Importance of resistance and conductance in Electrical Circuits,Types of resistor

Introduction

It is essential to comprehend the ideas of resistance and conductance in the field of electrical engineering. The way electricity moves through a circuit is largely dependent on these two ideas. Understanding the nuances of resistance and conductance can help anyone, from novice engineers to seasoned professionals, better understand how electrical circuits work. Let's set out on a quest to clarify these ideas and illuminate their importance.

Illustration contrasting a resistor and conductor. Left: a resistor with colored bands and a zigzag symbol. Right: red, green, yellow, and blue copper wires.

Resistance

We define resistance as the opposition a substance offers to the flow of electric current.

A resistor is a passive, two-terminal electronic component specifically designed to limit or control the flow of electric current within a circuit. In essence, it resists the movement of electrons, thereby converting electrical energy into heat. As a result, resistors play a vital role in managing energy within electronic systems. Moreover, they are crucial for several reasons, including voltage division, current limiting, and providing stability to electronic circuits. In addition, resistors help protect sensitive components from excessive current and contribute to the overall performance and reliability of electronic devices.

Free electrons create current as they flow through a substance. The atoms and molecules in the material obstruct this flow and create resistance. Metals like silver, copper, and aluminium allow electrons to pass easily and act as conductors. On the other hand, materials such as glass, rubber, mica, and dry wood block electron flow and function as insulators.

Note that resistance creates electrical friction, which generates heat as electric current flows. The moving electrons collide with the atoms or molecules in the substance, and each collision releases a small amount of heat.

Unit of resistance;The practical unit of resistance is the ohm, represented by the symbol Ω.

It is defined as under :

A wire has a resistance of 1 ohm when a potential difference of 1 volt across its ends causes a current of 1 ampere to flow through it (see Figure below).

Circuit diagram with current from point A to B through a 1-ohm resistor, labeled "R=1 Ω." Current is 1 A, voltage is 1 V.

There is another way of defining ohm.

A wire has a resistance of 1 ohm if it releases 1 joule of energy (or develops 0.24 calorie of heat) when a current of 1 ampere flows through it for 1 second.

A little reflection shows that second definition leads to the first definition. Thus 1 A current flowing for 1 second means that total charge flowing is Q = I × t = 1 × 1 = 1 coulomb. Now the charge flowing between A and B (See above Figure) is 1 coulomb and energy released is 1 joule (or 0.24 calorie). Obviously, by definition, p.d. between A and B should be 1 volt.

Factors Upon Which Resistance Depends

The resistance R of a conductor

is directly proportional to its length i.e.

R ∝ l

2. is inversely proportional to its area of X-section i.e.

R ∝ `frac1a`

3. depends upon the nature of material.

4. depends upon temperature.

From the first three points (leaving temperature for the time being), we have,

R ∝ `frac1a` or R= ρ `frac la`

where ρ (Greek letter ‘Rho’) is a constant and is known as resistivity or specific resistance of the material. Its value depends upon the nature of the material.

Specific Resistance or Resistivity

We have seen above that R= ρ `frac la`

If l = 1 m, a = 1 `m^2` then, R = ρ

Hence specific resistance of a material is the resistance offered by 1 m length of wire of material having an area of cross-section of 1 `m^2`.

A horizontal cylinder with a 1 square meter cross-sectional area and 1 meter length is shown. The end is hatched, indicating the area measurement.

Figure 1

Cube diagram with dimensions labeled 1m on each side. Arrows show electrical current entering and exiting. Dotted lines indicate hidden edges.

Figure 2

Specific resistance can also be defined in another way.Take a cube of the material having each side 1 m.Considering any two opposite faces, the area of cross-section is 1 `m^2` and length is 1 m

[See Fig. 2] i.e. l = 1 m, a = 1 `m^2`

Hence specific resistance of a material may be defined as the resistance between the opposite faces of a metre cube of the material.

Unit of resistivity. We know R = `frac{rho l}a` or ρ = `frac{Ra}l`

Hence the unit of resistivity will depend upon the units of area of cross-section (a) and length (l).

If the length is measured in metres and area of cross-section in `m^2`,then unit of resistivity will be ohm-metre (Ω m).

ρ= `frac{ohmtimes m^2}m`=ohm-m

If length is measured in cm and area of cross-section in `cm^2`,then unit of resistivity will be ohm-cm (Ω cm).

Substances exhibit a wide range of resistivity values. To give you an idea of this variation, refer to the following table:

S.No.	Material	Nature	Resistivity (Ω m) at room temperature
1	Copper	Metal	1.7 × 10^-8
2	Iron	Metal	9.6 × 10^-8
3	Manganin	Alloy	48 × 10^-8
4	Nichrome	Alloy	100 × 10^-8
5	Pure silicon	Semiconductor	2.5 × 10³
6	Pure germanium	Semiconductor	0.6
7	Glass	Insulator	10¹⁰ to 10¹⁴
8	Mica	Insulator	10¹¹ to 10¹⁵

The reader may note that resistivity of metals and alloys is very small.Therefore, these materials are good conductors of electric current. On the other hand, resistivity of insulators is extremely large. As a result, these materials hardly conduct any current. There is also an intermediate class of semiconductors. The resistivity of these substances lies between conductors and insulators.

