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Resistor - Symbol, Types, Unit, Color Code, FAQs

Resistor - Symbol, Types, Unit, Color Code, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:51 PM IST

Resistor –An electric friction

Let’s understand what is a resistor? And, also analyze its underlying concepts that involve topics like how resistor works and different types of resistors i.e. whether a resistor is fixed or a variable resistor, is it linear or non-linear and so on. We know that there are 3 classifications of materials based on whether they can conduct electricity or not- Conductors, semiconductors and insulators. Conductors can further be good conductors or poor conductors while insulators are the bad conductors. Metals (such as silver, copper, aluminium), salt solutions, acids are good conductors of electricity. This is because a large number of free electrons or ions (in solutions or acids) are present in them.
When an electric potential is applied across two terminals of a conductor, these electrons/ions start moving in a particular direction. But while moving, these electrons/ions collide with molecules/atoms of the conductor, which produce heat. So this property of hindrance or restriction of flow of electrons/ions is called resistance and the substance which is a metal or a salt solution or an acid itself is called a resistor and the resistor symbol looks like a zigzag line. Symbol of resistance is ‘R’ and the unit of resistance in which it is measured is called ‘ohm’.

This Story also Contains
  1. Resistor –An electric friction
  2. How to define resistance?
  3. Types of resistors
  4. what does a resistor do?
  5. Relation between temperature and resistance
Resistor - Symbol, Types, Unit, Color Code, FAQs
Resistor - Symbol, Types, Unit, Color Code, FAQs

A typical resistor looks like this (see fig-1) with these colour bands also called colour codes which tell us about the resistance value along with its tolerance.

(Fig-1)

In the below circuit (see figure 2) a resistor is connected to a battery , a switch and a bulb and with the potential applied, a current flows through the resistor which can be calculated using ohms law (V= IR), where V is the potential applied, R is the resistance of the resistor and I is the current flowing through it.

(Fig-2)

However symbol of a resistor in a circuit is (see fig- 3)

symbol of a resistor in a circuit

(Fig-3)

Thus in a circuit diagram the above symbol is used for resistors rather than drawing a resistor. They can be in combinations or connected to different circuit elements like capacitors, inductors, switch, ammeter, voltmeter, bulb etc.

There are various types of resistors which we will discuss later in this article along with its purpose in a circuit that is what does a resistor do in a circuit?

Also read -

Background wave

How to define resistance?

Resistance can be defined as property of a substance due to which it resists/hinders/opposes the flow of electricity/current comprised of electrons/ions through it.

Those substances which offer more resistance are poor conductor of electricity. E.g. - bakelite, mica, dry wood etc. Substances that offer too much resistance are good insulators and hence bad conductors of electricity. E.g.-paper, plastics (except P.V.C which is a poor conductor of electricity), distilled water, etc.

Unit of resistance

Resistance is measured in ohm (Ω). We can define 1 ohm of resistance as – A conductor offers 1 Ω of resistance if it allows 1A of current to flow through it when 1V of potential is applied across its terminals ( by ohms law- (V= IR) => R= V/I ).

For conductors, value of resistance is very small, of the order of 10-3 Ωwhile for insulators, it can be as high as 106Ω.

Concept of resistivity through laws of resistance

There are some factors on which resistance of a conductor depends .These factors are-

  1. Resistance is directly proportional to the length l of that conductor.

  2. It is inversely proportional to the cross sectional area A of that conductor.

  3. It depends on temperature of the conductor.

  4. It depends on type/nature of the material of the conductor.

Resistance depends on length and cross section area of a conductor

(Fig-4)

Thus from 1st, 2nd and 4rth factors –

R∝l/A => R= ρl/A -eq (1)

Where ρ is a constant called resistivity of a conductor which depends on nature of the material of the conductor, It is also called specific resistance.

Thus by eq (1) we can define resistivity of a conductor as the resistance of a conductor of length 1m and cross section area of 1 m2. Since if l=1m and A=1m2,R=ρ.

Unit of resistivity is Ω-m.

See the table below with values of resistivity of various materials-

values of resistivity of various materials

The reciprocal of resistance is called Conductance denoted by G. Thus conductance measures how easily the electrons/ions can pass through the conductor.

G=\frac{1}{\rho}.\frac{l}{A}=\sigma \frac{A}{l} , where σ is the reciprocal of resistivity also called conductivity or specific conductance.

Unit of Conductance is Ω-1 called mho(ʊ) or most commonly Siemens (S) and unit of conductivity is Siemens/meter (S/m).

