refresh DOM after append element

Answer 1

For dynamically added elements you need event delegation, use the other version on jQuery on(), you can delegate event to static parent of the dynamically added elements. In your case you can use #main_body

$('#main_body').on( "click", "#but", function() {
    alert( "bla bla" );
});    

Delegated events have the advantage that they can process events from descendant elements that are added to the document at a later time. By picking an element that is guaranteed to be present at the time the delegated event handler is attached, you can use delegated events to avoid the need to frequently attach and remove event handlers, jQuery Docs

Your code works here as it is because you are adding the dynamically element before binding the event but using event delegation will free you from the sequence you use to add the dynamic elements.