Home » Nuclear Physics

Category Archives: Nuclear Physics

Principle of Working of Microwave Oven

Manisha Chowdhury asked:

Why is it called a microwave oven?
Who invented it?
What is the principle of its working?

Answer:

A microwave oven is used to cook (or heat) food with the help of microwaves produced by magnetron – the device producing microwaves in the oven. Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food.

Who invented Microwave Oven?

Percy-spencerPercy Spencer is generally credited with inventing the modern microwave oven after World War II from radar technology developed during the war. Named the “Radarange”, it was first sold in 1946. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, and their use has spread into commercial and residential kitchens around the world.

Working of microwave oven

microwave ovenA microwave oven, commonly referred to as a microwave, is a kitchen appliance that heats and cooks food by exposing it to electromagnetic radiation in the microwave spectrum. This induces polar molecules in the food to rotate and produce thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer25–38 mm (1–1.5 inches) of a homogenous (high water content) food item; food is more evenly heated throughout (except in heterogeneous, dense objects) than generally occurs in other cooking techniques.

Magnetron2A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency higher than ordinary radio waves but lower than infrared light. Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are reserved for this use, so they don’t interfere with other vital radio services. Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in)—while large industrial/commercial ovens often use 915 megahertz (MHz)—32.8 centimetres (12.9 in). Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e. as both potential energy and kinetic energy of atoms), is heat.

For more details refer to :

 

expression for magnetic moment of the electron

An electron is moving with velocity v in an atom with orbital radius R.  Derive the expression for magnetic moment of the electron.

Posted by Sankhadip Das

Answer

 

Nuclear Fission and Fusion – A Facebook conversation

Find below a conversation between Master Deepak, PUC student and Mr. Mathew Abraham, Physics teacher.

Deepak asked: “nuclear fusion or fission in which of these energy released is more and why?”

 

Energy released per fission is more than energy released per fusion. But energy released per unit mass during fusion is very much greater than energy released per unit mass during fission

 

But why energy released is more

 

The fission of a U-235 releases about 200 MeV of energy
whereas the fusion of 4H to form He releases only about  20 MeV
But, the 200 MeV is obtained from 236 amu
and the 20 MeV is obtained from just 4 amu

 

Then why dont we use nuclear fusion as source of energy and electricity

 

10:38am

Nuclear fusion requires very high temperature to start. It is difficult to maintain such a huge temperature (of the order of million degrees).

Tokamak magnetic field and plasma current.

Tokamak magnetic field and plasma current. (Photo credit: Wikipedia)

Research is going on to construct nuclear fusion reactor

The international prototype is named tokamak
In that the nuclear fusion is carried out in plasma state held at high temperature confined in magnetic field produced by huge torroidal solenoids

 

A question from Atoms & Nucleus Decay

Plutonium decays with a half-life of 24000 years.If plutonium is stored for 72000 years.What fraction of it remains?

Asked Vikas

Usage of energy in nuclear fission and fusion

I read in a biographic book about Albert Einstein that the energy produced from burning materials is much less than the energy stored in them (the authored suggested the book itself as an example – he wrote that it can power a ship for about 100 years).
I would like to know how much of the energy of a material is used in nuclear fission and fusion.

Asked Avishag

Answer:

Nuclear explosion

Nuclear Explosion (Source: WIkiPedia)

Energy obtained by burning a substance is meagre compared to the energy contained in it. According to Einstein’s mass energy relation (E=mc2), the energy released by converting 1 gram of any substance completely into energy = 0.001 (mass) x 300,000,000 x 3,00,000,000 (square of velocity of light).

But, in nuclear reactions, the entire mass is not converted into energy. The tremendous energy liberated during a nuclear explosion is the result of a small portion of the mass of nucleus undergoing the reaction getting converted into energy.

For example, when a uranium 235 atom undergoes nuclear fission the enrgy liberated is 200 MeV = 200 x 1.6 x 10 -19 J

In 235 g of U-235 there are 6 x 1023 atoms.

So if that much of U-235 atoms undergo fission, the energy liberated will be about 1,00,00,000 J.

Hope that you might have had an idea of it.

Thoughts on Higgs Boson

When it is not contested scientifically that change is a continuous phenomenon,how is that Higgs boson, if found,can be considered as the ultimate building block of matter.

Sure, it may prove a mathematically arrived conclusion… yet the contradiction remains.
An ever changing, at the same time, ever stable (because it is the ultimate one) is the contradiction.
One may find changing or divisible particle only…

(Posted by K.C. Muralidhar)

Please post your thoughts and reactions as comments to this post

Is it possible to stop a nuclear reaction once started?

Is it possible to stop a nuclear reaction,after starting it ( I’m not only talking about reactor, but in nuclear bombs too ). If it’s possible then which laws will fail or If It is impossible then Why? …………. have any country have prepared about It? (Asks George)

Ask a Doubt

Post your Physics Doubts here

Name (required)

Email (required)

Subject

Message/Doubt/Question

Upload a file if required

Archives

askphysics@Twitter