Question

Why terminal voltage is less than emf ?

Answer 1

Because emf is the potential difference across the two terminals of a voltagesource when it is not connected to any circuit. And once it is connected to a circuit, the voltage reduces slightly because of the internal resistance of the voltagesource

Answer 2

EMF is the total amount of work done in moving a unit charge for one cycle for the whole path including the work done inside the cell.But terminal potential difference is work done in moving a unit charge from one terminal to other.Moreover some work is done in moving electrons inside the cell and so terminal potential difference is less than emf.