What is Ohm’s Law and how is it related to resistors?

What is Ohm's Law?

Ohm's Law is a fundamental principle in electrical engineering and physics that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit.

It states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them.

This simple yet powerful relationship is almost always expressed by the formula:

V = I × R

Where:

• V is Voltage, measured in Volts (V)

• I is Current, measured in Amperes or Amps (A)

• R is Resistance, measured in Ohms (Ω)

By rearranging this formula, we can solve for any one variable if we know the other two:

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

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

How is it Related to Resistors?

The relationship is absolute and defining. Here’s how:

1. Resistors are the "R" in V=IR

A resistor is a component specifically designed to provide a known amount of resistance (R) in an electrical circuit. The sole purpose of a resistor is to impede the flow of electric current. Ohm's Law is the mathematical tool we use to predict exactly how much it will impede the current for a given voltage.

2. Resistors are "Ohmic" Devices

Most standard resistors are "ohmic" devices. This means they obey Ohm's Law perfectly—their resistance value (R) is constant and does not change with the voltage applied or the current flowing through them.

• If you double the voltage (V) across a resistor, the current (I) through it will also double.

• If you halve the voltage, the current will halve.

• The resistance value itself remains unchanged.

This predictable, linear relationship is what makes resistors so essential for controlling circuits.

3. Practical Applications in Circuit Design

Ohm's Law allows engineers and hobbyists to use resistors for specific purposes:

• Limiting Current: This is the most common job of a resistor. For example, an LED might only be able to handle 20mA (0.02A) of current. If you have a 9V power supply, you can use Ohm's Law to calculate the required resistor to protect the LED.

  • R = V / I = 9V / 0.02A = 450Ω

  • You would place a 450Ω resistor in series with the LED to limit the current to a safe 20mA.

• Creating Voltage Drops: Resistors are used to create specific voltages at different points in a circuit (this is the principle behind a "voltage divider" circuit). Ohm's Law tells you what voltage will appear across each resistor.

• Dissipating Power: Resistors convert electrical energy into heat. The power (P) dissipated by a resistor is also derived from Ohm's Law and is given by:

  • P = I² × R or P = V² / R

  • This tells you if the resistor is the right size (wattage rating) to handle the power without burning up.

A Simple Analogy: Water in a Hose

A common analogy to understand V, I, and R is a water hose:

• Voltage (V) is like the water pressure pushing the water.

• Current (I) is like the flow rate (how much water is flowing per second).

• Resistance (R) is like the width of the hose. A narrow hose (high resistance) restricts the flow, while a wide hose (low resistance) allows more water to flow.

Ohm's Law says: Flow (Current) = Pressure (Voltage) / Hose Restriction (Resistance)

A resistor is like putting a specific, precisely-sized nozzle on the end of the hose to control the water flow exactly how you want it.

In a nutshell: Ohm's Law (V = I × R) is the mathematical formula that defines exactly how a resistor will behave in a circuit. The resistor is the physical component that provides the resistance (R), and Ohm's Law is the rule that governs it.