# Capacitor's dielectric and voltage?

1. May 1, 2015

### mohamed el teir

suppose a battery connected to a capacitor only (and no dielectric inserted), the potential difference of the capacitor will increase till it becomes equal to that of the battery, suppose we put dielectric in the capacitor, this will decrease potential difference of the capacitor, now the capacitor would have less potential difference than the battery, what will happen next, will the potential difference of the capacitor increase again till it becomes equal to that of the battery again, if it will not increase where will the rest of voltage supplied by the battery go, and if it will increase then the voltage and Q are the same as without dielectric so the capacitance will be the same as without dielectric while this is not true as permittivity increased, or will the potential difference increase again and the Q increase and the idea of Q being constant before/after inserting dielectric is while we aren't connecting a battery only

2. May 1, 2015

### x86

If I understood you correctly, you have a battery connected in series with a capacitor. The voltage across the capacitor will be fixed, that of the battery. The only effect the dielectric will have is changing the amount of charge stored on the capacitor.

3. May 1, 2015

### phinds

By what magic do you believe that this will happen?

4. May 1, 2015

### phinds

Exactly. As the dielectric is inserted the battery will supply more charge to maintain the voltage.

5. May 1, 2015

### mohamed el teir

thanks !

man, this will actually decrease the potential difference of the capacitor because k (dielectric constant) = v node / v, but what will happen yes that the potential difference of the capacitor will increase again to be equal to that of battery and therefore Q will increase to give the correct capacitance (higher one after inserting dielectric), what confused me was something i read that says Q is constant before/after inserting dielectric, i concluded that this is true only when there is no charging or discharging for the capacitor (ex: no battery connected to charge and no earthing to discharge), and i wanted to make sure that this conclusion is true

Last edited: May 1, 2015
6. May 1, 2015

### phinds

That's fine, but making sure that something is true by making a false statement about it is not really the best way to go. Best just ask your question directly. It will NOT decrease the potential difference on the capacitor unless you can magically cause the dielectric to be inserted faster than the battery can supply charge. For an actual battery that may be physically possible and not require magic, but for an ideal battery it is not possible and WOULD require magic.

7. May 1, 2015

### mohamed el teir

what false statement ? you can assume that inserting the dielectric would be after removing the capacitor and then connecting it again, if i didn't say the potential difference part i wouldn't ask the question, because when the potential of capacitor doesn't change then Q is not constant and this was the thing i wanted to make sure of because of the not-well-explained thing i read about Q being constant in absolute manner

8. May 1, 2015

### davenn

we cant assume anything ..... if you don't state all the information, we can only go by what you have written

9. May 1, 2015

### phinds

As davenn pointed out, that is NOT what you said. You may have felt that you fully implied it but I would not agree and in any case you have to say exactly what you mean if you expect to get any answer based on exactly what you are thinking. We are not mind readers.

10. May 1, 2015

### mohamed el teir

you know that the person writes this types of question at the moment he is thinking of it, so yes maybe not 100% of info is given clear, anyone can ask what's clear and then i will make my point more clear for sure

11. May 1, 2015

### mohamed el teir

as i said maybe not all the info is 100% clear, but you know as this question is based on confusion so it may have confusion in its info as well, the false idea of Q being constant in absolute manner would make one think of the question like this : how come the voltage will be constant in the capacitor while its Q is constant all the way ? if i could give the info 100% clear then i wouldn't have the confusion, that's what i mean

12. May 1, 2015

Fair enough.