Why canvas doesn't work with jQuery selector?

I have made simple example of using canvas and then I saw that my code doesn't work when I use jQuery selector.

Examples:

Javascript

    window.onload = function() {
        var canvas = document.getElementById('myCanvas');
        var ctx = canvas.getContext('2d');

        ctx.fillRect(10,50,100,200);
    };

JQuery

   window.onload = function() {
        var canvas = $('#myCanvas');
        var ctx = canvas.getContext('2d');

        ctx.fillRect(10,50,100,200);
    };

So I have no idea why it happened. Is there any limitations about it?