Related Topics Link,

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Types of resistors

There are mainly 2 types of resistors –Linear resistors and Non-linear resistors. Linear resistors are those resistors whose resistance is constant irrespective of current flowing through it or voltage applied to it. These resistors follow ohms law, the graph of current vs. voltage gives a straight line. However resistance of a nonlinear resistor depends on current and voltage as well as temperature. They do not follow ohms law and the graph of current vs. voltage is not a straight line. Example of a nonlinear resistor is a varistor.

Graph of current vs. voltage in case of linear and non-linear resistors-


V-I graph in linear resistors give a straight line and vice versa

(Fig-5)

In our day to day life, we deal with linear resistors generally. The linear resistors can be further classified into-

  1. Fixed value resistors- These resistors have fixed value and thus we can use a particular value of resistor in a circuit according to our requirement. A typical fixed value resistor looks like this-

Fixed value resistors Symbol for a fixed resistor

  1. Variable resistors- These types of resistors can change their values or can be adjusted between a range of minimum to maximum values. There are different types of variable resistors but typically these are 3 terminal devices. Potentiometers and rheostats are common variable resistances we use in our labs.

image of a potentiometer

(Fig-6)

Variable resistor symbol is-

Variable resistor symbol


what does a resistor do?

Resistor is a very important component of a circuit. They are mostly used to limit the amount of current flowing in a part of circuit. However role of a resistor in a circuit is not limited to this only. They can be used as potential/voltage dividers, in impedance matching to maximize power transmission at high frequencies, in biasing purposes in transistors, etc.

Effect of temperature on resistance

With increase in temperature-

  1. resistance of pure metals increase.

  2. resistance in alloys increase although this increase is relatively small and irregular compared to pure metals. In some high resistance alloys like manganin , this increase is negligible for a range of temperature.

  3. resistance in insulators (like glass, mica, rubber) and electrolytes decrease.

Also Read:

Relation between temperature and resistance

Let metallic conductor have resistance R0 at 0 ̊C. If we heat it to t ̊C, let its resistance be Rt. Then in normal ranges of temperature, the increase in resistance that is ΔR=Rt-R0 depends on initial resistance and rise in temperature directly.

Therefore, ΔR∝R0 × t

  • Rt-R0 = αR0t - eq (2)

  • Rt=R0(1+ αt) -eq (3)

  • α= (Rt-R0)/R0t , where is called the temperature coefficient of resistance

Thus we can define temperature coefficient of resistance as the increase in resistance per unit original resistance per degree Celsius/Kelvin rise in temperature.

  • If in Kelvin, then Rt=R0( 1-α)(T-T0)

  • If in Celsius, then Rt=R0(1+ αt)

Similarly if l/A is divided both sides in the equation (3), we get-

l=100m, A=0.1mm2, ρ=5×10-8Ω-m

ρt=ρ0(1+ αt) -eq(4)

The graph below shows change in resistance with temperature-

(Fig-7)

Also check-

NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is the resistance of a 100m length wire having a uniform cross-sectional area of 0.1mm2.If the wire is made up of manganin having a resistivity of 5×10-8 Ω-m.


 l=100m, A=0.1mm2, ρ=5×10-8Ω-m

               As, R=ρl/A => putting all the above values we get-  R=500Ω

2. If a wire is drawn out to 3 times its original length, by how many times its resistance is expected to be increased?

Since volume is same in both cases => l'A'=l×A => 3l×A'=l×A =>A' =A/3

                R'=ρ×l'/A' = (ρ×3l)/ (A/3) =>R'=9ρl/A = 9R

  • resistance increases by 9 times

3. Find the factor by which resistance of a wire increases if its diameter is doubled.

 R= ρl/A    => R∝1/πr2 , where r is the radius of the cross section of wire.

     R∝1/ D2     =>If D doubles, R becomes ¼.       

4. What is reactance?

Reactance is the resistance in a circuit or a part of a circuit if the current flowing through it is alternating or time varying.

5. What is impedance?

Impedance is a broader term which encompasses both reactance and resistance. It is often expressed as a complex number and hence considered as a vector.

6. What is the SI unit of resistance?
The SI unit of resistance is the ohm, symbolized by the Greek letter omega (Ω). One ohm is defined as the resistance that allows one ampere of current to flow when one volt of potential difference is applied across it.
7. How does the length of a wire affect its resistance?
The resistance of a wire is directly proportional to its length. Doubling the length of a wire doubles its resistance. This is because electrons have to travel a longer distance, encountering more collisions with atoms in the material. This relationship is described by the equation R = ρL/A, where R is resistance, ρ is resistivity, L is length, and A is cross-sectional area.
8. How does the cross-sectional area of a wire affect its resistance?
The resistance of a wire is inversely proportional to its cross-sectional area. Doubling the cross-sectional area halves the resistance. This is because a larger cross-section provides more space for electrons to flow, reducing collisions and thus resistance. The relationship is described by the equation R = ρL/A, where R is resistance, ρ is resistivity, L is length, and A is cross-sectional area.
9. How do you calculate the equivalent resistance of resistors in series?
To calculate the equivalent resistance of resistors in series, simply add the individual resistance values. For example, if you have three resistors R1, R2, and R3 in series, the equivalent resistance Req = R1 + R2 + R3. This is because in a series circuit, the same current flows through each resistor, and the voltage drops across each resistor add up to the total voltage.
10. How do you calculate the equivalent resistance of resistors in parallel?
For resistors in parallel, the equivalent resistance is calculated using the reciprocal of the sum of reciprocals. The formula is 1/Req = 1/R1 + 1/R2 + 1/R3 + ..., where Req is the equivalent resistance and R1, R2, R3, etc. are the individual resistances. This is because in a parallel circuit, the voltage across each resistor is the same, but the current is divided among the resistors.
11. How does temperature affect a resistor's resistance?
Temperature can significantly affect a resistor's resistance. For most materials, as temperature increases, resistance increases. This is due to increased atomic vibrations at higher temperatures, which impede electron flow. However, some materials, like certain semiconductors, can exhibit decreased resistance with increased temperature.
12. What is a thermistor and how does it work?
A thermistor is a type of resistor whose resistance changes significantly with temperature. There are two main types: Negative Temperature Coefficient (NTC) thermistors, whose resistance decreases as temperature increases, and Positive Temperature Coefficient (PTC) thermistors, whose resistance increases with temperature. Thermistors are widely used in temperature sensing and control applications.
13. What is power dissipation in a resistor?
Power dissipation in a resistor refers to the electrical energy converted into heat energy as current flows through it. It's calculated using the formula P = I²R or P = V²/R, where P is power in watts, I is current in amperes, R is resistance in ohms, and V is voltage across the resistor. Understanding power dissipation is crucial for selecting appropriate resistors for a circuit.
14. How does the material of a resistor affect its properties?
The material of a resistor significantly influences its properties. Different materials have varying resistivities, temperature coefficients, and power handling capabilities. For instance, carbon composition resistors are inexpensive but less stable with temperature changes, while metal film resistors offer better stability and precision. Wirewound resistors, made of metal wire, can handle high power but have more inductance, making them less suitable for high-frequency applications.
15. How do you determine the appropriate wattage rating for a resistor?
To determine the appropriate wattage rating for a resistor, calculate the power it will dissipate in the circuit using P = I²R or P = V²/R. Then, choose a resistor with a wattage rating higher than this calculated value, typically at least double for safety. This ensures the resistor can handle the power without overheating or failing.
16. What is the purpose of a current-limiting resistor?
A current-limiting resistor is used to restrict the amount of current flowing through a circuit or component. It's often employed to protect sensitive components from excessive current, which could cause damage or malfunction. For example, when using an LED with a power source that exceeds its voltage rating, a current-limiting resistor is necessary to ensure the LED receives the appropriate current.
17. What is a shunt resistor and how is it used?
A shunt resistor is a low-value resistor used to measure electric current indirectly. It's connected in parallel with an ammeter or in series with the circuit being measured. The voltage drop across the shunt resistor is proportional to the current flowing through it, allowing current measurement by measuring voltage. Shunt resistors are particularly useful for measuring high currents that would be impractical to measure directly.
18. What is a varistor and how does it protect circuits?
A varistor (variable resistor) is a type of non-linear resistor whose resistance decreases as the voltage across it increases. It's commonly used for surge protection in electronic circuits. When a voltage spike occurs, the varistor's resistance drops dramatically, shunting the excess current away from sensitive components. Once the voltage returns to normal, the varistor's resistance increases again. This behavior makes varistors effective in protecting against transient voltage surges.
19. What is the purpose of a bleeder resistor in a power supply?
