Two cells are connected in opposition as shown. Cell $\mathrm{E}_1$ is of $8 \mathrm{~V}$ emf and $2 \Omega$ internal resistance; the cell $\mathrm{E}_2$ is of $2 \mathrm{~V}$ emf and $4 \Omega$ internal resistance. The terminal potential difference of cell $\mathrm{E}_2$ is __________ V.