How can I set lateral margins to every paragraph, but not other elements?

Question

I would like to obtain narrow paragraphs within wide pages, so that section titles, figures, tables, may all be wide, and paragraphs may all be narrow.

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\setlength{\parindent}{0pt} % so we can see better what is happening

\newenvironment{narrowpar}% I’d like every paragraph to follow these rules.
{\begin{list}{}{%
\setlength{\listparindent}{\parindent}%
\setlength{\itemindent}{\parindent}%
\setlength{\leftmargin}{2cm}%
\setlength{\rightmargin}{2cm}%
}\item }%
{\end{list}}

\begin{document}    

\section{A section title that ranges across the page}

This is a paragraph (Loret Ipsum So On). And below, an equation that can make use of the whole page width:

\begin{equation}
p v = R T
\end{equation}

Here is another paragraph.

\begin{narrowpar}
This very text looks exactly the way I would like my paragraphs to look. But unfortunately it forces me to insert code manually before and after every paragraph in the source file.
\end{narrowpar}

\end{document}

Said in a different way, I am trying to control lateral space ("padding left and right" in HTML/CSS speak) for all paragraphs, without using page margins. I would appreciate any help...

Answer 1

You can try to redefine paragraph shape with \parshape command:

\parshape 1 .15\hsize.65\hsize

Argument 1: first line, argument pair: .15\hsize.65\hsize left and right margin. All other lines will be indent as the first line.

It works only for the paragraph where the command is present. So you must include \parshape to redefinition of \par, i.e.:

\def\par{\ifhmode\ownpar\fi\endgraf}
\def\ownpar{\parshape 1 .15\hsize.65\hsize}