Can I create script tag by jQuery?

To load from a url:

$("body").append($("<script />", {
  src: url
}))

To load from existing source code:

$("body").append($("<script />", {
  html: code
}))

Why are you not using jQuery.getScript(url,[callback])?

The error is in the selector:

$("body").append("<script>alert('hello world');<\/script>");

Note that you have to escape the unallowed characters in the appended string.