What are Diodes?

A Diode is a semiconductor which can be used to change Electric Supply from Alternating Current to Direct Current. They allow current to pass through them in one direction.

Schematic Symbol

Rectifier diodes in a circuit

A Diode has two parts; the P-type semiconductor and the N-type semiconductor. The P-type semiconductor has holes (Positive charges) as the majority charge carriers and N-type has electrons (Negative charges) as the majority charge carriers.

Silicon or Germanium are the materials used to manufacture diodes. During manufacture, some impurity atoms are added to silicon. This is called doping. To produce the P-type semiconductor silicon is doped with Boron, for example, which introduces holes as majority charge carriers. Phosphorus can be added to silicon to produce N-type semiconductor. Phosphorus atoms offer their electrons to silicon in a covalent bond. There are also minority charge carriers in both semiconductors; the P-type has electrons as minority charge carriers and the N-type has holes as minority charge carriers.

P-type doping

N-type doping

Diode Bias

Diode bias is the condition of a diode.

No Bias

The No Bias condition, this is when there's no voltage applied. The layers of ions in the depletion region of the diode repel majority charge carriers, holes and electrons, of the P-type and N-type semiconductors and prevent them from crossing the junction.

Forward Bias (VD )
In the Forward Bias condition, also known as 'on' condition of the diode, a positive potential is applied to the P-type semiconductor and a negative potential is applied to N-type semiconductor. The ions in the depletion region get neutralized and eventually they will allow heavy flow of electrons. This is due to the pressurizing of electrons in the N-type semiconductor.


The forward bias voltage of a germanium diode is 0.3 volts and that of a silicon diode is 0.6 volts. It is important to note the maximum forward current which can be allowed to pass through the diode. If too large current is allowed to pass through a diode, it can  easily get damaged.

Reverse Bias
In the reverse bias, the depletion region enlarges. When a positive potential is applied to the N-type semiconductor and the negative potential to the P-type semiconductor, the uncovered positive and negative ions in the depletion region increases. This is because the negative terminal of the voltage supply repels electrons in the P-type semiconductor and the positive terminal attracts electrons in the N-type semiconductor. Therefore conduction is not possible.

How do Electromagnets work?

An Electromagnet is that magnet which attracts metals only when it is connected to an electric supply. It is made up of a solenoid (a coil of insulated copper wire wound on soft iron).  When an electric current flows through a wire, it produces magnetic flux around the wire.

Magnetic flux produced by current. Magnetic flux is clockwise 

when current is moving away through the wire (from right to left).

Since the coil has many turns, that is 500 or more, the magnetic fluxes in each turn join each other to form larger lengths of  magnetic fluxes.These magnetize the iron which also produces fluxes due to its ferromagnetic property. As a result, the soft iron and the solenoid wire together produce very strong magnetic fluxes. But it should be noted that the soft iron will quickly lose its magnetism if the electric current supply is switched off.

According to Ohm's law R = V/I. Therefore in order to harvest magnetic flux, resistance (R) must be greater than Current (I) and whenever resistance increases voltage also increases. This means if a solenoid of 500 turns is connected in series with a battery of 12 Volts, one will need to connect a solenoid of about 700 turns if the battery voltage is increased to 15 Volts. If the number of turns are not increased at 15 volts, then the solenoid will just heat up and it will not produce magnetic flux.

You might also need to read about Relationship between the Resistance and Dimensions of a Conductor

Who invented Electromagnets?
When William Sturgeon, a British scientist, was trying to magnetize soft iron permanently he found out that the iron could only be strongly magnetized when current was flowing through the solenoid wire. Therefore he discovered/invented an Electromagnet.

History of Magnets

In 600 B.C, the lodestone also known as Magnetite was already known to the Greek. It is an iron ore which has the property of attracting metals especially small pieces of iron. Chemically, a lodestone is made up of iron oxide with the formula Fe3O4. The place where magnetic iron ore was first discovered is called Magnesia.

The word lodestone is got from an old English word way, this refers to the property, of the stone, of being able to show the direction of the earth's North pole and South pole. During middle ages, navigational compasses were made by joining a piece of  lodestone to a wooden splint then this was made to float on water in a small container. These two could point in N-S direction.

Uses of Electromagnets

Electromagnets are due to the magnetic effect of current in a wire or a conductor.   There are quite a number of applications of magnetic effect of current in a conductor and among these are:

• Electric Bells
• Lifting magnets
• Solenoid switch for the car starter motor
• Magnetic circuits of generators and motors

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.