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6/6/2015  Anonymous asked What is a vacuum in physics? Does it have a temperature?



Ans.Well, our idea of a vacuum is a bit of space with nothing in it. We don’t know of any examples of a perfect vacuum, but know some bits of space that get pretty close. Space beyond the Earth’s atmosphere isn’t a bad approximation to a vacuum, but it is filled with solar wind particles, light from the sun, cosmic rays and cosmic microwave background radiation. It’s probably also filled with dark matter which doesn’t interact with other stuff (except gravitationally, and possibly only through the feeble weak interaction), as well as neutrinos.

If you manage to pump all the air out of a steel can, for example, you will have a vacuum in there, but there will be photons constantly radiated off of the walls and re-absorbed by them. This soup of photons will be in thermal equilibrium with the walls, and therefore will have a defined "temperature". In fact, even the deepest of deep space (outside the galaxy, for example), is in a radiation bath of temperature 3K, left over from the Big Bang. There may be other stuff, like the neutrinos, for example, which are not in thermal equilibrium with the 3K radiation because they don’t interact with it, and so space may have two or more "temperatures".

But we said a vacuum is a region of space with nothing in it, and that means those photons have to go. Cooling the walls down to as close to absolute zero as you can get (and the limit here is that photons of energies that would be radiated by a wall of a cold temperature would have wavelengths longer than the size of the can -- that’ll let you freeze out all of the photons) will give you a vacuum. You have to also shield it from outside sources of energy. There’s little you can do about the neutrinos and dark matter -- they penetrate ordinary matter, but also don’t really interact with it so to a good approximation you can neglect them.

1/8/2015 14:4:56 Nadine Shampine asked "My husband went to a house that had been froze up since the middle of December. Every thing in this house was froze. Pipes and toilet and any thing that could freeze was froze. Sitting on the table was an unopened bottle of water which was still in liquid form .How can water in pipes and in the toilet freeze and not this bottle of unopened water?"

Ans.In order to freeze into ice, the water has to form an ordered crystal lattice structure, which it ordinarily does not do unless it has something to freeze "around," often called a nucleus or seed crystal. Ordinary water has lots of impurities in it like dust around which it can freeze, so the water in your ice trays has no problem. However, an unopened (which is the key quality, here) bottle of water contains little to no particles that are large enough around which the water can form a crystal (the bottled water was probably filtered to make it this pure). In fact, this is the entire basis around the science of cloud seeding, the process of "inspiring" clouds to rain by pumping dust into them. When clouds contain high amounts of supercooled water, we can use dry ice (silver iodide is commonly used as well) to "seed" the clouds with crystals in order for the water to cling to them, and then fall as snow, then melt again into rain.

There are some really cool videos on youtube demonstrating this supercooling effect, like this one:
http://www.youtube.com/watch?v=DpiUZI_3o8s

3/15/2014 11:25:41  Asad: Give difference between inertial and non inertial frames of references?


1/28/2014 14:14:56 kc trivedi asked How in common collector BJT configuration voltage gain is less than unity ?
Ans.The most striking characteristic of this configuration is that the input signal source must carry the full emitter current of the transistor, as indicated by the heavy arrows in the first illustration. As we know, the emitter current is greater than any other current in the transistor, being the sum of base and collector currents. In the last two amplifier configurations, the signal source was connected to the base lead of the transistor, thus handling the least current possible.


Because the input current exceeds all other currents in the circuit, including the output current, the current gain of this amplifier is actually less than 1 (notice how Rload is connected to the collector, thus carrying slightly less current than the signal source). In other words, it attenuates current rather than amplifying it. With common-emitter and common-collector amplifier configurations, the transistor parameter most closely associated with gain was β. In the common-base circuit, we follow another basic transistor parameter: the ratio between collector current and emitter current, which is a fraction always less than 1. This fractional value for any transistor is called the alpha ratio, or α ratio.
1/26/2014 9:08:15 Zulfiqar : asked Commutator is the only difference b/w AC &DC motors or some thing else.pl reply which will be helpful in coming examine and viva in practical.
Ans. Following the the three main differences between AC and DC motor.

1.The most basic difference is the power source. A.C. motors are powered from alternating current (A.C.)
D.C. motors are powered from direct current (D.C.), such as batteries, D.C. power supplies or an AC-to-DC power converter.


2.A.C. induction motors do not use brushes; they are very rugged and have long life expectancies.
D.C wound field motors are constructed with brushes and a commutator, which add to the maintenance, limit the speed and usually reduce the life expectancy of brushed D.C. motors.

3.The final basic difference is speed control. The speed of an A.C. motor is controlled by varying the frequency, which is commonly done with an adjustable frequency drive control.
The speed of a D.C. motor is controlled by varying the armature winding’s current.

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1/22/2014 9:08:15 Zulfiqar : Thank you for reply of question in details.my second question is that how the capacitor is work in electrostatic induction?

Ans.You are most welcome Zulfiqar, here is the link of your second question.
http://talhaphysicsacademy.blogspot.com/2014/01/how-capacitor-is-work-in-electrostatic.html

1/18/2014 12:24:47 Zulfiqar asked In ohmic law the voltage is directly proportional to current while in case of electric transformer the situation is quite reverse voltage is inversely proportion to current.clarification is required.
Ans.Actually, according to Ohm,s Law I= V/R, clearly Current is directly proportional to the Voltage, But according to P=VI or I=P/V, it shows that current is inversely proportional to the Voltage.

It depends on how you increase the voltage if you increase it by keeping the power of the source constant or not,if the power of the source is constant then the current would decrease when voltage increasing ....if you don't care about the power and just simply replace the battery with a new one's with higher power rating this can increase the current.

In Transformer, when voltage increases then current decrease because power remains constant...both side power is P=VI

By Ohm's Law, Current (I) is directly proportional to the Voltage (V) if Resistance (R) and Temperature remain same.
I = V/R.....or...R=V/I.....or......V=IR.

According to P=VI...or...I=P/V....or ...V=P/I,..... It says that Current inversely proportional to the voltage if power remain same.As we know that in Transformer, If power remain same, and voltage increase, then current decreases in Step Up Transformer. also Voltage decreases when current increases as in Step Down Transformer.

1/16/2014 19:31:20 Faiza asked Is there difference between configuration of common base amplifier and common base transistor?
Ans.No both has same characteristics.Common base configuration can be used as many operations like switch and amplifier while amplifier is a circuit which is used to increase the strength of input signal.As far as configuration is concerned both are same.
Common Base transistor


Common base amplifier

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