# Capacitance Definition and 147 Discussions

Capacitance is the ratio of the amount of electric charge stored on a conductor to a difference in electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. In this case the electric potential difference is measured between the object and ground. A material with a large self capacitance holds more electric charge at a given potential difference than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components (along with resistors and inductors). In a typical capacitor, two conductors are used to separate electric charge, with one conductor being positively charged and the other negatively charged, but the system having a total charge of zero. The ratio in this case is the magnitude of the electric charge on either conductor and the potential difference is that measured between the two conductors.
The capacitance is a function only of the geometry of the design (e.g. area of the plates and the distance between them) and the permittivity of the dielectric material between the plates of the capacitor. For many dielectric materials, the permittivity and thus the capacitance, is independent of the potential difference between the conductors and the total charge on them.
The SI unit of capacitance is the farad (symbol: F), named after the English physicist Michael Faraday. A 1 farad capacitor, when charged with 1 coulomb of electrical charge, has a potential difference of 1 volt between its plates. The reciprocal of capacitance is called elastance.

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1. ### Energy stored in a LC circuit

Homework Statement Energy in the circuit remains constant. When the current is flowing, the energy stored is all stored in the inductor. When the current stops flowing, it is because all the energy is stored in the capacitor. Find the fraction of energy stored in the inductor when half of the...
2. ### Capacitance Question - Total Energy, Potential Difference

Homework Statement Two identical parallel-plate capacitors, each with capacitance 10.0 μF, are charged to potential difference 50.0 V and then disconnected from the battery. They are then connected to each other in parallel with plates of like sign connected. Finally, the plate separation in...
3. ### Capacitor Energy With Different Dielectrics

Homework Statement Two parallel plates are connected to a battery which maintains a constant potential difference of 12.0 V. The distance between the plates can be varied and there are three plates of different dielectric materials available: rubber (0.40 mm, κ = 6.7), glass (4 mm, κ = 7) and...
4. ### Dielectric in a capacitor in equilibrium

Homework Statement Consider the situation shown in the figure. The width of each plate is b. the capacitor plates are rigidly clamped in the lab and connected to a battery of emf E. All surfaces are frictionless. Calculate the value of M for which the dielectric will stay in equilibrium...
5. ### Net capacitance

Homework Statement Find the potential difference between points a & b. Diagrams attached below. I have doubts in part (b) and (c). Homework Equations Q = CV, kirchhoff's laws The Attempt at a Solution I honestly don't get how the current can flow in opposite directions in part(b). if we...
6. ### Capacitance and impedence?

Homework Statement This is not necessarily a homework question, just a conceptual one. I was reading up on capacitors, and an example said that a 25uF capacitor has TWICE the impedence of a 50uF capacitor. Can someone explain what this means? I would have thought that a capacitor with higher...
7. ### Thevenin Equiv with just Capacitors

Homework Statement imgur link: http://i.imgur.com/TWB5rRu.png Homework Equations The Attempt at a Solution I can find the equivalent capacitance, 10F, but I'm not really sure how to find the equiv. Thev. voltage. I figured that with the voltage source present, half the voltage will be...
8. ### Capacitors in Series and Parallel

Homework Statement Fig. 25-39 shows a 12.0 V battery and four uncharged capacitors of capacitances C1 = 1.00 µF, C2 = 2.00 µF, C3 = 3.00 µF, and C4 = 4.00 µF. If only switch S1 is closed, what is the charge on (a) capacitor 1, (b) capacitor 2, (c) capacitor 3, and (d) capacitor 4? (figure at...
9. ### Question regarding Energy in Circuits with Capacitors

Hey everyone! I have a doubt that relates to energy stored in different capacitors (when they're in a circuit). I understand how in capacitors connected in parallel, the capacitor with the biggest capacitance stores the most energy because: E = 1/2 CV2 So naturally the bigger the capacitance...

in deriving the capacitance of a parallel plate capacitor why do we take the field between the capacitors as σ/∈ and not 2σ/∈,when there are two plates and in the region between the capacitors, field must add up so shouldn't it be 2σ/∈?
11. ### Marginal inductance and capacitance of a real wire?

Speaking just of long straight wires, not loops, coils or solenoids. Ideal wires have no resistance. Of course they would have no capacitance. And real wires have some very small amount of inductance, no? Firstly, would an ideal wire have some ideal nonzero inductance? Why/why not? After all...
12. ### Self-Capacitance of an Insulating vs. Conductive Sphere

Hey, guys! I had a brief electromag question. If capacitance is defined as the amount of charge that can be stored in a capacitor per unit of potential difference, then technically can capacitors made from insulators still have a capacitance? The problem I'm considering is calculating the...
13. ### Finding Voltage from capacitance, current and time

Homework Statement The question asks the current through a 2uF capacitor is show. sketch the capacitor voltage Vc between t = 0ms give that the voltage at t=0ms is 0V. Your graph must be correctly scaled with numerical values. There is a graph included showing a current rise for 0-2...
14. ### Capacitors in a series circuit W/ one dielectric

Problem: When a dielectric slab is inserted between the plates of one of the two identical capacitors in Fig. 25-23, do the following properties of that capacitor increase, decrease, or remain the same: (a) capacitance, (b) charge, (c) potential difference (d) How about the same properties of...
15. ### How does surface capacitive touchscreen work in detail?

