Resistance
The opposition offered by a substance to the flow of electric current is called its resistance.
A resistor is a passive two-terminal electronic component designed to limit or control the flow of electric current in a circuit. It resists the flow of electrons, converting electrical energy into heat. Resistors are crucial for several reasons, including voltage division, current limiting, and providing stability to electronic circuits.
Since current is the flow of free electrons, resistance is the opposition offered by the substance to the flow of free electrons. This opposition occurs because atoms and molecules of the substance obstruct the flow of these electrons. Certain substances(e.g. metals such as silver, copper, aluminium etc.) offer very little opposition to the flow of electric current and are called conductors. On the other hand, those substances which offer high opposition to the flow of electric current (i.e. flow of free electrons) are called insulators e.g. glass, rubber, mica, dry wood etc.
It may be noted here that resistance is the electric friction offered by the substance and causes production of heat with the flow of electric current. The moving electrons collide with atoms or molecules of the substance ; each collision resulting in the liberation of minute quantity of heat.
Unit of resistance. The practical unit of resistance is ohm and is represented by the symbol Ω.
It is defined as under :
A wire is said to have a resistance of 1 ohm if a p.d. of 1 volt across its ends causes 1 ampere to flow through it (See below Figure).
There is another way of defining ohm.
A wire is said to have a resistance of 1 ohm if it releases 1 joule (or develops 0.24 calorie of heat) when a current of 1 A 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`.
Figure 1
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).
1.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
2.If length is measured in cm and area of cross-section in `cm^2`,then unit of resistivity will be ohm-cm (Ω cm).
The resistivity of substances varies over a wide range. To give an idea to the reader, the following table may be referred :
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 terms of electricity, a conductor is a substance that facilitates the smooth circulation of electric charge. These substances possess high conductivity, indicating a minimal hindrance to electron flow. Copper and aluminum are frequently utilized conductors owing to their exceptional conductivity and advantageous characteristics.
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.
The reciprocal of resistivity of a conductor is called its conductivity. It is denoted by the symbol σ.If a conductor has 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 component whose function in a circuit is to provide a specified value of resistance is called a resistor. The principal applications of resistors are to limit current, divide voltage and in certain cases, generate heat. Although there are a variety of different types of resistors, the following are the commonly used resistors in electrical and electronic circuits :
1.Carbon composition types
2.Film resistors
3.Wire-wound resistors
4.Cermet resistors
1.Carbon composition type
This type of resistor is made with a mixture of finely ground carbon, 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.
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
In a film resistor, a resistive material is deposited uniformly onto a high-grade ceramic rod. 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.
Figure 2
Metal film resistors have better characteristics as compared to carbon film resistors.
3.Wire-wound resistors
A wire-wound resistor is constructed by winding a resistive wire of some alloy around an insulating rod. It is then enclosed in an insulating cover. Generally, nickle-chromium alloy is used because of its very small temperature coefficient of resistance. Wire-wound resistors can safely operate at higher temperatures than carbon types. These resistors have high power ratings ranging from 12 to 225 W.
Figure 3
4.Cermet resistors.
A cermet resistor is made by depositing a thin film of metal such as nichrome or chromium cobalt on a ceramic substrate. They are cermet which is a contraction for ceramic and metal. These resistors have very accurate values.
Figure 4
Functions of Resistors
1. 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.2. Current Limiting: In most applications, the amount of advanced flowing by means of a point is necessary to be in commanded. Resistors are asked to play the role of limiters nowadays administering correct amounts of current in order not to burn out touchy elements, like LED chips.
3. Signal Conditioning: Notably, resistors are widely used in sign processing and conditioning circuits wherein they reduce or amplify indicators functioning to match impedances and keep the signals integrity.
Applications and Importance of Resistors
In every digital device, simple as flashlights to the most complex computer systems one can see resistors.
1. Audio Equipment: Use of resistors can be observed in some components, such as volume controls the modified tones and signal filtering.
2. Power Supplies: Voltage law and modern days prescribing both use resistors in energy supply circuits.
3. Transistors and Amplifiers: Resistors establish bias, and gain control are performed using amplifier circuits by resistor.
4. 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.
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