When I was explaining static electricity to 8th-standard students, as an introduction I explained how charges and their attracting and repelling properties are responsible for the lovely attraction of tiny papers on a comb that is charged by rubbing against your head.
Suddenly a question from a curious mind arose "Sir, are these charges causing the magnetic behaviour of electromagnets? How do they only become magnetic when electricity is applied?"
The answer lies in Einstein's special theory of relativity. Yes, the theory explains the time dilation length contraction. But we know if these particles are reaching the velocity of light i.e, around 3 lakhs km per second. For example, If you are waiting at a bus stop and watching a bus of length 15m traveling at a velocity of 2 lakhs 25 thousand km per second then its length will appear to be 10 m for you. But someone who is sitting inside the bus doesn't feel that except that person will feel you became very thin. This is known as length contraction.
The time dilation is yet another effect of the special theory of relativity. When compared to a stationary clock, someone moving at a speed close to the speed of light will experience a slower ticking of the timepiece. So we are really safe that we can't approach the velocity of light. Hence, we experience neither length contractions nor time dilations in our daily life.
Ok! So how does it relate to the magnetism in a conductor? Absolutely, everyone is aware of that. Electrons flow when a conductor is subjected to electrical potential, although their speed is very low. That is comparable to the speed of ants. Yet, even for that low a velocity, the distance between electrons gets contracted. Yes, that length contraction is very very small but the electric attraction between electron and proton is 10^39 times greater than the gravitational attraction between them. Hence, this relative motion may have large consequences. Also, there are so many electrons so the electron density matters a lot in other words total effect is considerable. (see the figure)
So more number density of electrons will be higher than that of ions in a given area leading to extra charge distribution. This will provide an electric field around the conductor. that can attract a positive charge outside the conductor.
This phenomenon is our magnetism. How amazing is this!!
The most interesting thing is that when anyone learns the theory of relativity it seems that we are learning why we are traveling at a speed of 60 km/h. but in our daily life, we often use the theory of relativity without even realizing it.
Figure courtesy Wikipedia
-Renji
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