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# The concept of resistance in Junior engineer exam

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As an Electrical Engineer, today I am going to share basic concepts. In any engineering competitive exam, syllabus is started from basic concepts. Junior Engineer (Electrical) exam also includes basic concepts as its first topic and here I will explain about resistance from the basic concepts.

Resistance:

It is a property of the material due to which it restricts or opposes the flow of electrons passes through it. All materials have the resistance. It varies from zero from infinity.

Factors affecting resistance:

In electrical conductor, it depends upon the following factors:

1) Length of the conductor: It directly proportional to the length of the conductor i.e., it increases with increase in length of the conductor and decreases with decrease in length of the conductor.

2) Cross-sectional Area of the conductor: It also varies with the variation of cross-section of the conductor. It inversely proportional to the cross-section of the conductor. If cross-section of the conductor increases, its resistance is decreased. If cross-section of the conductor decreases, its resistance is increased.

3) Nature of the conductor:  the nature of all conductors are not same.

For example, A conductor may be made up of copper or aluminium or silver or alloy etc. Each material has its own resistance and it remains constant. It is called specific resistance of the material. Hence if we change the nature of the conductor, its resistance is also changed.

4) Temperature: it can easily be understood that resistance rises with increased in temperature and reduces with decreased in temperature. Although, some insulators have the negative temperature-coefficient of resistance. The rise in temperature decreases the resistance of electrolytes, partial conductors such as carbon and insulators such as mica, glass, rubber, paper etc.

If we neglect temperature, an expression can be derived as

R=þ. l/A   where, R=Resistance, þ=Specific Resistance or Resistivity of the material, which is constant, A=Cross-sectional area of the material.

Temperature coefficient of Resistance: It is a constant value of particular material at a given temperature and It is denoted by α. It can be observed by subtracting initial resistance at 0 degree C from the final resistance at a given temperature. Temperature coefficient ( at 20 degree C )of some important materials are given below.

Aluminium: 40.3
Copper : 39.3
Silver: 38
Brass: 20
iron: 65
Gold: 36.5
Nickel: 54
Platinum: 36.7
Tungsten: 47
Carbon: -5 (Negative TC)
Eureka or Constantan: 0.1 to -0.4
Nichrome: 1.5

Resistivity (at 20 degree C) in ohm- of some important materials are given below.

Silver: 1.64
Copper: 1.72
Gold: 2.44
Aluminium (Commercial): 2.8
Tungsten: 5.5
Nickel: 7.5
Brass: 6 – 8
Iron: 9.8
Carbon: 3000 – 7000

[Ohm (Ω) is the SI unit of Resistance and ohm-metre (Ω-m)  is the SI unit of Resistivity.]

Resistances in Series: When two or more resistances connected in series, their sum provides the equivalent resistance. Some important facts in series connection:

• Current remains same across all resistances in series connection.
• Voltage is varied across different resistances connected in series and it can be calculated by Ohm’s law.
• Sum of all voltage drops provides the source voltage or applied voltage or supply voltage.

Resistances in Parallel: In parallel connection of two or more resistances, their reciprocal’s sum gives conductance and its reciprocal provides equivalent resistance. Some important facts in parallel connection:

• Voltage remains same across all resistances in parallel connection.
• Current is varied across different resistances connected in parallel and it can be calculated by Ohm’s law.
• Sum of all individual currents provides the total circuit current or supply current.

[Note: The reciprocal of resistance and resistivity are called conductance and conductivity respectively. 1/R=Conductance(G) and it’s SI unit is siemense or mho. The SI unit of conductivity is mho/metre.]

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