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# Monthly Archives: March 2014

## JEE 2014 IS NEAR … How to do the last minute preparation?

The Engineering aspirants of India are now towards the last round of preparation JEE 2014. The paper pen phase of the 2014 edition of the JEE-Mains is scheduled for April 6 and the online examinations will be conducted on April 9, 11, 12 and 19.

The duration of the examination is three hours and will test the candidates on physics, chemistry and mathematics.

Here we will have a brief discussion on how to go for the last round of preparation in Physics. The visitors are welcomed for participating in the discussion through the comment system. Just post your questions and queries as comments to this post.

In Physics the most important topic is MECHANICS, which includes kinematics, laws of motion, work power energy, rotational motion. Electrostatics, Current electricity, magnetic effects of current, electromagnetic induction and alternating current circuits along with the entire OPTICS are to be taken seriously. If you can make a complete thorough revision during the week ahead, you can certainly score very good marks in Physics.

After that if you get time, revise Bernoulli’s principle, Surface Tension, Elasticity with special attention to Young’s modulus.

These are the portions which need to be understood and applied rather than memorizing.

PLEASE MAKE IT A POINT TO SOLVE ALL THE EXERCISES FROM THE NCERT TEXT BOOK. (Including the additional exercises)

Do not neglect the chapter UNITS AND DIMENSIONS, THE ERROR ANALYSIS ETC.

Practice well, solve maximum number of problems and success will be yours. Have some doubts in Physics? Feel free to discuss them here. The questions can be posted here for discussion and hope to get a suitable answer.

## Single phase, two phase, three phase and multiphase electric generators – which one is more efficient?

I started referring to the above issue when one students asked, “Why do we use three phase generation of power and not two phase or four phase?”

Three-phase has properties that make it very desirable in electric power systems:

The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to reduce the size of the neutral conductor because it carries little to no current; all the phase conductors carry the same current and so can be the same size, for a balanced load.

Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.

Three-phase systems can produce a rotating magnetic field with a specified direction and constant magnitude, which simplifies the design of electric motors.

Refer the following links for more …

http://en.wikipedia.org/wiki/Two-phase_electric_power

http://en.wikipedia.org/wiki/Polyphase_system

http://en.wikipedia.org/wiki/Three-phase_electric_power

## Attraction, Gravitation and some doubts

“In universal law of gravitaion what is  the word “attraction” actually refers to?
I am already on the earth surface and attached to it.Then what is the meaning that earth still attacting me accounts for.
Does that mean earth want me to attarct me towards its center point (ie.., into the inner core of the earth’s interior).

Explain with reasons…”

Yes, Earth continues to attract you until you reach its centre.

Every body attracts all other objects towards its centre, not to its surface!

## Space Time

First, I am a physics ‘hobbyist’; I have only rudimentary (read college classes) in physics and I absolutely recognize that I know very little about the subject except what I have studied and can find online.  However, I have a theory that the flow of time causes space in much the same way that the flow of electricity causes magnetism (the ‘right hand thumb rule’, for example).  This would explain the supposed curvature of space since it would flow in a circular motion nn degrees against the flow of time.  This idea could explain, in a way, the faster-than-light expansion of space nanoseconds after ‘The Big Bang’ since it wasn’t space that exploded but rather time that exploded meaning time ‘flowed faster’ immediately after the big bang causing a massive, near instantaneous expansion of space at that time.  (As you can see, I have far too much ‘time’ on my hands…)  The problem I have – and hope someone can answer – is that I can think of no way to prove or disprove this experimentally; not so much my ancillary thoughts of time and the big bang but rather if time causes space, how can this be disproved or proved experimentally?

## Speed of Light

How can the speed of light be a constant, when time is not?
If the rate of time can change depending of speed of matter or extreme gravity, the speed of light is 186,000 miles per second, but a second could flow at different rates.
If light from a star passes close to a black hole where time flows slower, this changes the speed of light we perceive?

## Entropy

The latest thinking about black holes is that they may not collapse into a singularity at all, but may actually explode back with  a “big bounce” once the diameter of the black hole reaches the Planck Scale or maximally compact state and once the “big bounce” diameter expands to that of the event horizon, it would release all of the information that till then had been trapped inside the black hole, back into the universe.

My question is this, if in fact the “big bang” that led to the creation of our current universe was the result of the “big bounce” associated with the collapse of a prior universe, is there any measure of entropy that could detect this. In other words would any measure of entropy differ between the universe caused by “big bounce” n and the universe caused by “big bounce” n+k?

The question is left for the visitors to answer

## Variation of Resistance with Temperature

Pieces of Cu and Ge are cooled from temperature to 80k what will be the effect ofon their resistance

Copper is a metal and Germanium is a semiconductor.

The resistance of a metal increases with increase in temperature and the resistance of a semiconductor decreases with increase in temperature.

In the question, The temperature is being decreased to 80 K from room temperature. Therefore the resistance of Cu will decrease and that of Ge will increase.

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