How is political geography different from political science?

Political
geography is different from political science in that it is a much more limited area of study. 
Political geography, like political science, is concerned with the study of politics.  However,
political geography mainly looks at the connections between politics and physical space. 
Because of this, it does not pay attention to many of the issues that political scientists look
at.

Political geography is defined in the link below as the study of the
spaces of politics and the politics of space.  In a human geography book I have (Human
Geography: People, Place, and Culture 9th Edition by Erin H. Fouberg, et. al.) we are
told (p. 240) that political geographers study the spatial manifestations of political
processes.  In other words, political geography is only concerned with politics as it relates to
space.  Political geography studies how states are created and how they come to control a given
area of land.  Political geography studies why certain political systems are distributed in
given ways across the face of the Earth.  Political geography studies how countries divide up
political power between their various regions.  All of these things have to do with politics and
space.

By contrast, political science goes far beyond the study of politics
as it relates to space.  Political scientists study all aspects of politics.  For example, a
political science department will offer courses in political philosophy.  It will offer courses
in how public opinion is formed.  Political scientists will study the ways in which interest
groups influence members of Congress or the ways in which social movements behave in their
efforts to achieve their goals.  All of these are important aspects of politics, but they have
very little to do with politics and physical space.  This means that they are part of the study
of political science but not part of the study of political geography.

In
short, political geography is a much more limited area of study because it studies only those
aspects of politics that are in some way related to space.  Political science is a field that
studies all aspects of politics.