Conductivity and resistance of materials

Some materials conduct electricity better than others. Materials with low resistance conduct electricity well, while materials with high resistance conduct electricity poorly, or not at all. Let's look at the conductivity and resistivity of different types of materials:

Conductive materials

Metals have low resistance and conduct electricity well. These materials are called conductors. For this reason they are used in electrical cables. 

Insulating materials

Plastic and ceramics have a very high resistance and do not conduct electricity at all. They are called insulators. This is why non-conductive materials, such as plastic or rubber, are used on the outside of cables. Contact with the wire does not cause an electric shock because electricity cannot travel through these materials Insulators are also used to prevent short circuits when two wires touch.

Electrical resistance in the wires

Each material has a specific resistance (or resistivity). It is measured in Ω.m. and is represented by the Greek letter ρ (rho), so it is of great importance for the material used for electrical cables.

The table below shows different conductive materials, their electrical conductivity and resistivity. As can be seen from the table, copper conducts electricity well and has a low resistivity, and that is why electrical cables are made with copper. But, for example, titanium does not conduct electricity well and therefore has a higher resistivity, so it is not very suitable as an electrical conductor.

Cable resistance factors

There are two other factors that determine the electrical resistance of the cable in addition to the type of material. These are the length and thickness of the conductor (the cable): 

• A thin wire has more electrical resistance than a thick wire of the same length. 
  

• A long cable has more electrical resistance than a short cable of the same thickness. 
  

The resistance of a given length of cable can be calculated with the formula: 

As can be seen, there are three factors that determine the electrical resistance of the cable. Namely:

•  The electrical resistivity of the material used. 

• Cable length (longer cable = higher resistance). 
  

• Cable diameter (thinner cable = higher resistance). 
  

It is important to know the resistance of the wire. When current flows through a wire, its resistance causes these two effects: 

• Voltage drop (loss) along the cable. 

• Cable heating. 

If the current increases, these effects are intensified. An increase in current will increase the voltage drop and cause the cable to heat up even more. 

The cable resistance is calculated as follows: 

Question: What is the resistance of a 1.5 m long cable with a cross section of 16 mm2? 

Being: ρ copper= 1.7 x 10-8 Ω/m

l = 1,5 m

A = 16 mm2 = 16 x 10-6 m2 

Answer:

R = ρ x l/A

R = 1,7 x 10 -8 x 1,5/(16 x 10-6 )

R= 1,7 x 10-2 x 1,5/16

R = 0,16 x 10-2 = 1,6 x 10-3

R = 1,6 mΩ 

Effect of cable length on electrical resistance

Resistance = Rho x length/area R= ρ x l/A. Let's use the above example to do the calculation for a 5 m long cable. The result is that the resistance is 5.3 mΩ. If the cable is longer, the resistance increases. 

Effect of wire thickness on electrical resistance.

Let's use the previous example to make the calculation for a cable of 2.5 m2 section. The result is that the resistance is 10.2 mΩ. If the cable is thinner, the resistance increases. 

Conclusion of electrical resistance in a cable

Both cable thickness and length have a considerable effect on cable strength.