A Guide to How Joule's Law Works

Joule's Law describes the relationship between heat (or power), current, and resistance in a conductor, where heat produced is proportional to the square of the current, the resistance, and the time when current is flowing.

James Joule formulated this law at the age of 22 between 1840 and 1841, his first of many laws. Through scientific experimentation, Joule studied the relationship between heat produced and electric current in a conductor, discovering one of the fundamental laws of electrical circuits.

Joule's Law Explained in Simple and Physical Terms

Imagine a block sliding on a desk. If you were to slide the block back and forth, you wouldn't notice an increase in heat. If you were to slide it faster and harder however, you would notice that the block is getting hotter due to the friction between the desk and the block. This is converting mechanical energy to thermal energy.

This example is a useful way to understand Joule's Law, which describes how electrical energy is converted into thermal energy. In this analogy,

• Heat due to friction would be like the heat generated from electrical resistance
• How often and how forcefully the block is pushed is similar to electric current
• Friction between the block and the desk is similar to electrical resistance

There is an important clarification for beginners, however:

Electric current is not similar to the speed of the block. Current is more like how much water flows per second, or electrically, the rate at which charge flows.

Joule's Law Mathematically Explained

$Q = I^2Rt$

Where

• $Q$ is the thermal heat produced, measured in Joules ($J$)
• $I$ is the current flowing through the conductor in amperes ($A$)
• $R$ is the resistance of the conductor in Ohms ($\Omega$)
• $t$ is the period of time the current is flowing in the conductor in seconds ($s$)

To quickly calculate heat, current, or resistance without doing the algebra by hand, try out our Joule's Law Calculator.

What Joule's Law Teaches Us

Just by looking at Joule's Law, you may notice a few things:

• Doubling the current produces 4 times as much heat
• High resistance materials heat up easier
• Circuits with less resistance don't lose as much energy to heat

This is why electronics engineers try to design circuits that have a minimal amount of resistance in order to keep as much energy as possible in the form of electricity, not heat.

Of course, there are circuits whose sole purpose is to generate heat, such as blow dryers, space heaters, and toasters. Below is a video of a piece of nichrome wire heating up as the current in it increases:

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The reason the wire was able to get hot with so little electricity was because of its resistance. This particular wire segment had a resistance of about $2\Omega$. This way more than other traditional conductors whose resistance would have been almost 0.

If you want to understand why increasing the voltage in the wire increases the resistance, check out this article on Ohm's Law.

Joule's Law Put Into Practice

It's important to know how to apply Joule's Law in real life. Below are a few practice problems to familiarize yourself with Joule's Law.

Beginner Problem (Solving for Current)

A wire of resistance $5\Omega$ dissipates $500J$ of energy in $10$ seconds. How much current was applied to the wire?

Intermediate Problem (Solving for Energy Dissipated)

An electric heater has a resistance of $12\,\Omega$ and operates with a current of $4\,\text{A}$ for $5$ minutes. How much thermal energy is dissipated by the heater?

Harder Problem (Using Ohm's Law and Joule's Law Together)

A resistor connected to a $24\,\text{V}$ power supply dissipates $2{,}400\,\text{J}$ of energy in $2$ minutes.
What is the resistance of the resistor?

Conclusion

Joule's Law reveals how electrical energy is converted into heat, making it essential for understanding power loss, thermal limits, and efficiency in real circuits. However, Joule's Law does not operate in isolation. To fully understand any electrical system, it must be used alongside other fundamental circuit Laws that govern voltage, current, and energy flow.

For example, Ohm's Law explains how voltage, current, and resistance are related, providing the current values that Joule's Law depends on. Kirchhoff's Voltage Law and Kirchhoff's Current Law extend this analysis to complex circuits by enforcing energy and charge conservation across loops and junctions. Together, these Laws form a complete toolkit for predicting circuit behavior under real operating conditions.

To continue building a deeper understanding of how electrical circuits work as a unified system, explore our detailed guide on Ohm's Law and its real-world applications, where voltage–current relationships are developed step by step and connected directly to practical circuit analysis.