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.

Relationship between the Resistance and Dimensions of a Conductor

With a uniform wire/conductor of a given material and a given length, the resistance obtained by dividing Potential difference between any 2 points by Current is directly proportional to the distance between them. That is to say if the distance between the 2 points (on the conductor) increases the resistance also increases.

If 2 resistors each having resistance R ohms are connected in parallel the equivalent resistance Re is given by Product / Sum (Product over Sum)

Re = R*R/R+R
Re = R*R/2R
Re = R/2
Re = (1/2)*R
Each resistor will offer half of its resistance.

See the working in the slide. Click the previous button then after click on the slide screen.

Therefore, if two wires of the same material, having the same length and diameter are connected in parallel their resistance is half that of one wire. The effect of connecting two wires in parallel is similar to doubling the area of conductor. Connecting 7 wires in parallel is the same as increasing the cross section area of a wire 7 times and this reduces the resistance to a 1/7 (seventh) of one wire. Generally, the resistance of a given length of a conductor is inversely proportional to the cross section area. 


Other factors that influence resistance are; nature of the material (different materials have different resistances) and temperature (the resistance of some materials increase with an increase in temperature). So everything we have discussed above is true if temperature is constant.

Resistance of a wire = (Length of wire (L)/Cross Sectional Area (A))*a constant for a given material (ρ)

See the equation in the slide.

The constant of a material is called Resistivity of a material and it is represented by a Greek symbol rho (ρ). Resistivity is measured in ohm meter (Ωm)

Definition for Resistivity

This is the resistance of the specimen having one meter long and one meter square of cross sectional area.

Calculating Current in Two Parallel Resistances

Like I have already stated in one of the previous posts, voltage is the same in parallel resistor connection.
Voltage for resistor 1 = Voltage for resistor 2

That is to say (Voltage supplied), V = (I1R1 = I2R2).
Not forgetting that V = IR
Also Is = I1 + I2. (Is is the current supplied)
I-I1 = I1-I1+I2
I2 = I-I1
Substitute I2 in the equation V = (I1R1 = I2R2)
I1R1 = R2(I-I1)
I1R1 = IR2 - I1R2
I1R1 + I1R2 = IR2
I1(R1+R2) = I*R2
I1 = I*R2 / R1+R2
Similarly
I2 = I*R1 / R1+R2