Isomorphism of $k$-Algebras Let $k$ be a field an $A$ a finitely generated $k$-algebra (think of the polynomial ring).
Suppose I have an isomorphism of $k$-algebras
$$
A \cong A/I
$$
for some ideal $I$. If $A$ were a finitely generated $k$-vector space we can conclude $I=0$, as opposed to the non finitely generated case. So can we still get $I=0$, even though $A$ is not f.g. as a vector space over $k$, only as an algebra?

The answer is yes for finitely generated commutative $k$-algebras (Mariano's answer deals with the non-commutative case). More generally, if $A$ is a Noetherian ring and there is an isomorphism $f\colon A/I \to A$, then $I = (0)$.
Why? Let $\pi\colon A\to A/I$ be the quotient map. If $I\neq (0)$, then $f^{-1}[I]$ is a nonzero ideal in $A/I$, so $\pi^{-1}[f^{-1}[I]]$ is an ideal in $A$ which properly contains $I$. Repeating, we can build an infinite ascending chain of ideals.
Edit: Here are a few more details. We start with $I_0 = (0)$ in $A$, and we set $I_{i+1} = \pi^{-1}[f^{-1}[I_i]]$ for all $i$. Let's check by induction that we get a proper increasing sequence $I_0 \subsetneq I_1 \subsetneq \dots$ of ideals in $A$. In the base case, $I_1 = \pi^{-1}[f^{-1}[(0)]] = \pi^{-1}[(0)] = I$, and we've assumed that $(0) \subsetneq I$. And given the inductive hypothesis $I_{i-1} \subsetneq I_i$, we have $I_i = \pi^{-1}[f^{-1}[I_{i-1}]] \subsetneq \pi^{-1}[f^{-1}[I_{i}]] = I_{i+1}$.
