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Sunday, April 29, 2012

What is Loss of coolant accident in a Nuclear Reactor?

A loss of coolant accident (LOCA) is a mode of failure for a nuclear reactor ; if not managed effectively, the results of a LOCA could result in reactor core damage.

Every Nuclear power plant's Emergency Core Cooling System(ECCS) exists specifically to deal with LOCA.

When coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown is designed to stop fission chain reaction. However due to radioactive decay the nuclear fuel will continue generate a significant amount of heat.

The decay heat produced by reactor shutdown from full power is initially equal to about 5% to 6% of thermal rating of reactor.

If all independent cooling trains of ECCS fail to operate as designed, this heat can increase the fuel temperature to the point of damaging the reactor.  

Sunday, April 22, 2012

How is blood pressure measured?

A person's arterial blood pressures are usually measured with the help of an inflatable cuff wrapped around the upper arm at the level of heart. A stethoscope is used to monitor the sound of the blood flowing through an artery below the cuff. The cuff is first inflated until the flow of blood stops then the pressure of the cuff is gradually reduced until the blood just begins to flow, which is recognized by a gurgling sound in the stethoscope. This pressure, called systolic, represents the maximum pressure the heart produces in the artery. The pressure in the cuff is then further reduced until the gurgling stops, which corresponds to the restoration of normal blood flow. The pressure at this time, called diastolic, represents the pressure in the artery between the contractions of the heart.   

Physicians express blood pressures in torr, where 1 torr is the pressure exerted by a column of mercury 1 mm high; it is equivalent to 133Pa. The torr was formerly referred to as the "millimeter of mercury", abbreviated mm Hg. The unit is named after Evangelista Toricelli (1608-1647), the Italian Physicist who invented the barometer, which measures atmospheric pressure. Average atmospheric pressure is 760torr. In a healthy person the systolic and diastolic blood pressures are, respectively, about 120 and 80torr.   
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Saturday, April 21, 2012

How does particular metal crystallize to particular structure?

The reason for a particular metal to crystallize in a particular structure must be sought in the fact that the free energy E-TS for this system is lower than that for any other structure.

The same remark may be made with reference to those metals which have different structures in different temperature regions(Allotropy). This phenomenon is exhibited especially by three and four valent metals and by the transition metals.

For instance, Alpha Iron(BCC) is stable upto 910 deg.centigrade; between 910  and 1400 degree Centigrade the stable structure is gamma Iron(FCC); between 1400 and the melting point (1530) the structure is again BCC( Delta iron).

The transformation from one structure to another is dictated by the requirement of minimum free energy. This does not mean that such transformations takes place as soon as the existing structure becomes unstable.

In fact, a transformation of structure involves a rearrangement of atoms, and such a process may take a long time. The reason is that even though the free energy after the transformation is lower than in initial state, the two states are usually separated by an energy barrier or activation energy.

Thermodynamics specifies only the equilibrium condition but doesn't give any information about the velocity of reaction or processes involved in establishing equilibrium. From the atomic point of view, the stability of crystal structures is a problem of cohesive energy, involving the interaction between the atoms.      

Difference between "Mass" "Matter" and "Energy"

Well the fancy of the subject "physics" is unexplainable. Understanding of basic terminology gives immense satisfaction. 

How many of you could differentiate the three terms "Matter", "Mass" & "Energy" which are essential for beginning understanding of physics? 

Let us start explaining things:- "Mass" is an attribute and not a physical thing. Mass is the attribute "inertia" of a physical object or of what ever is contained in a specified region of space. 

"Inertia" is the inherent reluctance to undergo a change in velocity. If one consults the research documents of decade 1900-10, when Einstein and others developed the equation E=mc2, one finds that for those physicists- the word "Mass" was synonym for "inertia". 

"Matter" is tangible stuff; What you can touch and sense with your hands and so a physical thing.Hence all matter has inertia and hence has mass. 

 "Energy" is the ability to do work. It is an attribute to physical object or of whatever is contained in a specified region of space. Three general forms for existence of energy are "Kinetic", "potential", "Radiant". "Kinetic" is the energy associated with motion. 
"Potential energy" is the energy that has potential for being converted to kinetic energy. 
"Radiant energy" is energy of Electromagnetic waves. 

Take a stone and throw into air. The stone which you are able to collect it from ground has got matter as you have touched and sensed it. While moving in air the stone doesn't get blown by wind breeze etc and does possess "inertia". It is moving with a velocity and does has Kinetic energy, at the same time it is at a varying height from ground and hence possess gravitational potential energy.Thus the stone in air has got partly kinetic energy and partly potential energy. Thus "mass' and "energy" exist as attributes of a physical system, just as do color or shape. Mass and energy exist in the same fashion and matter exists in different fashion. Hope this benefits the student community.......Have a nice day

How do we define Glass?

There are number of definitions of glass which have different approaches. 

There is no universally accepted or universal definition of glass. 

It is a fusion product of materials which have been cooled to a rigid condition with out crystallization(It need not be inorganic). 

Examples of familiar glasses made from cane sugar are lollipops and cotton candy. The former are in shape of rigid block and latter are flexible fibers.

Other definitions: 

1) An Amorphous solid or glass is one in which long range order is absent and the array of equilibrium atomic positions is strongly disordered. 

2) Glass is an uncrystallized solid material. 

3) Glass is an inorganic product of fusion which has been cooled to a rigid condition with out crystallizing. 

4) A general definition including all the aspects of glass is not possible; glass is to be conceived on one hand as a physical chemical condition and on other hand as a technical material.

Thursday, April 19, 2012

Why to use Fuse?

Any electrical or electronic appliance is designed to take specified current. Under some circumstances, there may be drawing of larger currents by then then what they are rated for.

For instance too many appliances connected simultaneously in a circuit causing the total current drawn to be higher than can be handled by wires. This is called overloading.

The higher current due to short circuits or overloading can damage the appliances and the electric wires may even catch fire. To avoid this risk, a safety device called "Fuse" is used in series with the circuit.

A fuse is a short piece of wire of low melting point which is connected in series between Mains and the Appliance. When the current rises above the safe limit the fuse wire gets melted and makes the circuit open.
Design aspects of fuse like: Material of  wire, Dimensions etc are selected based on the safe limiting of current required.


Electric power & Energy

Electric Power

Electric power is the electric work done per unit time.

1 Watt = 1 Joule / 1 Second

Though Watt is unit of power, it is too small for measuring domestic consumption of electricity. Thus we use a larger unit of power called Kilowatt.

Electrical Energy

It is usually measured in a unit called Kilowatt hour(KWh) which is amount of electrical energy consumed when an electrical appliance with a power rating of 1 KiloWatt is used for 1 hour.

1 KWh= 3.6 x 10 ^ 6 Joules

Power Rating of Domestic Appliances

The power rating of an electrical appliance tells us the rate at which the appliance uses electrical energy.
An electric iron box marked 220V, 1000W has a power consumption of 1000W at 220V AC, i.e it consumes electrical energy at the rate of 1000Joules per second when a potential difference of 220V is applied across it (which is potential difference at which electricity is supplied to our homes).  

The electrical energy consumed (in KWh) by an electrical appliance is given by the product of its power rating (in KW) and the time for which it is used(in hrs).

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