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How to Calculate Resistance in Physics

What is Resistance in Physics?

Definition of Resistance: Resistance is the opposition to the flow of charges (electrons) or current. It is important to know how to calculate resistance to design and build electrical circuits. The resistance of a conductor determines the amount of current that flows through it. It is measured in Ohms (Ω).

Resistance Formula

To calculate resistance, you need to have an understanding of Ohm’s Law, which states that the current through a conductor between two points is directly proportional to the voltage across the two points. The formula for Ohm’s Law is I = V/R, where I is the current, V is the voltage, and R is the resistance. The SI unit of Resistance is in Ohms.

Therefore, the resistance formula is: R = V/I

How to Calculate Resistance
How to Calculate Resistance

How to Calculate Resistance: The Basics

Resistance is a fundamental property of any electrical component that opposes the flow of electric current. It is measured in ohms (Ω) and helps in determining the behavior of circuits. To calculate resistance, you need to know two values: voltage (V) and current (I). The relationship between these quantities can be described using Ohm’s Law:

Resistance (R) = Voltage (V) / Current (I)

Now, we will see the various methods and techniques used to calculate resistance in different scenarios.

1. Using Ohm’s Law

Ohm’s Law, named after the German physicist Georg Simon Ohm, provides a straightforward way to calculate resistance when you know the voltage and current. By rearranging the formula, you can solve for resistance:

R = V / I


  • R is the resistance in ohms (Ω)
  • V is the voltage in volts (V)
  • I is the current in amperes (A)

For example, if you have a circuit with a voltage of 12V and a current of 2A, you can calculate the resistance as follows:

R = 12V / 2A = 6Ω

By using Ohm’s Law, you can quickly determine the resistance in various circuits and configurations.

2. Color Coding on Resistors

Resistors, the most common components used to introduce resistance in circuits, often have color-coded bands to indicate their resistance values. The color bands follow a specific pattern, allowing you to determine the resistance without the need for complex calculations.

Each color represents a numerical value, and the order of the colors determines the magnitude and tolerance of the resistance. You can refer to a color code chart or use online calculators to interpret the color bands accurately.

For instance, looking at a resistor with the following color bands: yellow, purple, red, and gold. Referring to the color code chart, we find that yellow represents 4, purple represents 7, red represents 2, and gold indicates a tolerance of ±5%. Combining these values, we get the resistance as:

47 x 102 ± 5%

Thus, 470 Ω ± 5%

Color coding simplifies resistance identification and makes it easier to calculate resistance in a circuit.

3. Series and Parallel Resistors

In circuits, resistors are often connected in series or parallel configurations. Calculating the total resistance in such cases requires a slightly different approach.

Series Resistors

When resistors are connected in series, the total resistance (R_total) is equal to the sum of individual resistances:

Rtotal = R1 + R2 + R3 + ……

For example, if you have three resistors with values 2Ω, 3Ω, and 4Ω connected in series, the total resistance is:

Rtotal = 2 + 3 + 4 = 9 Ω

Parallel Resistors

In a parallel configuration, the reciprocal of the total resistance (1/Rtotal) is equal to the sum of the reciprocals of individual resistances:

1/Rtotal = (1/R1) + (1/R2) + (1/R3) + …..

To calculate the total resistance in parallel, you can use the formula:

Rtotal = 1/ [(1/R1) + (1/R2) + (1/R3)]

For example, if you have two resistors with values 2Ω and 3Ω connected in parallel, the total resistance is:

Rtotal = 1/ [(1/2) + (1/3)] = 1.2Ω

Series and parallel configurations will help you to accurately calculate resistance in more complex circuit setups.

4. Wheatstone Bridge

The Wheatstone bridge is a circuit configuration commonly used to measure unknown resistances accurately. It consists of four resistors connected in a diamond shape, with an unknown resistor connected between two of them. By adjusting the values of the other three resistors, you can achieve a balance where the current through the galvanometer becomes zero.

The formula to calculate the unknown resistance using the Wheatstone bridge is:

Runknown = (R3 / R4) x ( R2)


  • Runknown is the unknown resistance.
  • R2 is the known resistance connected in parallel to the unknown resistor.
  • R3 and R4 are the two remaining resistors forming the bridge.

The Wheatstone bridge provides an accurate method for resistance measurement and is widely used in laboratory settings.

How to Measure Resistance: Example

To calculate resistance, you must use the formula R = V/I, where R is resistance in Ohms, V is voltage in volts and I is current in amperes. For example, if you have a circuit with a voltage of 12V and a current of 2A, you can calculate the resistance as R = 12/2 = 6 Ω.

Another way to calculate resistance is to use the color code on the resistor. Resistors are usually marked with colored bands that indicate the resistance value. Each band represents a digit, and the colors of the bands determine their values. The first band represents the first significant digit, the second band represents the second significant digit, and the third band represents the multiplier.

For example, a resistor with the colors yellow, purple, orange, and gold represents a value of 4.7 kΩ. The yellow band represents the first significant digit (4), the purple band represents the second significant digit (7), the orange band represents the multiplier (103), and the gold band represents the tolerance.

Tool for Measuring Resistance

Another useful tool for measuring resistance is an ohmmeter. An ohmmeter is a device that measures the resistance of an electrical circuit. To use an ohmmeter, you need to disconnect the circuit from the power source, connect the leads to the circuit, and read the resistance from the display.

Resistance vs Resistivity

The relationship between resistance and resistivity is: R = (ρL)/A


R = resistance (si unit in ohms, Ω)

ρ = Resistivity (si unit in ohm-metre)

L = length (si unit in metre)

A = area (si unit in metre squared) = πr2

r = radius (si unit in metre)


Find the resistance of a wire of length 0.65m, radius 0.2 mm, and resistivity 3 x 106 ohm-metre.


R = (ρL)/A = (ρL)/(πr2) = (3 x 106 x 0.65) / (3.142 x (2×10-4)2) = 15.5 Ω

Therefore, the resistance is 15.5 ohms (Ω)

FAQs (Frequently Asked Questions)

Q1: How do I calculate resistance if the voltage is not given?

A1: If the voltage is not provided, you cannot calculate resistance directly using Ohm’s Law. However, you can measure the current passing through the resistor using an ammeter and use that value along with the resistance formula.

Q2: Can resistance have a negative value?

A2: No, resistance is always a positive value. It represents the opposition to current flow, and a negative resistance would imply a current amplification rather than a restriction.

Q3: What happens if I exceed the power rating of a resistor?

A3: Exceeding the power rating of a resistor can cause it to overheat, potentially leading to failure or even a risk of fire. It is crucial to choose resistors with power ratings suitable for the circuit’s requirements.

Q4: Are there any practical applications of resistance calculations?

A4: Absolutely! Resistance calculations are fundamental in circuit design, electronic troubleshooting, power distribution, and various other fields related to electrical engineering.

Q5: Can I use a multimeter to measure resistance?

A5: Yes, multimeters are versatile tools that allow you to measure resistance accurately. Make sure to select the appropriate range on the multimeter and connect the leads properly for reliable measurements.

Q6: What is the significance of resistance in electronic circuits?

A6: Resistance plays a crucial role in controlling current flow, voltage division, and power dissipation in electronic circuits. It enables the precise operation of components and ensures the stability and safety of the entire system.


In conclusion, you can calculate resistance using Ohm’s Law, the color code on the resistor, or an ohmmeter. Understanding how to calculate resistance will help you design and build electrical circuits with accuracy and precision.

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