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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

INTENSIVE VARIABLES AND EXTENSIVE VARIABLES

A thermodynamic variable may be intensive or extensive.

What are intensive variables?

The variables which are independent of size or amount of substance are called intensive variables. 

Example: Density is an intensive variable. (because it does not depend on the amount of that substance)

What are extensive variables?

The variables which depend on the size or amount of the substance are called extensive variables. It is additive for independent, noninteracting subsystems

Example: Mass and volume are extensive variables. (Because they are directly measuring the amount of substance)

Jameel Leers idea on Atom Bomb! What do you think?

Jameel Leers Posted “I’m a carpenter with a bit of an interest in physics. Today while eating lunch on a  building site i realised something that i need confirming or de-bunking. Here it is: I believe an Atomic Bomb wouldn’t work in space. Here’s why: Take 90 kgs of tnt, when it detonates it is self-oxidising and instantly sublimes into rapidly expanding gases. The TNT also releases energy in the form of heat, light, sound and more importantly, kinetic energy in gases given as Ek =  1/2 MV^2. An atomic bomb only realeases heat and light. All the expanding gas, the kinetic energy, is provided by heating the atmosphere. The atmposphere around the A-blast is super heated expands, providing the destructive energy. This is why an atomic bomb blast appears to ‘suck back’. Because the bomb doesn’t make its own gas, and gas that it displaces by expansion cools and returns to ground zero. In the vacuum of space however, the atom bomb will produce only heat and light. The heat will instantly vaporise the supercritical elements and turn to gas, and they will expand, but it will only be the same explosive force as a conventional explosive of the same weight. eg 100kg’s of U-235 will produce 100kg of expanding gas. Just as 100kg’s of TNT will produce 100kg’s of gas. So thats what i think. A-bomb won’t really work in space. It is still a nuclear chain reaction. Not a ‘bomb’ though… What do you think?”

Thermal Expansion Problem

Meenu asked:

A metre scale is calibrated at 0 degree celsius to give correct reading. Find the distance between 50 cm and 51 cm if the scale is used at 10 degree celsius. Coefficient of linear expansion of steel is 1.1×10-5

Answers given in two books are 1.00011cm and 0.99988 why this contradiction?

(The question is being posted at PhysicsForum.PlusTwoPhysics.com too so that visitors can readily reply with their answers and explanations)

Specific Heat capacity and Molar specific Heat capacity

Chaitanya asked:

“how to calculate molar the heat capacity of a gas
given cp and cv of the gas?”

 

Answer:

Specific heat capacity of a substance is the quantity of heat required to raise the temperature of 1g of that substance through 1oC. It can be measured at constant pressure (Cp) and at constant volume (Cv).

Molar heat capacity of a substance is the quantity of heat required to raise the temperature of 1 mole of the substance by 1oC.

Molar heat capacity = Specific heat capacity × Molecular weight,

i.e.,

Cv = cv × M and Cp = cp × M.

 

Extinguishing fire : Which is better? – Hot water or cold water

“What’s the difference if we add hot water or cold
water to extinguish fire?”

Sreeda asks:

Answer: The thing to be noted here is that the fire extinguishing involves absorption of heat and the absorption of heat in converting hot water to steam is very much more than the heat absorbed in heating cold water to the boiling temperature.  Hot water readily converts to steam absorbing enormous amount of heat and therefore hot water is better for fire extinguishing purpose than cold water.

It is true that cold water absorbs more heat than hot water (say about 1% more than boiling water, but it takes more time than the cooling effect produced by  vaporisation of hot water.

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