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'.

Extension Cords are only intended for temporary use

Extension cords are used to bring power to electrical devices that need to be used in areas that are a bit far from the wall socket. But this should not be permanent. There are various kinds; Power strips, Surge protector and multi tap.

Power strip (extension cord)

Most of the time extension cords are improperly used:

Since an extension cord is a length of cable with a plug on one end and three or more sockets on the other end, considering the fact that the longer the conductor the bigger the resistance heat energy can develope in the cord (cable) whenever current goes against that resistance. This gradually makes its insulation weak and therefore it should not be used for more than 90 days otherwise it will turn a threat to its user.

Due to many electrical needs and few power outlets people overload extension cords, since they have more than one socket. This can lead to fire outbreak as more current is drawn through the extension cord. The fuse may not blow immediately but the cord can get hot enough to ignite its insulation, the nearby cloths or carpet and fire starts. In most cases even the devices connected to it will get damaged if they are not protected.


Safety Practices

• Extension cords are not meant for permanent use. Look for a licensed electrician to install more wall socket outlets.
• Avoid overloading the extension cord. Whenever adding another electrical device on the cord, first check the current rating or power rating and calculate the total current drawn through the extension cord. Most extension cords have fuses with 13A current rating so if you have one with 13A, make sure you don't exceed this current.
• Worn out and damaged cords should no longer be used and be destroyed to prevent reuse.
• When buying an Extension Cord, verify that it is tested and labelled by a recognised testing laboratory.
• Extension cords should be visually inspected for damage before use on any work.

Wiring a Three Pin Plug

Wiring a three pin Plug

Three Pin Plugs (or 3-pin plugs) are designed to protect the electrical appliance and its user from electrical hazards. They vary in size; there are those whose pins are moderately sized, also known as UK plugs, and those with large pins known as non-UK plugs. Someday or today you might need to change the 3-pin plug of your electrical appliance for some varying reasons. Though some electrical appliances are manufactured with the 3-pin plug molded on the AC in put cable, you might still need to change it. Perhaps you bought your electrical appliance when the plug pins are larger than the socket outlets. You will need to cut off that plug and replace it with the suitable one. In this post we will look at how to wire a three pin plug.

To wire a 3-pin plug, one has to be familiar with the colour codes of the wires:

• the live wire is brown
• the neutral wire is blue
• the earth wire is green and yellow striped

If you can get a new Universal 3-pin plug like the one in the picture below, unscrew the top cover of the plug, the 3 terminals and the cable grip.

Universal 3-pin Plug

Cut away about 1 cm of the insulation from each wire using a cable insulation stripper or a razor blade if the former is not in your possession. Make sure not to sever the wire strands.

Put the flex under the cord grip and screw it tightly in order to hold it in place.
Note: Do not leave the flex outside the plug to prevent the wire strands from breaking off their respective terminals as a result of strain.

Do not leave flex outside plug

Twist the wire strands so that there are no strands straying.

Connect the wires to their respective terminals. 3-pin plugs have their pins marked E for earth, N for neutral and L for live. Screw tightly all the terminals.

The connections should be like this.

Put back the cover and screw it tightly.
Disclaimer

Current, Potential difference and Resistance

In an electric circuit, there are three things to measure; Current, Potential difference and Resistance.

Current (I)
An electric current in a wire is a drift of electrons, but according to the convention adopted in 1800, current is regarded as the flow of positive electricity.
Current can also be defined as charge flowing in a circuit per unit time. Current (I) = Charge (Q)/time (t).
Current is measured in Amperes.
An Ampere is the Current which, if flowing in two straight parallel and infinitely long wires, placed one meter apart in a vacuum with a negligible cross sectional area, will produce on each of the wires a Force of 0.0000002 N per meter length of the wire.

Potential difference

Potential difference between 2 points x and y, in a circuit, is the work done when moving a unit of charge from x to y.

The SI unit is the Volt. A volt is the potential difference between 2 points of a circuit carrying a constant current of one ampere when the power dissipated between these points is one Watt.

The circuit consists of resistors R1 and R2 each having a potential difference, in volts, V1 and V2 depending on the value of their resistances. The arrow shows the path of convention current.

Resistance

This is the opposition to the flow of Current through a conductor. The SI unit of Resistance is the Ohm. An Ohm is the resistance in which current of one ampere flowing for one second generates one joule of thermal energy.