Append div to end of document with jQuery?

I would like to write a function using jQuery to append a div to the end of a web page. I would like to be able to use the function on many different pages.

I've written the following code, but it doesn't work:

    $(document).append('<div id="helloDiv"></div>');
    $('#helloDiv').html('hello'); // does nothing
    $('#helloDiv').css('top','100') // throws an error

How do I fix this?