A more general picture of Newton`s third law is simply to say that the total momentum is conserved. This is important for cases where electric and magnetic fields are present. In these cases, the addition of forces to ordinary « objects » does not give zero, because the dynamics in the fields themselves change a little. The magnetic force is very different. The moving proton contributes a zero magnetic field at the electron site and therefore no magnetic force acts on the electron, while the electron introduces a non-zero magnetic field in the side at the proton site, so that a magnetic force acts on the proton in the +y direction. The magnetic force does not have the property of reciprocity. In other words, forces do not obey Newton`s third law, which means that this « law » is not fundamental, but a relationship that applies to electrical and gravitational interactions, but not to all kinds of interactions. This raises an interesting question. Choose the proton plus the electron as the system of interest. The net force on this apparently isolated system is non-zero, meaning that the total momentum of the system changes, meaning that the momentum is not a quantity conserved in the presence of magnetic forces. In a 1942 paper by J.

M. Keller, « Newton`s Third Law and Electrodynamics » (American Journal of Physics 10, 302-307, doi.org/10.1119/1.1990405), the author discusses this situation and shows that when the momentum of the field is added to the momentum of the particles, this total momentum does not actually change. When a live rectangular loop is placed in a magnetic field, the forces acting on either side of the loop perpendicular to the field provide torque that rotates the loop. I can`t give a tutorial here about electromagnetic fields that carry both energy and momentum. Here is a Wikipedia article on the « radiation pressure » related to field pulse: I read this explanation in your book on magnetic materials (nice explanation, by the way, really). « We will give two more examples of pulses in the electromagnetic field. We have pointed out in section 26-2 the failure of the law of action and reaction when two charged particles move on orthogonal trajectories. The forces on the two particles do not balance, so the action and reaction are not the same; Therefore, the net dynamics of matter must change. It is not kept.

But the dynamics in this area also change in such a situation. If you calculate the momentum given by the Poynting vector, it is not constant. However, the change in particle momentum is formed only by the momentum of the field, so the total momentum of the particles plus the field is conserved. It depends on the form. Two magnetic bars line up next to each other in opposite directions. Two magnetic disks are aligned, as you point out. In the last chapter (radiation) of our manual is a brief analysis of why light can change the momentum of an object. The electric field in light displaces the charge, and the magnetic field in the light exerts a force on the moving charge that turns out to be in the direction of the propagating light. The momentum of the field decreases and the momentum of the particles increases. If the fields change in light (for example: by changing the acceleration of the emitting charges), then, of course, the momentum of the field changes. However, according to Newton`s 3rd law, an equal and opposite force should act on the object exerting the force. But in the fields, it is the field that exerts forces on objects.

Does this mean that the loop exerts an equal and opposite force on the magnet or on the magnetic field? How do these forces affect and how do they affect the magnet? Newton`s third law makes that magnets have equal and opposite effects on each other, whether they are electromagnets or permanent magnets. The same and opposing forces are transmitted by the magnetic field. Just like in an electric motor or generator, field magnets press/pull on the armature via their magnetic fields, the armature also pushes/pulls the field magnets. If you hold a small electric motor in your hand when you start, you can feel the anti-spin torque of the motor housing when it applies rotational torque to the armature. I think that`s true, if they can *only* shoot. But if they can move spatially, I think it would be like putting two magnets next to each other, they pinch each other. 1. It is not entirely obvious to me what the final state would be, but note that if you place two magnetic bars next to each other, with their magnetic moments in the same direction, they rotate in such a way that their magnetic moments are opposite to each other; This is the stable position. At the moment I am holding 2 magnets, disc-shaped, so I can find a stable position when they are side by side (as you mentioned), due to the shape of the disc. But they usually want to be parallel to each other. It is obviously also dependent on form. In magnetic interactions between closed current loops, the reciprocity of forces holds (Newton`s third law).

It is interesting to see how this happens in detail. In the Web-VPython program shown below, the two current-carrying wire loops are divided into many short segments. The Biot-Savart law is used to calculate the magnetic field that one loop contributes to the position of each segment of the other loop and the magnetic force exerted on this segment. It also calculates the torque of this force around the center of the scene (at the left edge of the right loop, which is the origin of the coordinate system, where x is on the right, y is at the top, and z is outside the side). The cyan arrows represent the magnetic field at one place around a loop, due to the other loop, and the red arrows represent the magnetic force on that segment, due to the magnetic field of the other loop. Taking into account the tiny components due to numerical rounding, the calculation shows that the net forces on the two loops have the same size and opposite directions. The result is not obvious if we consider the very different distribution of forces between the two loops. Yes, the problem lies not in your understanding of magnetism, but in Newton`s third law. The usual language about « action » and « reaction » is simply confusing.

The law is very simple. The force between two objects is the same for each and points in opposite directions on each. In other words, the sum of forces is zero. Take two magnets and they repel each other (opposite directional forces) or attract each other (opposite directional forces), depending on their orientation. In both cases, the third law is followed. There you go. That both possibilities exist has nothing to do with the third law. It also applies to gravity, for which only forces are attractive.

Since the net force on the left loop is in the -z direction and the net force on the right loop is in the +z direction, one would expect the torques around the center of the scene to both be in the -y direction. The forces on the segments of the left loop are all in the -z direction, so the torque must necessarily be in the -y direction. However, in the right loop, the force is only about a third of the loop in the +z direction, and the distance to the center of the screen is small (remember that the center of the screen is on the left edge of the right loop). There are smaller forces on the rest of the loop, but they are all in the -z directions and the distances from the center of the screen are large. The net effect is that the net torque at the right loop is actually in the +y direction, with the same amplitude as the torque on the left loop. The net torque of the combined system is zero. Could you explain in simpler language what exactly « field pulse » means? It`s a bit of a vague term in my head, I can`t really understand what it means. And, the resulting question, how can the dynamics in this area change? I already use vpython in my laptop.

How do I embed a diagram I create with vpython into a website? A very simple way is to simply use a screenshot tool to get the diagram image and insert it into your web page. If you are using Web VPython (webvpython.org or glowscript.org), you can click « Share or Export This Program » while editing to get an HTML file that you can insert into an iframe on your website, as in my article under brucesherwood.net/?p=191 « Magnetic forces do not work (network) ». Thank you for pointing that out. I thought I remembered Feynman laying this riddle, but when I searched for it recently, I didn`t find it. Another mystery is whether it was reasonable to imagine that even a very good Caltech student could find the answer. Thank you for your quick response. Vpython is awesome! I hope you find time to answer, all the best Professor Bruce! Consider the following interaction between a proton and an electron: 2. I don`t really know the nature of the proof of reciprocity for closed circuits. I was told that this could be proven, and I only quote the result, but then I show the details of the fields and forces that I had not seen before. The electrical forces between the proton and the electron exhibit « reciprocity ».

That is, the electric force exerted by the proton on the electron is equal and opposite to the electric force exerted by the electron on the proton. The reason, of course, is that the electric force is proportional to q1q2 (which corresponds to q2q1) and is also proportional to the vector pointing from one charge to another. (Similarly, gravitational forces also exhibit reciprocity, since the gravitational force is proportional to m1m2 (which corresponds to m2m1) and is also proportional to the vector pointing from one mass to another.) If the loops can move freely and start from rest, the left loop receives an impulse in the –z direction in the first short time interval and the right loop receives the same impulse force in the +z direction.