How to call javasscript_tag in rails?

I followed the each step of above question and here are my changes:

In show.html.erb (http://localhost:3000/posts/24)

<% javascript_tag do %>
 $(function() {
   setInterval(function(){ $.get("/posts/<%= @post.id   %>");   }, 5000);
    });
<% end %>

<span id="changed_data">
 It should be change after every 5 seconds
</span>

controller class

def show
 @post = Post.find(params[:id])
 respond_to do |format|
   format.js
   format.html
 end

end

Added a new file

app/views/posts/show.js.erb
$("#changed_data").html("<h2> Changed </h2>");

But it sounds like that javascript_tag is not calling jquery method. What am I doing wrong here?