concatenation error in jquery

Hi following is a working code

var j=1;
$('.btn').click(function (e) {
                e.preventDefault();
                dyntable = "";
                j += 1;                
                count += 8;                
                dyntable += '<tr id=' + j + '>';
                
                for (i = i; i <= count; i++) {

                    dyntable += '<td><input  width="70px;" class="cp" type="text" id="textbox' + i + '" /></td>';

                   
                }
                
                 dyntable += '<td><input type="button" value="Add" class="btn"/></td>';
                dyntable += '</tr>';
                 $(dyntable).insertAfter('#1');
                $('.cp').width(70);
                
            });

My requirement is to concatenate following statement with j

$(dyntable).insertAfter('#1');

like  $(dyntable).insertAfter('#'+j);

i have tried several times like

$(dyntable).insertAfter("#'+j+'");

but not working

how to solve this

Regards

Baiju