Learn English – When should I capitalize the word “principal”

When using the word 'department' (or 'group', 'committee', and the like) as part of a recognized name it would be capitalized:

Bill recently joined the Advertising Department.

If using 'department' not as part of a specific name it would be lower case:

We had a party to welcome Bill to the department.

As a further note, you could also drop the 'Department' and re-write the first sentence as:

Bill recently moved to Advertising.

But if using 'advertising' not as the name of a specific department, but rather as a field of expertise, you would write:

Bill works in advertising.

In general text, I would use lower case. In a title, I would use capital. Which do you consider this to be?