Facebook
Google
GitHub
LinkedinDC Electric Circuits
Capacitance
19 questions By Tony R. Kuphaldt
-
Question 7 of 19
The amount of capacitance between two conductors may be calculated by the following equation:
C = εA dWhere,
C = Capacitance in Farads
ε = Permittivity of dielectric (absolute)
A = Conductor area, in square meters
d = Separation distance, in meters
How far away from each other would two metal plates, 2 square meters in area each, have to be in order to create a capacitance of 1 μF? Assume that the plates are separated by air.
Reveal answerIf you calculated a distance in the order of 2 million meters (2 ×106 meters), you made a common mistake! The proper answer is 17.71 micro-meters (17.71 ×10−6 meters), or 0.01771 millimeters.
Notes:This problem is first and foremost an algebraic manipulation exercise. Then, it is merely a matter of solving for d given the proper values. Finding ε could be difficult, though, and this is by design: I want students to learn the significance of absolute permittivity!
-
Question 8 of 19
Capacitance exists between any two conductors separated by an insulating medium. Given this fact, it makes sense that a length of two-conductor electrical cable will have capacitance distributed naturally along its length:
There should be a way to prove the existence of such “stray” capacitance in a substantial length of two-conductor cable. Devise an experiment to do this.
Reveal answerIt is the nature of capacitance to store electrical charges, manifested in the form of static voltage. Testing for the presence of a stored charge between the two conductors of a cable would be one way to prove the existence of capacitance within the cable. I’ll leave the details of testing for a stored electrical charge to you!
Notes:The purpose of this question is to make students think critically and creatively about capacitance. There is more than one way to test for capacitance in a cable, so do not limit students to one method only!
-
Question 9 of 19
Suppose you wished to build a component with no other purpose than to provide capacitance in an electric circuit (a capacitor). How might you design such a device to perform this function, and how could you maximize its capacitance?
Reveal answerI’ll let you determine how a capacitor is constructed, from your own research.
To increase capacitance:
- Increase plate area
- Decrease spacing between plates
- Increase permittivity of dielectric
Notes:These factors affecting capacitance are very hypothetical when dealing with fixed-value capacitors. After all, few people will ever have to design or construct a capacitor. However, these factors are very practical and important to understand when dealing with stray capacitance between conductors, where conductor layout and placement are well within the control of those building an electrical system!
These factors are also important to understand for comprehending the function of variable capacitors. Be sure to bring up the subject of variable capacitors in your discussion with students.
