what is the difference between public affairs and public relations especially in a government department

Robert Lawrence heath define public affairs as the management function responsible for interpreting an organisation's external environment or in a case of a corporation its non commercial environment and managing an effective and appropriate response to that environment.

this was question was brought out when i was conducting a research on government and public relations .and then i came to realize that most people especially in the government refer to public activities as public affairs .and i also got to figure out that public affairs only comes where there are issues of politics.then the question is ,is public affairs part of PR or they are totally different by from robert's definition it does not seem that different...

then i wonder was i in a public affairs department of pubic relations department .