Question

Give reasons why the terminal voltage is less than the emf of a cell.

Answer 1

It is because of presence of internal resistance.

Detail explanation

EMF is the potential difference across the two terminals of a voltage source when it is not connected to any circuit. Once it is connected to a circuit, the voltage reduces slightly because of the internal resistance of the voltage source. The terminal voltage of the voltage source is the potential difference across the terminals when it is connected to a circuit. This is why the terminal voltage is always less than the EMF of the same voltage source.
When the current starts to flow in the circuit, then there is a voltage drop due to internal resistance... Thus V is always less than E. This can also be observed from the following eqn:
V=E-Ir