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how can you find the direction of a magnetic field at a given point in the magnetic line of force?

Fadzilah asks:

“How can you find the direction of a magnetic field at a given point in the magnetic line of force?”

Ans: The direction of the north pole of a magnetic needle on any magnetic line of force gives the direction of magnetic field at that point.

Learn SHM online from Vidhyadrishti

A sample Powerpoint presentation from VidhyaDrishti – a site run by group of IITians for the benefit of IIT aspirants.

Download here.SHM Part 1

Ask Questions for Board Exam too

Do not wait for foolish questions! You can ask questions from Board Exams too

A problem from Transformation of Energy

Rosie asks:

“I can’t figure out how to solve this problem:

The total mass of a motorcycle and rider is 250kg. During braking, they are brought to rest from 16m/s in a time of 10s. What is the maximum amount of energy converted into heat by the brakes?

I don’t know whether to use WD = F x d or another set of formula?”


This question is based on conservation of energy. The heat energy liberated is equal to the change in kinetic energy. Since the bike is brought to rest, the change in KE is equal to its initial KE.

The answer assumes that the entire KE is converted into heat energy alone.
Heat =Change in KE =

Origin of colours and Atomic Spectra

Ramu asked:

“we know about absorption & emission spectra. We get colours; so what is orgin of colurs? why the atoms emit colour spectra”


Light is emitted when an excited atom returns to its ground state. When excited, the electrons jump from lower level to higher level absorbing the energy equal to the difference in energy levels corresponding to the orbits. When these electrons come back to the original states the energy is emitted.

As we know E = hν=hc/λ

Accordingly light of different wavelengths (and hence different colours) are emitted depending on the energy difference between the levels of transition.

For more details, please refer to these links.

  1. http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
  2. http://en.wikipedia.org/wiki/Spectrum
  3. http://www.archive.org/stream/originofspectra00footuoft#page/n5/mode/2up
  4. http://en.wikipedia.org/wiki/Color
  5. http://www.newton.dep.anl.gov/askasci/phy05/phy05192.htm

PowerPoint Based Lessons for Class X Physics






Photoelectric Effect

Puneet Asked:

“In [wiki]photoelectric effect[/wiki] electrons emit out leaving behind positive charge then why [wiki]electron emission[/wiki] not stop after sometime.”


The photoelectric current can continue for long only if the lost electrons are replenished by some means. Unless, the positive potential will go on increasing until a stage is reached when no more electrons can be released with the the particular frequency of incident radiation.

Photoelectric effect takes place in the case of metals which have plenty of free electrons. The electrons lodged off by light could easily be regained as there is not much energy difference from a stray electron and a free electron on the surface of a metal.

Further, in a metal, the free electrons does  not belong to a single atom, each atom is surrounded by a sea of electrons. SO it is not right to say that each electron released creates an ion each – but creates a net positive charge for each electron lost.

(Comments and further explanations from visitors solicited)

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January 2011

Schrodinger’s Cat in Daily Life

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