Question
Thu February 24, 2011 By: Annie Joseph

why during discharging: Terminal p.d. V emf E. V = E+Ir?? why???why??

Expert Reply
Tue March 01, 2011
Dear student,
 
Electromotive force is the voltage of your battery.
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.
 
1) during discharge: terminal V < emf, due to the voltage drop from internal resistance
2) during recharge: terminal V > emf, due to restoration of potential difference across the terminals of the battery (i.e. voltage source)
 
Hope this helps.
Team
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