Difference Between Law and Principle in Physics

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Almost every aspect of physics can seem confusing at first. Sometimes, even the language seems more abstract than need be. For example, your teacher might nonchalantly introduce a principle of physics and later start talking about a related law, all without explaining the difference between the two terms. But even if your teacher tends to gloss over such details, knowing the difference can help you in your progress toward understanding the rules behind the physical world.

Principles Are All Around

Principles, in physics, are more like guiding ideas than anything. According to F. Woodbridge Constant, former chairman of the physics department at Trinity College and author of both “Fundamental Principles of Physics” and “Fundamental Laws of Physics,” a principle tells physicists how the world works in general. Principles tend not to be specific rules that you can write down in mathematical terms. Instead, they are more like guiding theories that allow scientists to make predictions about new phenomena and develop new laws that clearly explain the phenomena.

Laws Are on Paper

In contrast to the sometimes wishy-washy wording of principles, laws are perfectly clear. This is because physicists define laws with mathematical statements. You cannot subjectively interpret mathematical statements, which makes them more reliable than principles in clearly defining the rules of nature in a way that all physicists can agree on. One key difference between a law and a principle is that laws have no exceptions. Physicists prove a law via the mathematics of physics, and proofs make no room for exceptions.

Uncertainty and Principles

Principles have a tint of uncertainty, mainly arising from their lack of clear mathematical definition. For example, the uncertainty principle, a curiously named principle of quantum physics, states that when you measure two aspects of a physical particle, such as in measuring both the position and velocity of an object, you can never know both of the aspects with perfect precision. While this principle does not have a mathematical form of its own, it has guided -- as all good principles do -- physicists to develop specific mathematical statements for specific cases, such as for the position and momentum of a particle moving in a box.

Ohm My Law!

Just like laws in mathematics, laws in physics are unbreakable. Take Ohm’s law, for example, which states that current is equal to the potential difference divided by the resistance in a conductor. Written mathematically, Ohm’s law says that I=V/R, where “I” is current, “V” is potential difference and “R” is resistance. Because a physical law is always true, if you know two of the values in Ohm’s law, you can always calculate the third. Ohm’s law applies to specific situations, making it useful for unambiguous tasks, such as determining how to design a circuit. This is why laws are so important in applied physics and engineering.


  • Fundamental Principles of Physics; F. Woodbridge Constant
  • Fundamental Laws of Physics; F. Woodbridge Constant
  • Physics of Everyday Phenomena; W. Thomas Griffith and Juliet Brosing

About the Author

Having obtained a Master of Science in psychology in East Asia, Damon Verial has been applying his knowledge to related topics since 2010. Having written professionally since 2001, he has been featured in financial publications such as SafeHaven and the McMillian Portfolio. He also runs a financial newsletter at Stock Barometer.

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