why during discharging: Terminal p.d. V emf E. V = E+Ir?? why???why??
It is the MAXIMUM voltage the circuit can attain. So, when you place a voltmeter in parallel to your battery/cell, the reading shown is your Electromotive Force, or EMF.
Whereas, PD or potential difference is simply the difference of the voltage before and after experiencing a resistance effect. So basically, the potential difference can never be equal to the EMF in a circuit because there is always energy lost as heat, or some other form.
2) during recharge: terminal V > emf, due to restoration of potential difference across the terminals of the battery (i.e. voltage source)