# Homework Help: 3 plate capacitor

1. Mar 31, 2012

### bbakshan

Hi,

I have a capacitor in which the first plate is connected to 3.3V and the second plate is connected to ground.
A third plate which is connected to 2.5V is inserted in between the two plates.

Will the capacitance between 3.3V and ground remains? If yes, will it be larger?

Thank you guys,
Boris.

2. Mar 31, 2012

### Staff: Mentor

Hi bbakshan, Welcome to Physics Forums.

In an ideal capacitor, where does the electric field manifest and what shape does it take? What is the field elsewhere?

3. Mar 31, 2012

### bbakshan

Hi,

Yes, this is an ideal capacitor.
There would be 2 electrical fields between the 3.3V plate and 2.5V and between 2.5 and 0V.

else where it should be zero. Correct?

4. Mar 31, 2012

### Staff: Mentor

Well, I wasn't referring to this case in particular. I was referring to an ideal capacitor in general. The answer is that the field exists between the plates and nowhere else.

In your first post you posed the question, "Will the capacitance between 3.3V and ground remains? If yes, will it be larger?" You should be able to answer that question now, knowing where the fields begin and end.

5. Apr 1, 2012

### bbakshan

O.k . but isn't the 2.5V plate somehow affects the capacitance between 3.3V and 0V ?
Doesn't it behave like a shielding plate so that plate 3.3V doesn't "see" the 0V plate ?

6. Apr 1, 2012

### Staff: Mentor

That was the point of the electric field discussion above

7. Apr 1, 2012

### bbakshan

Hi again,

What is the answer? I don't quite understand what is it that you're trying to say...
sorry

8. Apr 1, 2012

### Staff: Mentor

If the electric field is confined to the region between plates, then there is no field "beyond" a plate to "see" another plate. Only the plate in the middle can "see" the two outer plates.

The setup behaves as two separate capacitors connected in series.

9. Apr 1, 2012

### bbakshan

Thanks.
From that I conclude that if a step function is (voltage spike) occurs on the 3.3V and since the voltage on the 3.3V-2.5V capacitor "wants" to remain constant then the voltage spike would go to the 2.5V plate. correct?

What I mean is that if the capacitors are connected in series then there would still be a connection between 3.3V and 0V. What kind of connection (if not capacitance) would it be?

10. Apr 1, 2012

### Staff: Mentor

If the 2.5V supply is ideal and there's no resistance involved, then the spike would not affect the potential (w.r.t. ground) on the 2.5V plate. In the real world where there is resistance (and inductance too) and it takes time for charges to move, then the 2.5V plate would experience a transient. It would also be passed along to the 0V plate to some extent that depends upon the relative capacities. Remember that two capacitors in series are equivalent to a single capacitor of value C = C1*C2/(C1 + C2).