Btukfyl

What are the possible alternatives to move within an environment, the second equation all on the left?
Here there is my MWE:

documentclass[a4paper,12pt]{article}

usepackage{mathtools}



begin{document}

begin{align*}

F_{x}+qleft[frac{partial A_{x}}{partial t}+frac{partial A_{x}}{partial x}frac{dx}{dt}+frac{partial A_{x}}{partial y}frac{dy}{dt}+frac{partial A_{x}}{partial z}frac{dz}{dt}right]= -qfrac{partialvarphi}{partial x}+q u_{x}frac{partial A_{x}}{partial x}+q u_{y}frac{partial A_{y}}{partial x}+q u_{z}frac{partial A_{z}}{partial x}& notag\

intertext{cioe'}

F_{x}=qleft{ -left[frac{partialvarphi}{partial x}+frac{partial A_{x}}{partial t}right]+u_{y}left[frac{partial A_{y}}{partial x}-frac{partial A_{x}}{partial y}right]-u_{z}left[frac{partial A_{x}}{partial z}-frac{partial A_{z}}{partial x}right]right}&.notag\

end{align*}

end{document}

I would place both & alignment points at the very beginning of each row.

Two additional remarks: (a) Placing the & symbols at the very end of each row is equivalent to not providing any alignments points at all which, in turn, means that that the equations will be aligned at the right-hand ends. (b) The notag directives do nothing but create code clutter when used inside an align* environment.

documentclass[a4paper,12pt]{article}

usepackage[italian]{babel}

usepackage[margin=2.5cm]{geometry} % choose suitable page size parameters

usepackage{mathtools}



begin{document}

begin{align*}

&F_{x}+qleft[frac{partial A_{x}}{partial t}

  +frac{partial A_{x}}{partial x}frac{dx}{dt}

  +frac{partial A_{x}}{partial y}frac{dy}{dt}

  +frac{partial A_{x}}{partial z}frac{dz}{dt}right]= 

  -qfrac{partialvarphi}{partial x}

  +q u_{x}frac{partial A_{x}}{partial x}

  +q u_{y}frac{partial A_{y}}{partial x}

  +q u_{z}frac{partial A_{z}}{partial x}\

intertext{cio`e}

&F_{x}=qleft{ -left[frac{partialvarphi}{partial x}

  +frac{partial A_{x}}{partial t}right]

  +u_{y}left[frac{partial A_{y}}{partial x}

  -frac{partial A_{x}}{partial y}right]

  -u_{z}left[frac{partial A_{x}}{partial z}

  -frac{partial A_{z}}{partial x}right]right}.

end{align*}

end{document}

There's no reason for using align without aligning things.

I'd go with a standard

documentclass[a4paper,12pt]{article}

usepackage{mathtools}



begin{document}



begin{equation*}

F_{x}+qleft[frac{partial A_{x}}{partial t}+frac{partial A_{x}}{partial x}frac{dx}{dt}+frac{partial A_{x}}{partial y}frac{dy}{dt}+frac{partial A_{x}}{partial z}frac{dz}{dt}right]= -qfrac{partialvarphi}{partial x}+q u_{x}frac{partial A_{x}}{partial x}+q u_{y}frac{partial A_{y}}{partial x}+q u_{z}frac{partial A_{z}}{partial x}

end{equation*}

cioè

begin{equation*}

F_{x}=qleft{ -left[frac{partialvarphi}{partial x}+frac{partial A_{x}}{partial t}right]+u_{y}left[frac{partial A_{y}}{partial x}-frac{partial A_{x}}{partial y}right]-u_{z}left[frac{partial A_{x}}{partial z}-frac{partial A_{z}}{partial x}right]right}.

end{equation*}



end{document}

If you want to align both F_x, the way is to fix the alignment point at them:

documentclass[a4paper,12pt]{article}

usepackage{mathtools}



begin{document}



begin{align*}

&F_{x}+qleft[frac{partial A_{x}}{partial t}+frac{partial A_{x}}{partial x}frac{dx}{dt}+frac{partial A_{x}}{partial y}frac{dy}{dt}+frac{partial A_{x}}{partial z}frac{dz}{dt}right]= -qfrac{partialvarphi}{partial x}+q u_{x}frac{partial A_{x}}{partial x}+q u_{y}frac{partial A_{y}}{partial x}+q u_{z}frac{partial A_{z}}{partial x}\

intertext{cioè}

&F_{x}=qleft{ -left[frac{partialvarphi}{partial x}+frac{partial A_{x}}{partial t}right]+u_{y}left[frac{partial A_{y}}{partial x}-frac{partial A_{x}}{partial y}right]-u_{z}left[frac{partial A_{x}}{partial z}-frac{partial A_{z}}{partial x}right]right}.

end{align*}



end{document}

Note that notag is superfluous in align* that does not number equations in the first place. Also remember that there must not be a trailing \, which would cause unwanted vertical space (and a spurious equation number with align).

Here's a polished version, with a macro instead of all those partial commands, and the two options.

documentclass[a4paper]{article}

usepackage{mathtools}



newcommand{der}[2]{%

  frac{d#1}{d#2}%

}

newcommand{pder}[2]{%

  frac{partial #1}{partial #2}%

}



begin{document}



section{texttt{align}}



begin{align*}

&F_{x}+qleft[pder{A_{x}}{t}+pder{A_{x}}{x}der{x}{t}

      +pder{A_{x}}{y}frac{dy}{dt}+pder{A_{x}}{z}der{z}{t}right]

     =-qpder{varphi}{x}+q u_{x}pder{A_{x}}{x}+q u_{y}pder{A_{y}}{x}

      +q u_{z}pder{A_{z}}{x}

intertext{cioè}

&F_{x}=

  qleft{

    -left[pder{varphi}{x}+pder{A_{x}}{t}right]

    +u_{y}left[pder{A_{y}}{x}-pder{A_{x}}{y}right]

    -u_{z}left[pder{A_{x}}{z}-pder{A_{z}}{x}right]

  right}.

end{align*}



section{texttt{equation}}



begin{equation*}

F_{x}+qleft[pder{A_{x}}{t}+pder{A_{x}}{x}frac{dx}{dt}

     +pder{A_{x}}{y}frac{dy}{dt}+pder{A_{x}}{z}frac{dz}{dt}right]

    =-qpder{varphi}{x}+q u_{x}pder{A_{x}}{x}+q u_{y}pder{A_{y}}{x}

     +q u_{z}pder{A_{z}}{x}

end{equation*}

cioè

begin{equation*}

F_{x}=

  qleft{

    -left[pder{varphi}{x}+pder{A_{x}}{t}right]

    +u_{y}left[pder{A_{y}}{x}-pder{A_{x}}{y}right]

    -u_{z}left[pder{A_{x}}{z}-pder{A_{z}}{x}right]

  right}.

end{equation*}



end{document}