Question about Electromagnetic Forces

[lugita15]

On page 13-3 of his Lectures on Physics, Feynman says: "We have seen that there is a force on a wire in the presence of a magnetic field, produced, say, by a magnet. From the principle the action equals reaction we might expect that there should be a force on the source of the magnetic field, i.e., on the magnet, when there is a current through a wire.*" In the footnote, it says:"We will say later, however, that such assumptions are not generally correct for electromagnetic force." Does that mean that for electromagnetic forces, the law of action and reaction does not hold? If so, where does Feynman discuss this "later"? Also, is there a simple example where the law of action and reaction does not hold?

Any help would be greatly appreciated.
Thank You in Advance.

 

[Doc Al]

I don't have my Feynman lectures handy, but this thread might help: violation of action-reaction law?

 

[astrokreng]

I can provide some clarification on this topic. In this context, Feynman is referring to the electromagnetic force, which is one of the fundamental forces of nature. This force is responsible for the interaction between electrically charged particles, and it is also the force responsible for the interaction between a wire carrying a current and a magnetic field.

The law of action and reaction, also known as Newton's third law, states that for every action, there is an equal and opposite reaction. This law is a fundamental principle in classical mechanics and is applicable to many physical situations. However, when it comes to electromagnetic forces, this law does not always hold true.

Feynman mentions that this assumption is not generally correct for electromagnetic forces because the force on the source of the magnetic field, in this case, the magnet, is not equal in magnitude to the force on the wire. This is because the force on the magnet is dependent on the strength and direction of the current in the wire, while the force on the wire is dependent on the strength and direction of the magnetic field.

Feynman does not discuss this further in his lectures, but this topic is covered in more detail in his book "The Feynman Lectures on Physics, Volume II: The New Millennium Edition". In this book, he explains that the law of action and reaction does not hold for electromagnetic forces because they are carried by virtual particles, called photons, which do not have mass and therefore do not obey Newton's third law.

An example where the law of action and reaction does not hold for electromagnetic forces is when two electrically charged particles interact with each other. In this case, the force on each particle is not equal in magnitude and opposite in direction, as predicted by Newton's third law, but rather depends on the charges and distances between the particles.

In summary, while the law of action and reaction is a fundamental principle in classical mechanics, it does not always hold true for electromagnetic forces. This is because these forces are carried by massless particles and are dependent on the properties of the particles involved. Further reading on this topic can be found in Feynman's book mentioned above or in other textbooks on electromagnetism.