A bleeder resistor is a high-value resistor connected across the output of a power supply. It serves several purposes:
20. How do resistors affect the Q factor of an RLC circuit?
In an RLC (Resistor-Inductor-Capacitor) circuit, the Q factor (Quality factor) is a measure of the circuit's resonance characteristics. The Q factor is inversely
21. What is the difference between fixed and variable resistors?
Fixed resistors have a set resistance value that doesn't change, while variable resistors (also called potentiometers or rheostats) allow the resistance to be adjusted. Variable resistors are useful in applications where you need to control current or voltage, such as volume controls in audio equipment.
22. How do carbon composition resistors differ from metal film resistors?
Carbon composition resistors are made of a mixture of carbon and clay, while metal film resistors have a thin metal film deposited on a ceramic rod. Metal film resistors generally offer better precision, stability, and lower noise than carbon composition resistors. However, carbon composition resistors can handle higher power and voltage surges better.
23. What is the difference between linear and non-linear resistors?
Linear resistors obey Ohm's Law, meaning their resistance remains constant regardless of the applied voltage or current. Non-linear resistors, on the other hand, have a resistance that varies with the applied voltage or current. Examples of non-linear resistors include thermistors, varistors, and light-dependent resistors (LDRs). Non-linear resistors are often used in sensing and protection applications.
24. What is tolerance in resistors and why is it important?
Tolerance in resistors refers to the maximum deviation from the nominal resistance value, usually expressed as a percentage. It's important because it indicates how precise the resistor's actual value is compared to its stated value. Lower tolerance resistors are more accurate but typically more expensive. Understanding tolerance is crucial for designing circuits that require precise resistance values.
25. How do you read a resistor's color code?
A resistor's color code consists of colored bands that represent its resistance value and tolerance. The first two bands represent the first two digits of the resistance value, the third band is the multiplier, and the fourth band (if present) indicates the tolerance. Each color corresponds to a specific number or multiplier value.
26. Why are resistors important in electrical circuits?
Resistors are crucial in electrical circuits because they control the flow of electric current and voltage distribution. They help protect sensitive components from excessive current, divide voltage, and can be used to create heat or light in certain applications. Without resistors, many circuits would not function properly or safely.
27. Why are resistors often connected in series or parallel?
Resistors are connected in series or parallel to achieve specific total resistance values that may not be available as single components. Series connections increase total resistance, while parallel connections decrease it. This allows for precise control of current and voltage in complex circuits.
28. What is the purpose of a pull-up or pull-down resistor?
Pull-up and pull-down resistors are used to ensure a known state for a signal line when no other active device is driving it. A pull-up resistor connects the line to the positive supply voltage, while a pull-down resistor connects it to ground. This prevents the input from "floating" and potentially causing unpredictable behavior in digital circuits.
29. What does a resistor symbol look like in a circuit diagram?
A resistor is represented by a zigzag line or a rectangle in a circuit diagram. The zigzag symbol is more common and looks like a series of sharp peaks and valleys. This symbol visually represents the resistor's function of impeding the flow of electric current.
30. What is Ohm's Law and how does it relate to resistors?
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. Mathematically, it's expressed as V = IR, where V is voltage, I is current, and R is resistance. This law is fundamental in understanding how resistors behave in circuits.
31. What is a potentiometer and how does it work?
A potentiometer is a three-terminal variable resistor. It has a resistive element with a sliding or rotating contact (wiper) that moves along the element. As the wiper moves, it changes the ratio of resistance between its two outer terminals. Potentiometers are commonly used for adjusting volume, brightness, or other variable settings in electronic devices.
32. How do resistors affect the brightness of LEDs in a circuit?
Resistors are crucial for controlling the brightness of LEDs and protecting them from damage. LEDs require a specific forward voltage and have a maximum current rating. A resistor in series with an LED limits the current flowing through it, preventing excessive current that could burn out the LED. By adjusting the resistor value, you can control the LED's brightness while ensuring it operates within safe parameters.
33. What is the significance of the temperature coefficient of resistance?
The temperature coefficient of resistance (TCR) indicates how much a resistor's value changes with temperature. It's usually expressed in parts per million per degree Celsius (ppm/°C). A positive TCR means resistance increases with temperature, while a negative TCR means it decreases. Understanding TCR is crucial in applications where temperature variations could significantly affect circuit performance, such as in precision measurement equipment or temperature-sensitive environments.
34. How do surface-mount resistors differ from through-hole resistors?
