Why is the terminal voltage of a cell is less than its emf?

A cell consists of electrodes and electrolyte. When it is connected to an external circuit it sends a current through it.What happens to the terminal voltage of the cell when it sends a current through the extemal circuit?

A battery of emf 10V and internal resistance 3Omega is connected to a resistor.If the current in the circuit is 0.5A,what is the resistance of the resistor? What is the terminal voltage of the battery when the circuit is closed?

A battery of e.m.f. 10V and internal resistance 3W is connected to a resistor. If the current in the circuit is 0.5A, what is the resistance of the resistor? What is the terminal voltage of the battery when the circuit is closed?

A storage battery of emf 8.0 V and internal resistance 0.5 Omega is being charged by a 120 V dc supply using a series resistor of 15.5 Omega . What is the terminal voltage of the battery during charging? What is the purpose of having a series resistor in the charging circuit?