Concept of internal voltage drop: Every practical source offers some opposition to the current due to its internal parts or components. e.g. a 12V battery has electrodes made up of conducting material having finite low resistance. Such a resistance of internal parts of source is called ‘Internal resistance’ of source. When source delivers current to load, the current flowing through the internal resistance causes voltage drop across it. This voltage drop is called ‘Internal Voltage Drop’. Due to this internal voltage drop, the terminal voltage of practical voltage source is always less than its emf.