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Capacitance
19 questions By Tony R. Kuphaldt
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Question 4 of 19
Suppose two wires, separated by an air gap, are connected to opposite terminals on a voltage source (such as a battery). An electric field will develop in the space between the two wires: an invisible web of interaction, similar in some ways to a magnetic field. In this diagram, draw the invisible “lines of flux” for this electric field, showing their physical range:
Reveal answer
Follow-up question: explain how electric flux lines differ in geometry from magnetic flux lines.
Notes:Students may note that electric lines of flux do not follow the same paths that magnetic lines of flux would. Whereas magnetic lines of flux are always circular, electric lines of flux always terminate between points.
Note to your students the relevance of this fact in shielding: unlike magnetic shields which must divert the inevitable paths of magnetic flux lines, electric shields are able to terminate electric flux lines.
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Question 5 of 19
Electric fields may be described as “invisible webs” of interaction across space between electrically charged objects. Most people should be familiar with magnetic fields from playing with magnets as children: the forces of attraction or repulsion that act across open space between two or more magnetic objects. But electric fields are not the same as magnetic fields. The two different kinds of fields exert forces on entirely different objects.
Give an example of where an electric field manifests a tangible, physical force, like the magnetic fields we are all familiar with. Under what conditions are electric fields strong enough for human beings to detect without instruments?
Reveal answer“Static cling,” where articles of clothing are attracted to one another after being dried in a machine, is an example of an electric field strong enough to produce tangible, physical attraction over a distance. Another, similar effect is that of peoples’ hair standing on end prior to a lightning strike.
In both cases, what condition causes such a strong electric field to develop?
Notes:Electric field force is also used in some precision voltage measuring instruments (“electrostatic” meter movements), as well as the more common electroscope. If you happen to have either an electrostatic meter movement or an electroscope available in your classroom, use it to demonstrate the physical effects of electric fields.
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Question 6 of 19
Capacitance is a very important property in many types of electric circuits. Define what “capacitance” is, and what causes it.
Reveal answer“Capacitance” is the capacity of two separated conductors to store energy in the form of an electric field, resulting from an applied voltage. You may also find a definition of “capacitance” stated in terms of opposition to change in applied voltage over time.
Capacitance is caused by the establishment of an electric field between two conductors.
Notes:Ask students what unit of measurement capacitance is expressed in. Also, ask them if they think the capacitance of any given conductor pair changes with the applied voltage or stored energy, or if capacitance is a quantity independent of particular electrical conditions.
