# Q & A: origin of magnetism

Q:
This is in relation to following question:"Why do moving charges produce magnetic field?" which has been answered... But I have certain doubts to clarify. But before that I need to tell ya that for most Indian students, the basic of electrostatics is not done at age 16 and rativity is not even mentioned even at 12 standard(WHAT??- I KNOW!)... So first, in the second paragraph why is there a net current to the left..... (And I should have asked what a neutral wire is first)... And is is it the wire that consists of the two opposing infinite lines of charges? BTW doesn't the opposing charges cancel out such that there is no net current cause it's said that the opposing -ve charges are equal? And finally, the question ive been wanting to ask is this. Does a moving charge produce a magnetic field? Yes. But is the underlying cause of the magnetic field, the electric field that changes to the moving charge? Or does it have anything to do at all with the electric field of the moving charge? Consider an isolated positive charge initially at origin. It sets up an electric field in the surrounding. Let's consider a point C on the surrounding such that the field is constant as long as the charge is stationary. Now say the charge is made to move with a constant velocity in the x direction.(by some external means such that the field is not disturbed by the external force). The electric field at C must change.. does this cause a magnetic field to be set up? AND I DON'T REALLY UNDERSTAND HOW TO FIND ANSWERS TI MY QUESTIONS... SHOULD I JUST TYPE MY QUESTION IN THE SEARCH BOX OR WHATT? HELP ME....
- R Lucas (age 19)
Shillong, Meghalaya, India
A:

It's hard to follow the details of your question without pictures, but I can make some helpful suggestions.

A great book to learn about this is Electricity and Magnetism by E. Purcell. (I was very fortunate to take a course based on that book from its author my first semester in college.)

Your intuition that somehow it's the fields that transform between electric (E) and magnetic (B) is completely right. If you know (E,B​) in one frame you know it in all other frames using relativistic transform rules. The stories we tell about current in wires, sheets of charge, etc. are just very useful ways of helping us make sense of the rules, and Purcell pprovided such stories.

Mike W.

p.s. Here's another one of our threads on this https://van.physics.illinois.edu/qa/listing.php?id=2358
https://van.physics.illinois.edu/qa/listing.php?id=2358

(published on 01/07/2019)