# Capacitance lab

Discussion in 'Homework Help' started by LKim322, Feb 10, 2004.

1. ### LKim322 Thread Starter New Member

Feb 10, 2004
1
0
Hi!! i really need help on my physics lab questions which is due in a couple hours...i'm so screwed..maybe someone can please help me...please...anyways..here are the questions:

1) For some capacitors, a certain polarity must be observed when they are connected as part of a circuit. What type of capacitors are these, and why are they polarized??

2) On the outside of many capacitors, a certain voltage is indicated in addition to the capacitance value. What does this voltage mean?

3) Often, capacitors that are part of an integrated circuit are of sub-millimetric dimensions. How is it possible that capacitors of such a minute size can have any useful value if, by necessity, the area of their "plates" must be exceedingly small??

2. ### Battousai Senior Member

Nov 14, 2003
141
44
Remember C = k*epsilon*A/d

1. My guess is they are capacitors which are made of only one plate and some other conducting material on the other side that can't be contacted. Either that or the dielectric in between the capacitor plates is composed of a ferromagnetic material which contain domains that only line up in one direction. I'm not too sure about this question although I've read about it before but just can't remember.

2. The voltage listed on the outside of the cap is the breakdown voltage. A cap can only sustain a certain voltage across it before it breaks down and becomes a short. The listed voltage is the maximum voltage that can be sustained across the cap before breakdown.

3. The area of the plates may be small but also very often the distance between the plates is also very small, and since C ~ 1/d you can still achieve pretty high capacitance values on chip. However really big caps like uF caps are usually not made on chip. Another way to achieve high capacitance values is to use exotic dielectric materials like Al2O3 (I think) or organic materials (high k dielectrics).