Two cells are connected in opposition as shown. Cell \( E_1 \) is of \( 8 \, \text{V} \) emf and \( 2 \, \Omega \) internal resistance; the cell \( E_2 \) is of \( 2 \, \text{V} \) emf and \( 4 \, \Omega \) internal resistance. The terminal potential difference of cell \( E_2 \) is ____ \( \text{V} \).