Surface-mount resistors are designed to be mounted directly onto the surface of a printed circuit board (PCB), while through-hole resistors have leads that are inserted through holes in the PCB. Surface-mount resistors are generally smaller, allowing for higher component density and automated assembly. Through-hole resistors are easier to handle manually and can dissipate heat more effectively. The choice between them often depends on the specific application requirements and manufacturing process.
35. How do resistors contribute to voltage division in a circuit?
Resistors can be used to create a voltage divider circuit, which reduces a larger voltage to a smaller one. In a simple voltage divider, two resistors are connected in series across a voltage source. The output voltage is taken from the junction of the two resistors and is proportional to the ratio of the resistances. This principle is widely used in various applications, from simple level shifting to more complex analog-to-digital converters.
36. What is the difference between precision and general-purpose resistors?
Precision resistors have tighter tolerance (typically 1% or better) and lower temperature coefficients compared to general-purpose resistors. They maintain their resistance value more accurately over time and temperature changes. Precision resistors are used in applications requiring high accuracy, such as instrumentation, test equipment, and high-end audio. General-purpose resistors, with typical tolerances of 5% or 10%, are suitable for less critical applications and are more cost-effective.
37. How do wirewound resistors differ from other types, and where are they commonly used?
Wirewound resistors are made by winding a metal wire around an insulating core. They can handle high power and have very low tolerances, making them suitable for precision applications. However, they have higher inductance compared to other types, which can affect performance in high-frequency circuits. Wirewound resistors are commonly used in power supplies, motor controls, and other high-power applications where their ability to dissipate heat is advantageous.
38. What is the purpose of a fusible resistor?
A fusible resistor is designed to act as both a resistor and a fuse. Under normal operating conditions, it functions as a regular resistor. However, if excessive current flows through it, the resistor will open (like a fuse), protecting the circuit from damage. Fusible resistors are often used in power supplies and other applications where both current limiting and overcurrent protection are needed in a single component.
39. How do resistors affect the time constant in RC circuits?
In an RC (Resistor-Capacitor) circuit, the resistor plays a crucial role in determining the time constant. The time constant τ (tau) is calculated as τ = R * C, where R is the resistance in ohms and C is the capacitance in farads. This time constant represents how quickly the capacitor charges or discharges through the resistor. A larger resistance value results in a longer time constant, meaning slower charging and discharging of the capacitor.
40. How do resistors contribute to noise in electronic circuits?
All resistors generate electrical noise due to thermal agitation of charge carriers, known as Johnson-Nyquist noise. This noise is proportional to temperature and resistance value. In sensitive analog circuits, this noise can be significant and may affect the signal-to-noise ratio. Different resistor types have varying noise characteristics; for example, wirewound resistors generally produce less noise than carbon composition resistors. Understanding and minimizing resistor noise is crucial in designing low-noise amplifiers and other precision analog circuits.
41. What is the difference between a rheostat and a potentiometer?
While both rheostats and potentiometers are variable resistors, they are used differently in circuits. A rheostat is a two-terminal device used to vary current in a circuit by changing its resistance. It's typically used for high-power applications like dimming lights or controlling motor speed. A potentiometer is a three-terminal device used primarily for voltage division. It's commonly used in low-power applications like volume controls or sensor interfaces. The key difference is that a potentiometer is used to vary voltage, while a rheostat is used to vary current.
42. How do resistors affect the bandwidth of a circuit?
Resistors play a significant role in determining a circuit's bandwidth, especially in combination with capacitors or inductors. In an RC low-pass filter, for example, the cutoff frequency (which defines the bandwidth) is inversely proportional to the product of resistance and capacitance. Increasing the resistance lowers the cutoff frequency, reducing the bandwidth. In amplifier circuits, resistors in feedback loops can affect gain and bandwidth. Understanding this relationship is crucial in designing filters, amplifiers, and other frequency-dependent circuits.
43. What is a photoresistor (LDR) and how does it work?
A photoresistor, also known as a light-dependent resistor (LDR), is a component whose resistance decreases when exposed to light. It's made of a semiconductor material like cadmium sulfide. In darkness, it has high resistance, but when light falls on it, the energy from photons frees electrons, increasing conductivity and lowering resistance. LDRs are used in light-sensing applications such as automatic street lights, camera light meters, and light-activated switches.
44. How do resistors affect the gain of an amplifier circuit?
Resistors play a crucial role in determining the gain of amplifier circuits. In a basic operational amplifier configuration, the gain is set by the ratio of feedback resistor to input resistor. For example, in a non-inverting amplifier, the gain is given by G = 1 + (Rf / Rin), where Rf is the feedback resistor and Rin is the input resistor. By carefully selecting these resistor values, engineers can precisely control the amplification factor of the circuit.

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