Conductor

In electricity, a conductor refers to a substance that allows electric charge to flow smoothly. These substances conduct electricity well because they offer very little resistance to the movement of electrons. Engineers and electricians frequently use copper and aluminum as conductors due to their excellent conductivity and beneficial properties.

Conductance

The reciprocal of resistance of a conductor is called its conductance (G). If a conductor has resistance R, then its conductance G is given by ;

G = 1/R

Whereas resistance of a conductor is the opposition to current flow, the conductance of a conductor is the inducement to current flow.

Conductance is crucial for evaluating electrical circuits, as it indicates the ease of electricity passing through a conductor. High conductance materials facilitate current flow, while low conductance materials obstruct the flow of electricity.

The SI unit of conductance is mho (i.e., ohm spelt backward). These days, it is a usual practice to use siemen as the unit of conductance. It is denoted by the symbol S.

Conductivity

We call the reciprocal of a conductor's resistivity its conductivity, and we denote it by the symbol σ. If a conductor has a resistivity ρ, then its conductivity is given by ;

Conductivity, σ = `frac1rho`

We know that G = `frac1R` = `frac a{rho l}` = σ`frac al`.

Clearly, the SI unit of conductivity is Siemen `meter^{-1}` (S`m^{-1}`).

Applications of conductor

Conductors are present in all aspects of our daily lives, seamlessly integrated into our technology-based society. They play a crucial role in transporting electric energy, from power lines to electronic circuits, enabling the functionality of our homes and devices.

Types of Resistors

A resistor provides a specific resistance value in a circuit. Engineers primarily use resistors to limit current, divide voltage, and, in some cases, generate heat. Although many types of resistors exist, electrical and electronic circuits commonly rely on the following types:

1.Carbon composition types
2.Film resistors
3.Wire-wound resistors
4.Cermet resistors

1.Carbon composition type

Manufacturers create this type of resistor by mixing finely ground carbon, an insulating filler, and a resin binder. The ratio of carbon and insulating filler decides the resistance value [See Fig 1]. The mixture is formed into a rod and lead connections are made. The entire resistor is then enclosed in a plastic case to prevent the entry of moisture and other harmful elements from outside.

Diagram of a resistor with labeled parts: color bands, resistance material, insulation coating, and leads. The cutaway reveals a carbon composition.

Figure 1

Carbon resistors are relatively inexpensive to build. However, they are highly sensitive to temperature variations. The carbon resistors are available in power ratings ranging from 1/8 to 2 W.

2.Film resistors

Technicians uniformly deposit a resistive material onto a high-grade ceramic rod to create a film resistor. The resistive film may be carbon (carbon film resistor) or nickel-chromium (metal film resistor). In these types of resistors, the desired resistance value is obtained by removing a part of the resistive material in a helical pattern along the rod as shown in Fig 2.

Cross-section of a resistor, showing labeled parts: wire lead, metal end cap, insulating base, outer insulating coating, and metal or carbon film scribed helix.

Figure 2

Metal film resistors have better characteristics as compared to carbon film resistors.

3.Wire-wound resistors

Manufacturers create wire-wound resistors by winding a resistive alloy wire around an insulating rod and enclosing it in an insulating cover. They typically use nickel-chromium alloy due to its very small temperature coefficient of resistance. Wire-wound resistors safely operate at higher temperatures than carbon types and offer high power ratings ranging from 12 to 225 W.

Diagram of a resistor showing its internal parts: resistance wire, ceramic former, end connection, connection lead, and outer sealing.

Figure 3

4.Cermet resistors.

Manufacturers make a cermet resistor by depositing a thin film of metal, such as nichrome or chromium-cobalt, onto a ceramic substrate. They are cermet which is a contraction for ceramic and metal. These resistors have very accurate values.

Diagram showing four types of resistors: Carbon Composition, Carbon/Metal Film, Surface Mount, and Wirewound. Each type is labeled and illustrated.

Figure 4

Functions of Resistors

Voltage Division: To bias the transistor and to put some reference voltages in it, engineers use resistors as voltage dividers. By placing resistors in a particular circuit, you could deliberately produce voltage ranges.
Current Limiting: In most applications, controlling the amount of current flowing through a point is essential. Resistors serve as current limiters, regulating the flow to prevent damage to sensitive components like LED chips.
Signal Conditioning: Engineers widely use resistors in signal processing and conditioning circuits to reduce or amplify signals, match impedances, and preserve signal integrity.

Applications and Importance of Resistors

In every digital device, simple as flashlights to the most complex computer systems one can see resistors.

Audio Equipment: Audio equipment uses resistors in components such as volume controls, tone modification circuits, and signal filters.
Power Supplies: Voltage law and modern days prescribing both use resistors in energy supply circuits.
Transistors and Amplifiers: Transistors and amplifiers use resistors to establish bias and control gain within amplifier circuits.
LED Current Limiting: For LED circles, the resistors offer ensure for varying reasons a number of which are that LEDs copy will likely burn out.

Conclusion

In essence, resistance and conductance are key principles that form the basis of electrical circuit behavior. Understanding these principles enables engineers to create new solutions, solve intricate issues, and push the boundaries of electrical engineering. Whether you're working on circuit design or exploring electrical phenomena, remember that resistance and conductance are essential in the field of electrical engineering.