# What happens when an external agent changes charge on a working capacitor?

Consider an uncharged parallel plate capacitor having capacitance 'C' with a resistance 'R' connected to it in series and both of them connected across a ideal cell of emf E volts . If in between the process of charging of capacitor , let us say at that instant the charges on upper and lower plates of capacitors are +q and -q , an external agent comes and gives charge +2q to the lower surface of upper plate (having charge +q already ) . Then what will happen ? .

My prediction : At that very instant when +2q is given to the lower surface of the first plate , the charges will redistribute on the plates so that no field is created inside the conducting plates due to stationary charges . The redistribution is shown here

Charges present on outer surface of the plates will have no effect on current .Although due to the changed charges on inner surface of plates potential difference across capacitor will change . And thus current will also change .

Is this correct ?

This explaination seems correct to me but still i have a doubt in my own explaination . Wouldn't the flowing current restrict the redistribution of charges on plates ? The electric field in the plates will be downwards , so wouldn't the downward motion of free electrons to neutralize the given charge and hence redistribute the charge on the plate be restricted ?

So what exactly will happen in this situation ?

## 1 Answer

Adding charges on capacitor wouldn't change the charge configuration of capacitor at steady state because at steady state $$I =0$$ hence Voltage across capacitor would be equal to emf of battery which implies that charge on capacitor will be equal to $$Q_c = EC$$ where E is Emf of battery.

So where would additional charge go?

You can think of it as adding charge on the electrode of the battery and this charge will go into electrolytic solution of battery where charging of battery takes place but as we know charging of battery (in ideal case) doesn't change the emf of battery, hence no change in capacitor charge also (at steady state).

• i was asking for it at general time 't'. In between the process of charging Oct 3 '21 at 8:10
• @Raghav Madan , as soon as you the put the charge on the plate the charge distribute itself in such a way so that potential difference across capacitor can be at E (emf) hence the same phenomenon of charging phenomena occur which i already explained. Oct 3 '21 at 8:36
• Ok .But i have read battery cant store charges .So how will be the battery able to store that extra charge given to capacitor Oct 3 '21 at 8:41
• @Raghav Madan , charging doesn't mean storing charge, it simply means converting ions of electrolyte into actual electrolyte by chemical reaction, but i should have mention that it would be just half reaction because we are getting electron from one terminal only (so its not complete charging in true sense) . Oct 3 '21 at 9:30