[firedrake] Expession

Dear all, I have a very simple, yet not so obvious - to me at least - question: If I define an expression which does not depend on space variables, for example Q_expr = Expression("A*sin(2*pi*F*t)", t=t, A=A, F=F) Q.interpolate(Q_expr) shouldn't this be equivalent to just writing Q as a scalar, Q = A*sin(2*pi*F*t) ? This quantity Q is then used in a weak form to multiply the test function v on a boundary, e.g. Q*v*ds_v(1). I get different results for the two cases. Clearly, in the 1st case Q is a function while in the 2nd case it is a scalar, but I don't see why this would give different results. I might be missing something obvious here... Thank you, Anna.