (A polar molecule is one in which the centre of mass of positive charges and the centre of mass of negative charges do not coincide and have a net dipole moment.) When microwaves are passed through food materials, the water molecules contained in them tends to turn and this rapid churning of water molecules develops a lot of heat. The turn-table adds to the churning and randomization developing heat more rapidly.

The food materials contained in a closed metal vessel will not be heated up by the microwave oven as they do not allow microwaves to pass through them. The metals are opaque to microwaves.

Why the food materials heated using microwave dry up and become stiff when cool?

In a microwave oven, the water molecules are heated very fats that the water is converted readily into steam. If kept open, the steam will escape and the material will become hard when cooled due to the decrease in the water content.

Heat! Heat! Heat!

Thermal vibration of a segment of protein alph...
Thermal vibration of a segment of protein alpha helix. The amplitude of the vibrations increases with temperature. (Photo credit: Wikipedia)

A glass ball at -10 degrees C is placed in 500 ml of water at 20 degrees C. The final temp is 15 degrees C.
A. How much heat is lost by the water
B. what is the heat capacity of the water
C. How much heat is gained by the glass ball
D. What is the heat capacity of the glass ball
E. what is the ratio of the heat capacities of the water and the glass ball
F. What is the ratio of the temp changes of the water and glass ball
G. How are the answers to e and f related

Waves in a bucket!

Arvind MR asks:

“if a drop of water is dropped in a bucket we can notice that a wave is created and it moves away as a ring……but i see that it rebounds back and come to a point again but not the starting position ……so the number no times this happens depend upon the height we drop the water droplet…..my question is why the wave rebounds after touching the ends of the bucket?”

Answer:

See the animations here and try explaining yourselves

Watch an interactive animation on reflection of waves here.

http://wildcat.phys.northwestern.edu/vpl/waves/wavereflection.html

Physics in Irrigation

Somnath asked:

“water in a canal 6m wide n1.5m deep is flowing with a speed of 10km/hr.How much area will it irrigate in 30min if 8cm of standing water is need”

 

Answer (to be posted soon)

PhysicsWorld reports: Flowing water may exist on Mars!

Images from NASA’s Mars Reconnaissance Orbiter (MRO) reveal that dark, narrow, finger-like structures follow slopes in certain regions of the southern hemisphere of the planet during its summer months. The researchers believe that these could be caused by flowing salt water and say the finding raises the tantalizing prospect that there might be life on Mars.

Read more here
Also Read
Introducing the ‘antimagnet’
Photons tune in and shape up

 

Why water has more dielectric constant than mica?

Ashis asks:

“Why water has more dielectric constant  than
mica. Please explain”

A Falling drop! Watch the picture and explain it

You can see below the photograph of a drop of water falling on water surface. Can anyone explain the observation?

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