invalid assignment left-hand side

invalid assignment left-hand side

Hello friends I'm new at this forum.

I am trying to assign some value to text filed using jquery.
I am using this code.

$("#tb").val() = "alpha";

its giving me this error (invalid assignment left-hand side)

If i use this simple statement

documetn.getElementById("tb").value = "alpha";

this works..

Please tell how to assign value to an input filed using jquery.