Get featured image URL by page id

I am using WordPress 3.4.1. I need to display the featured image of a page. How can I get the featured image URL by particular page ID. Any help?

Did you try anything? Its always helpful to share what you have tried.

$url = wp_get_attachment_url( get_post_thumbnail_id($post_id) );

Or if you want to get the image by image size.

$src = wp_get_attachment_image_src( get_post_thumbnail_id($post_id), 'thumbnail_size' );
$url = $src[0];

http://codex.wordpress.org/Function_Reference/get_post_thumbnail_id

http://codex.wordpress.org/Function_Reference/wp_get_attachment_url

http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src