Home » 2011 » September » 15

Daily Archives: September 15, 2011


Electricfield, Magnetic Field and Photon

We know that photon has no charge. Then where from it gets electric and magnetic field in light?

Malik Sajad asks


Electric and magnetic fields are not produced by the photon; it is massless too.

But, photon is created by time varying electric and magnetic fields, usually produced by accelerated charges.

As you might have already learnt, a charge at rest produces an electric field around it. A moving charge (current) produces a magnetic field around it as well as magnetic field. A varying current produces a varying magnetic field and hence an induce emf or current. An accelerated charge produces a time varying electric and magnetic fields.

Thus under suitable situations, the electric and magnetic fields can reinforce each other and propagate out as pure electromagnetic form of energy and such a packet of energy is called photon.

What is twin paradox?

TWIN PARADOX is a paradoxical situation coming in while explaining the theory of relativity. “Time dilation” is one of the important consequences of relativity. It says that if a body is moving with relativistic speeds, time will dialate for it. That is, a duration of 1 second for him will be more than that for a person at rest or moving at ordinary speeds.

The relativistic time is given by the equation,

timedilation.gif (113×68)

Twin Paradox - Relativity

So, if we imagine twin brothers, one at rest at earth and other going on a relativistic interstellar trip (certainly he has to go out of solar system as the velocity of light is very much more than escape velocity of solar system, and further, he will reach the sun in just 8 minutes if traveled with the speed of light)

Let us take that his speed is 0.9 times the speed of light in vacuum. Then 1 second for him is equal to 27 s (approximately) for his brother on earth. So, if he returns after one year of space programme, he will be older by 1 year, but his brother who was at home will be older by 27 years or more; if alive.

This is the TWIN Paradox.


You are welcome to think of the possibilities of a relativistic travel and the related issues here.

Why direct currents aren’t used in Home circuits?

How can an alternating current change its direction while moving in a wire or any conductor? Why direct currents aren’t used in Home circuits?



Alternating current is produced by rotating a coil in a strong magnetic field (or by rotating magnet around the coil)

So, during every half rotation the direction of emf (voltage) induced in the coil reverses and hence the current too reverses its direction. The frequency of ac in India is 50 Hz and therefore in every 1 sec there will be  100 changes in direction.

Alternating currents are suitable for long distance as it can be stepped up using transformer to minimize energy loss and can be stepped down for use at home.


%d bloggers like this: