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Capacitance
19 questions By Tony R. Kuphaldt
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Question 10 of 19
What is a Leyden Jar, and how it its construction similar to the construction of all capacitors?
Reveal answerA “Leyden Jar” is a device used by early experimenters of static electricity to store electrical charges. It is made from a glass jar, lined inside and outside with metal foil. The glass insulates the two layers of metal foil from each other, and permits the storage of electric charge, manifested as a voltage between the two foil layers.
All capacitors share a common design feature of Leyden jars: the separation of two conductive plates by an insulating medium.
Notes:Encourage your students to find a picture of a Leyden Jar, or even to build their own. One can’t help but notice the functional equivalence between a capacitor and a jar: storing charge versus storing substance!
A jar is not the only object which may be transformed into a capacitor. Aluminum foil and paper sheets may also be used to make a rudimentary capacitor. Have your students experiment with building their own capacitors, especially if they have access to a capacitance meter which may be used to compare the capacitance of different designs.
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Question 11 of 19
Electric fields, like all fields, have two fundamental measures: field force and field flux. In a capacitor, which of these field quantities is directly related to voltage between the plates, and which is directly related to the amount of charge (in coulombs) stored?
Based on this relationship, which electric field quantity changes when a sheet of glass is inserted between these two metal plates, connected to a source of constant voltage?
Reveal answerField force is a direct function of applied voltage, and field flux is a direct function of stored charge.
If a sheet of glass is inserted between two metal plates connected to a constant voltage source, the electric field force between the plates will remain unchanged, while the electric field flux will increase (and along with it, the amount of charge stored on the plates).
Follow-up question: explain how the variable of electric permittivity is relevant to the described situation.
Notes:The concept of a field is quite abstract. Electric fields in particular are abstract because they cannot be tangibly perceived, at least not outside of dangerous voltage levels. Magnetic fields, which everyone should be familiar with from playing with magnets, may serve as an illustration of fields in general, but it is very important for students of electricity and electronics to understand that electric and magnetic fields are two different entities, albeit closely related (by Maxwell’s Laws).
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Question 12 of 19
The storage of electric charge in a capacitor is often likened to the storage of water in a vessel:
Complete this analogy, relating the electrical quantities of charge (Q), voltage (E or V), and capacitance (C) to the quantities of water height, water volume, and vessel dimensions.
Reveal answerElectrical charge ≡ Water volume
Voltage ≡ Height of water column in vessel
Capacitance ≡ Area of vessel, measured on a cross-section with a horizontal plane
Notes:Many students find this a helpful analogy of capacitor action. But it helps even more if students work together to build the analogy, and to truly understand it.
Perform some “thought experiments” with vessels of different size, relating the outcomes to charge storage in capacitors of different size.
