An electrical power supply has an internal resistance. It supplies a direct current $I$ to an external circuit for a time $t$. What is the electromotive force (emf) of the power supply?

total energy transferred to the whole circuit

$\frac{\text{total power transferred to the whole circuit}}{I \times t}$

$\frac{\text{total energy transferred to the external circuit}}{I \times t}$

total power transferred to the external circuit