Capacitance

My friend Justus asked these two questions:

1. What is capacitance?
2. What are conduits?

Let's discuss Capacitance first.
Capacitance is a term to do with capacitors, it is the ability of a Capacitor to store electric charge when its plates are at different potentials. 
Capacitance is measured in Farads. A Farad is the Capacitance of a capacitor which requires a potential difference of 1 volt to maintain the charge of 1 coulomb on that capacitor.

Capacitance (C) = Charge (Q)/p.d across capacitor (volts). 

Making Q the subject brings us to the term 'Electric Charge'. This is the potential difference of the capacitance of the capacitor and the applied voltage.
Q = CV

Electrolytic Capacitors

There are other smaller units of capacitance:

1µF = 10^-6 F

1nF = 10^-9 F

1pF =10^-12 F

What is a Capacitor? Still somebody else perhaps my friend might want to know what a Capacitor is.
A Capacitor is a device that has ability to store electric charge. It consists of two conductive plates separated by a dielectric which prevents charge from moving between the plates directly.
Capacitors are divided into two groups:

1. Fixed Capacitors
2. Variable Capacitors 

Unlike fixed capacitors, the capacitance value of variable capacitors can be altered. 

Capacitor symbols

In Fixed capacitor group, we have subcategories and these include:

• Non polarised capacitor. This type of capacitor can be connected either way. It has no positive pole or negative pole.
• Polarised capacitor. The polarised type have positive and negative terminal and must be connected so that there's Direct Current through it in the correct direction. A capacitor of this kind is electrolytic that is it has an electrolyte in between its plates as a dielectric. 

Variable capacitors are also subdivided into two groups:

1. Tuning capacitors. These are used in radio receivers, Television and Transmitter circuit for tuning.
2. Trimmers. These are used to make a fine tune in the circuit. The symbol for trimmers is a bit different.

 

Symbol for Trimmer capacitor

 Capacitors can also be divided into five groups according to their dielectric:

• Paper Capacitor. This capacitor consists of two metal foils with a waxed paper in between them.
• Air capacitors. These are used in radio receivers for tuning. They consist one set of fixed plates and another set of movable plates.
• Mica Capacitor. In this capacitor, mica is the dielectric seperating two plates. Mica capacitors are used in high frequence circuits.
• Ceramic Capacitor. A capacitor of this kind is made up of ceramic material like clay. It is also used in high frequence circuits for variation of temperature.
• Electrolytic Capacitor. This capacitor is made up of Aluminium foils with an oxide film. Capacitors of this kind can be used for power factor improvement in machines.

Capacitors in an electronic circuit

 

Symbol for Electrolytic Capacitor

 Capacitors are also used in power distribution to improve power factor.



Conduit System is a wiring system, read the post 'Wiring Systems: Conduits'.

How do Electric Cells work?

Dry cells

An electric cell is a device in which a chemical action produces electrical energy. I think we have just defined an electric cell, and this, definition, opens for us away to knowing 'HOW ELECTRIC CELLS WORK?' How does a chemical action produce electrical energy? 
Well, there are various kinds of electric cells and each produces electrical energy using a different method but the idea remains, 'chemical action producing electrical energy'. 
There are two categories of Electric cells and these are Primary cells and Secondary cells. Primary Electric cells cannot be recharged after being used up whereas Secondary Electric cells can be recharged.
The Primary Electric cell in details. 
If two dissimilar metals are immersed in an acid or a salt solution, called the electrolyte, an emf will be produced and the setting is called the Electric Cell or the Simple Electric Cell. The Simple Electric Cell was the first to be discovered before all other cells by a scientist called Alesandro Volta. When a cell of this kind is put to use, for a limited time the load will enjoy its e.m.f. This is because the electrolyte of the cell loses its chemical qualities with use.
This is how the chemical action may be explained. Suppose sulphuric acid is the electrolyte and when it is in its concentrated form, its formula is H2SO4. Now, in order for the cell to be able to supply electrons, the acid should be diluted by adding it to water.The SO4 group of atoms separates from the two hydrogen (H) atoms. The two hydrogen atoms lose their electrons to the SO4 group of atoms. Therefore the hydrogen atoms remain with a positive charge each. This is also called ionization, for the hydrogen atoms and the SO4 (sulphate) group of atoms have become ions.

H₂SO₄ = 2H⁺ + SO₄^2- 

In the figure below, a copper rod and a zinc rod are dipped in dilute sulphuric acid and both of them are connected to a resistor and an ammeter in series. In such a setting, the ammeter will deflect to show current in the circuit. Soon the zinc rod slowly starts to dissolve in sulphuric acid in form of zinc ions as bubbles form on the copper rod, these are hydrogen bubbles. When zinc dissolves, it leaves two electrons on the rod. These electrons go into the circuit to create a Potential difference. The zinc ions, Zn²⁺, join the sulphate ions, SO₄^2-, to form zinc sulphate.

When zinc dissolves in acid, the solution gets warm and this warmth is a result of the internal molecular energy produced. This is not the case in the simple cell, the action of acid on zinc produces electricity instead.

Hydrogen bubbles formed on the copper rod are as a result of hydrogen ions leaving the solution to receive an electron, each, from copper. They become neutral and form hydrogen gas bubbles.

Therefore, copper becomes positively charged and zinc negatively charged.

Illustration of a Simple cell

Simple cell

There are two problems that make the simple cell work for a short time:

1. Polarization. This is the formation of hydrogen gas bubbles around copper rod which acts as an insulator and prevents other hydrogen atoms from receiving electrons. That is to say the internal resistance of the cell is increased. This also prevents current from flowing in the circuit.
2.  Local action. If the zinc used is impure, the impurities such as iron or carbon form small anodes at the surface of zinc. These become local cells liberating hydrogen at their anodes. This simply wastes zinc. Local action can be prevented by cleaning the zinc plate in sulphuric acid and putting a thin film of mercury on the zinc rod. Mercury dissolves pure zinc forming zinc amalgam on the surface and the impurities remain covered.

A number of different cells have been invented with the elimination of Polarization considered important. Among the many invented cells, Laclanche cell managed to address this fault. Georges Laclanche, the inventor, decided to surround the anode with a depolarizer in a porous pot and this is the mixture of powdered carbon and manganese dioxide. He then put this together with a zinc rod in a glass jar containing ammonium chloride solution, the electrolyte.
Manganese dioxide oxidizes hydrogen to form water. However, the depolarizing action of manganese dioxide is slow.

Laclanche Wet Cell

  There is another form of Laclanche cell and this is the Dry cell.

Inside a Dry cell

Its assembly is almost the same as that of the wet cell. The ammonium chloride solution is replaced by ammonium chloride paste. This paste is made up of starch and ammonium chloride.The Dry was invented by a German scientist called Dr. Carl Gassner. It is the most commercially manufactured primary cell.