It's pretty easy to find the "working principle" of surface capacitive touchscreen like http://www.elotouch.com/Technologies/SurfaceCapacitive/howitworks.asp [Broken] Roughly speaking, I know that A surface capacitive touchscreen has a conductive layer to which some electrodes are attached...
16. ### Dielectric inserted in parallel plate capacitor

Homework Statement When a dielectric is partially inserted between two plates of a capacitor that is disconnected, what will be it's motion? Also what will be its motion if the capacitor is connected to a battery? Homework Equations The Attempt at a Solution For the first part. I think the...
17. ### Capacitance - sphere

Homework Statement 1) Two solid conducting spheres have radius r1 = 3 cm and r2 = 42 cm. The two spheres are connected by a thin conducting wire. Assume: - the wire is very long and thin, with negligible surface area, so it does not affect the capacitance of the system. - the switch is...
18. ### Why does the method of images work?

Why does the method of images work?
19. ### Electricity and Magnetism RC Circuit Conceptual Questions

Edit: sorry i realized i posted this in the wrong section. Can some move it to hw? Thank you. Hey i was studying intro E&M recently and i started learning about RC circuits. Up until this point everything, concept wise, has made sense for me. Then I learned how RC circuits work, and the math...
20. ### Tapped Transformer With a Capacitor

The question is the following: a) Determine all load voltages and currents b) Determine the resistance reflected into the primary. Answer a) R(l): 35V, 2.91A, X(cl): 15V, 1.5 A The answer to part (b) is apparently 34.5 Ohms. How did they arrive at these answers? I am not sure...
21. ### Double Paralell Plate Capacitor with Dielectric

Homework Statement Please see attached. Homework Equations The Attempt at a Solution I get a result of ## \alpha =\frac{1}{2} ## for part a) - which I think is correct. I'm stuck however on part b) - with the dielectrics. I have that the field in the region where the dielectric is...
22. ### Capacitance between two wires-not straight!

Hi, I would like to know how to calculate capacitance between two wires which are parallel but not necessarily straight.for instance the two wires might be parabolic..I would like to calculate for such general structures.i.e) two parallel wires that are not necessarily straight. Thanks in advance
23. ### Capacitance between two parallel wires not straight

Hi, I would like to know how to calculate capacitance between two wires which are parallel but not necessarily straight.for instance the two wires might be parabolic..I would like to calculate for such general structures.i.e) two parallel wires that are not necessarily straight. Thanks in...
24. ### Estimate the Dielectric Breakdown of Air

Homework Statement A capacitor is charged to V = 5 kV. A wire is then connected to one plate and is slowly connected to the other. Before actually touching the plate, a spark of length l = 1 cm shoots from the wire to the plate. Using this information, estimate the dielectric breakdown of air...
25. ### Is it possible to charge photons?

I was wondering if there is any way to electrically charge photons?
26. ### Can we calculate capacitance of an isloated meta plate?

i have done the capacitance of an isolated charged sphere (=4 pi E R).but can we find capacitance of an isolated ,infinite charged plate too??how?
27. ### Capacitor Network

Homework Statement Capacitors C1 and C2 are connected in series and this combination is connected in parallel with capacitor C3. Homework Equations Q=CV Ceq=C1+C2+C3 equivalent capacitance for capacitors in parallel 1/Ceq=1/C1+1/C2+1/C3 equivalent capacitance for capacitors in series The...
28. ### Capacitor with magnetic dielectric

Odd question, would anybody know what would happen if a parallel plate capacitor (good conductor) had a soft magnetic material (fairly poor conductor) as a dielectric? What would be the DC/AC electrical characteristics of this?
29. ### Ballistic Galvanometer Lab

My question is : What is the absolute capacitance? In my electricity lab, I was assigned to prepare a report on measuring absolute capacitance of a capacitor using ballistic galvanometer. I am familiar to the term self capacitance but not the absolute one. My mentor asked me to find it out by...
30. ### Charge flowing through/across a Capacitor?

