- #1

zhouhao

- 35

- 0

## Homework Statement

I think the in equation ##(28.2)##,##x^i## in ##\frac{dx^i}{dt}## and the ##x^i## decides ##\rho## is not the same,if they are equaivalent,##\rho## can not vary with position changing and time fixed, because ##\frac{dx^i}{dt}## indicate the ##x^i(t)## which means if position changed the time would also be changed.

##\frac{dx^i}{dt}## could define the velocity through a points' trajectory,##\rho## is not point charge,what's the meaning of the "velocity"?

## Homework Equations

The four electricity vector is defined as ##j^i=\rho\frac{dx^i}{dt}##,##\rho## depend on ##(t,x^1,x^2,x^3)##

## The Attempt at a Solution

I want to find some clues about the "velocity" in contents after the definition and only find LLs(Landau and Lifshitz) regarding the ##x^i## in ##\frac{dx^i}{dt}## as the same as the ##x^i## which decides ##\rho##-------In equation ##(28.5)## ##dS^i## was gotten through dividing ##dV## with ##dx^i##.