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Monthly Archives: July 2011

Electrons and flow of electrons – Did you have this doubt before?

“When an electric cell is connected to a circuit, electrons flow away from the negative terminal in the circuit. But within the cell, electrons flow to the negative terminal. Explain?

Answer: The question seems to be the result of some misconceptions. If we consider an electrochemical cell, it is the chemical reactions which develops and maintains the potential difference between the terminals of the cell. When externally connected, the electrons flow from the negative terminal (at lower (+) potential, to higher(+) potential). When the electrons start moving, the potential difference tends to decrease and therefore chemical reaction starts/speeds up to keep the potential difference constant.

But as the movable charges inside an electrolyte are ions, they cannot move as freely as the electrons in a metal. The opposition to the movement of ions results in a resistance inside the cell itself and is called internal resistance.

How to tackle problems with constraint relations?

“Can you suggest a way to tackle problems on constraint relations easily??” – Aritra posted this question.

Answer: The common way to solve problems involving constraints is to replace the constraints with their reaction forces. As the question is not specific, I cannot tell much now. If you have further doubts regarding this issue, please post as comments to this post.

What is the mechanical energy of a bird on top of a moving Ship?

A bird is at rest on d top of a stick of moving ship….!! what is the mechanical energy that exists in the bird ???” – This question was posted by Fatima

Answer: The mechanical energy of the bird is both Potential and Kinetic as the bird is at a height with respect to ground and the bird is in motion along with the ship.

(Any deeper discussion is welcome)

Change in resistance due to stretching a wire

“There is a wire whose resistance is R. it is streched, and its length gets increased to 2%. find new resistance.” – Knishka Agrawal asked

Answer: The resistivity does not change due to stretching, but the length and area of cross section changes; but the volume remains constant. Therefore, resistance changes which can be calculated from the formula for Resistance

What are Tachyons?

What are Tachyons?

This question was posted by Vimal Raj.

Tachyons are hypothetical particles which always travel faster than light.

The idea is that, if there is a particle which travels with a speed less than the speed of light, it will never cross the speed of light. But if there are some partricles which originally travel faster than light, it will never come to a speed less than that of light. Such hypiothetical particles are called tachyons.
They are not discovered.They may or may not exist. It’s just a matter of logic.

Super Hero crossing Mach1 speed and then?!!

Charles posted this interesting Question on a Super Hero crossing the Mach 1 Speed. Visitors are requested to respond with your ideas.

“I’m working on a novel with a “superhero” protagonist.  The guy puts on his spandex, and takes off across state, say. 🙂
OK, as he speeds up, can he tell when he approaches and passes Mach one?  I mean, do you think such a character would feel  vibrations as he nears the speed of sound, and can slow so he doesn’t boom people below?  I read where a condensation cloud appears around jets as they pass Mach 1.”

Answer: When an object crosses the speed of sound (Mach 1) boom is heard if he is traveling through air or another medium. No boom will be heard in vacuum no matter how fast he is traveling. (Sound requires a medium for propagation as it is a mechanical wave). When you write a novel, you can just assume special abilities and capabilities to the super hero. (That is why he is super)

Radius of an electron and Distance between electric field lines.

“What is the distance between two lines of force produced by a charge or the distance is zero between them and what is the diameter of electron?” – Ela posted.

The electric field lines are imaginary lines used to represent the electric field visually. The electric field lines are imagined or drawn relatively closer where there is greater electric field. (Though electric flux is often defined as the number of field lines per unit area, it is not so. Electric flux cannot be talked in terms of just numbers. That definition is just to make  a quick idea related to the lines in a given situation. To cross check, just calculate the electric flux from a proton and try to draw that much number of lines around a point)

The radius of electron has been calculated from Mass Energy relation as 2.817 940 3267 x 10-15m

Therefore, Diameter of an electron = 5.635880578916 x 10-15m

Please note that the above value is just a theoretical estimation. The actual size of electron is not experimentally determined.