<script type="text/javascript">
  jQuery(document).ready(function(){
    jQuery('#student_name').on('change',function(event){
    var totalmarks = jQuery("#marks_physics").val() + jQuery("#marks_chemistry").val()+ jQuery("#marks_chemistry");
    jQuery("#marks_total").val(totalmarks);
  });
});
</script>

<script type="text/javascript">
  jQuery(document).ready(function(){
    jQuery('#student_name').ajaxComplete(function(event,request,settings){
    var totalmarks = jQuery("#marks_physics").val() + jQuery("#marks_chemistry").val()+ jQuery("#marks_chemistry");
    jQuery("#marks_total").val(totalmarks);
  });
});
</script>