Homework Statement Dry air has a resistivity of about 3*10^13 Ωm. A capacitor has square plates 10cm on a side separated by 1.2mm of dry air. The capacitor is charged to 250V. Assuming the potential difference does not change as the charge flows, what fraction of the charge will flow across the...
31. ### Change in total energy of capacitance

Homework Statement When an initially charged capacitor is connected in a circuit with an uncharged capacitor, why would the total energy of both capacitors be less than the initial energy of the charged capacitor? Homework Equations This was a question asking for an explanation, so I'm not...
32. ### Find the capacitance from 2 energies stored, not knowing voltage

Hi there, new to this website so please bare with me if I break any rules and let me know for future. I am a maths and physics tutor but occasionally I come across a question I just cannot work the answer for. In this case I know the answer but I can't get to it. 1. Homework Statement...
33. ### How to switch on and off capacitance on a piece of conducting material?

I don't know if this is the right way of describing it, but I need to activate an area of capacitance on a touchscreen by applying some current to a piece of conducting material like tinfoil. I am needing to remotely turn pages on a touchscreen ereading device with a microcontroller, and this...
34. ### Capacitor equivalence help

http://<a [Broken] href="http://imgur.com/UwulXqg"><img [Broken] src="http://i.imgur.com/UwulXqg.png" title="source: imgur.com" /></a>[/PLAIN] [Broken] 1. Homework Statement Find equivalent capacitance of this formation from a to b, C1=4.1 uF, C2=6.86 uF, C3=5.03 uF, C4=5.92 uF...
35. ### Capacitance of an asymmetrical parallel plate capacitor

I've been looking all over and to my great surprise have not be able to find an answer to this question. What is the capacitance between to parallel plates if one plate is larger than the other? For example, the capacitance between a 1" diameter plate and a 4" diameter plate. Thanks.
36. ### Inserting metal to parallel plate capacitor

If there is a parallel plate capacitor with plate area, A, plate separation, d, and a slab of copper of thickness, b, is inserted exactly halfway between the plates. How to find it's new capacitance, C', and new potential difference, V' ? Answer: V' = E(d-b) and C' = εA / (d-b) I...
37. ### Three plate capacitor and charge distribution

In the figure given the capacitance for the two pair would be same. Can anybody please explain me how the charge would be distributed across each plate? I think that the middle plate being isolated should have +Q on one side and -Q on other, but how would be the charge distribution about the...
38. ### Where does inductive reactance go?

Lenz law states the induced emf in any circuit is always in a direction to oppose the effect that produced it. So my question is, 1. Does this mean xl creates a difference in potential and actual electrons flowing in the opposite direction or just oppose the current direction that created it...
39. ### Testing and Troubleshooting Capacitive Sensor Probes

I am working on a system that uses capacitance sensor probes, the system components are the probe, triaxial cable with LEMO connectors, BNC cable, oscillator, and a demodulator. I have had some trouble finding ways to troubleshoot the system other then basically testing continuity. Are there any...
40. ### Discharging a charged capacitor to another neutral capacitor

Homework Statement After a capacitor of capacitance ##C_1## is charged up to a voltage ##V_o##, the Battery is removed from the circuit. Another capacitor of capacitance ##C_2## is connected in series with the capaciotr ##C_1##. Find the charge on each capacitor after equilibrium is attained...
41. ### Effect of capacitor plates at different angle

Hi, I am interested to simulate the capacitor with plates having an angle of 195 degrees between them. https://dl.dropboxusercontent.com/u/32847378/cp1.jpg [Broken] I know that if the angle between of plates is 180 deg, then electric lines of forces will follow a elliptical path and...
42. ### Explanation of capacitance formula

Hello, I am new to electronics and am having trouble understanding this formula for capacitance: Could someone please break this down into layman terms? Interestingly π appears? I'd like to understand why that exists in capacitance. Thank you all in advance!
43. ### Moving capacitor plates

Let us take a charged capacitor and move the plates such that it the distance between them be constant. For example, one moves towards left and other moves upwards. Assuming only overlapping area affects capacitance... WILL THE CHARGE STORED IN CAPACITOR CHANGE? I know the capacitance changes.
44. ### Energy stored in capacitance

My question is why is the energy stored in a capacitance equals to half the energy supplied by the battery. I already know that there is heat loss in wires. But what I really want to know is that why is it only half of it ? It could have been any other fraction.
45. ### Capacitance and Electric Field

I've gotten myself in a bit of a muddle here, I'm hoping you lovelies can help me out! So, the capacitance relates to the amount of charge a capacitor can store. Higher capacitance, greater charge. Greater stored charge, greater stored energy (right? or not?). So a higher capacitance means you...
46. ### Help please -- Model of the capacitance and dielectric

Hi everyone, I'm new in physics forum and this is my first post. So basically, our lecturer grouped us and gave each group a project to carry out. And the only information that we got is the name ( The one we got is "Model of the capacitance and dielectric" ). With only this information, me...
47. ### Calculate the number of sodium ions entering an non-myelinated axon

Homework Statement Calculate the number of sodium ions entering a non-myelinated axon during the action potential, per metre of axon length. The change in potential is 100 mV, the axon membrane capacitance per unit length is 3 × 10−7 F/m, and the charge on an ion is 1.6 × 10−19 C. What...