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# Monthly Archives: November 2011

## why do electrostatics experiments not work well on humid days?

Humid air contains water droplets and make it conducting. Humid air defects static electricity experiments in two ways,

(1) It reduces friction

(2) It carries away the charges on rubbed bodies.

## How to walk through walls?

Illusion optics create an invisible gateway

Chinese researchers have devised an invisible gateway. Read more at http://physicsworld.com/cws/article/news/44576

## What accounts for the acceleration or deceleration of light when it travels from one medium to another?

“When light passes through different mediums speed changes but who provide acceleration to change velocity?”

This question was posted by Jishnu Answer: Just imagine you are participating in a cross country race. When the road is good you can travel faster; but if the path is a marshy place, your speed will decrease. This is just an analogy. To understand it better, you should know that the way light travels in a medium is different from the way it travels in vacuum.

## Momentum and Kinetic Energy

Two bodies one light and other heavy have equal momentum.Which of them has higher kinetic energy?

The lighter body will have greater kinetic energy.

Explanatiom

Momentum, p=mv

squaring both sides, p2 = m2v2

Kinetic Energy = 1/2 mv2 = p2/2m

Therefore, for bodies of equal momenta, kinetic energy is inversely proportional to mass.

## What is a tensor?

Stress is tensor,then,what is meant by tensor? (Rupasri asked)

Tensor is a mathematical object analogous to but more general than a vector, represented by an array of components that are functions of the coordinates of a space.

Tensors are used to represent correspondences between sets of geometrical vectors.

Refer to the following links for a more detailed analysis.

http://www.grc.nasa.gov/WWW/k-12/Numbers/Math/documents/Tensors_TM2002211716.pdf

Physically, vectors are used to represent locations, velocities, accelerations, flux densities, field
quantities, etc.

## magnetic force

Is magnetic force a kind of energy?if yes then why is not used to produce electricy continuously from a magnetsince attraction is continuous?

## HYSTERISIS

What is hysteresis? I only know its related to the frequency.

Ans:

Hysterisis mens “Lagging Behind”

Hysteresis phenomena occur in magnetic materials, (as well as in the elastic, electric, and magnetic behavior of materials,) in which a lag occurs between the application and the removal of a field (or force) and its subsequent effect

The lack of retraceability of the magnetization curve is the property called hysteresis

#### Magnetic hysteresis When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained: the material has become magnetized.

The relationship between magnetic field strength (H)(EXTERNAL) and magnetic flux density (B) (ACQUIRED) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.

If the magnetic field is now reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence (RETENTIVITY).

If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. (See the diagram) The ‘thickness’ of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material.

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