FORM 2-PHYSICS-STATIC ELECTRICITY
Capacitor is a device which is used for the storage of charges consisting of two conductors, parallel-nearly separated by air or any other dielectric.Dielectric is an insulating medium used between plates of a capacitor.
Mode of Action of a Capacitance
Explain mode of action of a capacitance
Consider two unequal metal cans which were made to stand on the caps of two identical electroscopes.These cans are given equal charges of Q units from an electrophorus disc. The charged disc is lowered inside a can until it touches the bottom. In this way the whole of the charge is given up to the can and goes to the outside.
It will be noticed that the leaf divergence is greater for the small can, showing that it has acquired higher potential than the larger can.In this case, the larger can is said to have a larger capacitance while the smaller can has a lower capacitance.When the two cans are joined by a wire electricity flows from the smaller can to the larger can until potentials are equalized.
The Action of a Capacitor
Explain the action of a capacitor
The positive charge on A induces an equal and opposite charges on opposite sides of B. These induced charges will respectively raise and lower the potential of all points in their neighborhood and in particular they will affect the potential of plate A.
As far as A is connected , however the negative induced charge will have the greater effect. The net result is is that the potential of A is slightly reduced.
B is next earthed either by touching it with a finger or by connecting it to the nearest cold-water pipe. Immediately the leaf shows a great decrease in divergence. This implies a big decrease in potential, and hence a big increase in capacitance of A.The presence of the earthed plate B results in a very large increase in the capacitance of A.
Construction of an Air-filled Capacitor
Describe the construction of an air-filled capacitor
This constitute two parallel metal plates with air band between them.A flat metal A is set up vertically on insulating legs and is connected to a gold leaf electroscope by means of a wire.
The plate is then given a positive charge by induction with a negatively charged ebonite rod. The divergence of the leaf indicates the potential of the plate.A second insulated plate B is now brought up slowly into a position parallel to A.
When B is very close to A but not touching it, it will be noticed that the leaf divergence decreases very slightly.We conclude from this that the potential of A has been decreased by the presence of B, and hence its capacitance has increased slightly.
Equivalence Capacitance of a Combination of Capacitors
Determine equivalence capacitance of a combination of capacitors
Factors affecting the capacitance of a parallel-plate capacitor.
There are three factors which affect the capacitance of a parallel-plate capacitor, namely;
- Area of plates
- Distance apart of the plates.
- Dielectric between the plates.
Relative permeability (dielectric constant) of a medium
Relative permeability is the ratio of the capacitance of a given capacitor with the medium as dielectric to the capacitance of the capacitor with a vacuum as the dielectric.
It has no units since it is a ration of similar quantities.Paraffin wax has a relative permeability of about 2 while that of mica is about 8.
