using Json in website contianing master page

Hi,
I had a website which has a master page.In the registration page i want to authenticate the emailId.Am trying to use json to check the existance of the record.How to trigger json on textbox text change,because of the master page i was not able to get the emailtext box id.. tis is the json code
<script language="javascript" type="text/javascript">
        $(document).ready(function() {

            $('#txtEmailId').change(function()
           {
           // my code here
            });
        });
    </script>


but i was not able to get the text box,can any one give me a ...