[jQuery] add child to <ul> very simple question!

[jQuery] add child to <ul> very simple question!


Ok, this probably just needs someone to slap me towards a tutorial,
however I couldn't seem to find one!
I'm trying to use jquery to add a child to part of an unordered list,
so can anyone explain how to make a new sub-child of the id 'test'
appear? (see below)
SCRIPT:
function populateAmounts(){
    $('#test').append(document.appendChild("name of new child");
    }
HTML:
<ul>
    <li><span class="folder">Subfolder 2.1</span>
        <ul id="folder21">
        <li id="test">File 2.1.1</li>
        <li>File 2.1.2</li>
     </ul>
    </li>
    <li><span class="file">File 2.2</span></li>
</ul>
Many thanks!