# Q & A: observer-dependent current

Most recent answer: 06/13/2015
Q:
Consider a place where there is nothing but two point-like charges and you are the observer. If charges are moving then electric current is said to be flowing. In such place where there is no other reference, it cannot be distinguished whether charges are moving or the observer. In that situation, how can we know if electric current is present or not?
- Razin A. Shaikh (age 16)
Navsari, Gujarat, India
A:

As you noticed, the electric current density in a region with charge depends on the choice of reference frame. So there is no absolute invariant answer to the question of whether there's a current present. Once you pick a reference frame, you can get an answer and calculate forces etc. correctly so long as you stick with that reference frame.

Mike W.

(published on 06/13/2015)

## Follow-Up #1: relaativity and motion

Q:
Does it mean that it doesn't matter whether I am moving or the charge and electric current is dependent only on relative motion between observer and the charge ?
- Razin Shaikh (age 16)
Navsari, Gujarat, India
A:

It not only doesn't matter, it doesn't even mean anything to say whether you are moving. Only relative motion has any consequences.

Mike W.

(published on 06/13/2015)