direct insert to video field

how to insert directly to field_video??

i use this php code. but it's not working.

$url="http://www.youtube.com/watch?v=xxxxxxxx";
.
.
.
  $node->body = $feed_item->description;
  $node->teaser = node_teaser($node->body);
  $node->field_video =  $url;
.
.
    node_save($node);