Checkpoint $27\mathrm{.}1\mathrm{.}3$A battery has an emf of $12$ V and an internal resistance of $2$ S$.$ Is the terminal$-$to$-$terminal potential difference greater than, less than, or equal to $12$ V if the current in the battery is $($a$)$ from the negative to the positive terminal, $($b$)$ from the positive to the negative terminal, and $($c$)$ zero?