# where does the kinetic energy come from for magnetic attraction/repulsion?

262 views

I understand the basics of magnetism; charge, fields, etc, but I don’t understand how a magnetic field can apply force seemingly forever.

For example, if I bring a magnet close to another magnet, that other magnet will eventually be drawn to the magnet in my hand, causing it to move. That kinetic energy “is caused by the magnetic attraction” but how?

If I keep my hand between two magnets, they will squeeze against my hand for seemingly forever. If I were to squeeze my own hand, I’ll get tired eventually from the exertion, but magnets don’t “get tired.” Are magnets somehow infinite energy machines? I’d assume they aren’t, but where is this energy coming from and why does it seem endless?

Edit: yes I understand the difference between “force” and “energy,” defining those terms doesn’t actually answer my question!

In: Physics

Normal magnets typically do “become tired” and lose their net magnetic field over time, especially if you heat them up.

You need to put energy into the system to bring a magnet close to another repulsive one. That energy is then reversed when it pushes it away. Same goes for magnets that attract, except that you’re giving energy to the system when you’re pulling them apart.

For the scenario when you’re holding the magnet, there’s no energy to talk about at that moment. Yes there is a constant force, but nothing is accelerating. You’ll get tired because of the way our muscles work, not because there’s some sort od energy flow happening. It’s the same scenario as holding something heavy in your hands, gravity is constantly pulling it down and you feel like you have to use energy to fight it, while at the same time you could just put the object down onto a table and suddenly there’s no energy being wasted to hold it in place. Energy is only used when you’re moving an object away from an attractive force (gravity, magnet) or accelerating it.

There is magnetic “potential energy” in the magnets when they are far apart. You bringing them closer together allows them to apply more force to each other, eventually overcoming friction and making the magnet move.

The force between the magnets is constant, if they are well made, and it will never get “tired”. When you pull the magnets apart, you use your muscles to put potential energy back into the system, this allows them to convert it back into kinetic energy in the next experiment.

Force isn’t really “energy”. Force times distance is energy, but the magnets can’t move together for an infinite distance because they will touch after a pretty small distance.

Adding to what people have said, energy isn’t force. In fact mechanical work, the definition of energy, is achieved by pushing a force through a certain distance. So if you have a 1 Newton weight being held down by gravity, there won’t be an exchange of energy until you start lifting that weight or let it fall against gravity. Lifting it 1 meter near the surface will cost 1 Joule of energy (and doing it in 1 second would have required the use of 1 watt of power.)

Gravity is a good force to demonstrate with, because it’s purely attractive and therefore easier to understand conceptually. If you consider that [gravity drops off with the inverse square] (https://chart-studio.plotly.com/~adelsbergc17/197/force-of-gravity-vs-separation-distance-both-masses-are-fixed-at-20kg.png) at long distances, and also consider that work/energy is defined as force integrated over some distance, you can actually find the total amount of energy that a gravity well can give a specific object in question. By getting the area under this curve all the way out to a distance approaching infinity where the attractive force becomes zero, it becomes apparent that this energy is finite. If you fire a bullet from a point mass which has this much energy, it will be slowed by gravitational pull until it comes to a complete halt at an infinite distance. This is referred to as escape velocity.

You may have seen explanations of gravity as distortions in the “fabric” of space, where massive objects cause a greater “dip” in this fabric, and objects moving toward them effectively fall “downhill”. It may help to think of magnets and magnetism in the same way.

If you were to roll a ball downhill, you’d be unlikely to ask why the ball and the bottom of the hill are propelling toward each other, but in some ways it’s the same thing.

Magnets aren’t propelling themselves toward each other, so much as “rolling downhill” until they collide.

Not directly answering the question but many people get force and energy confused.

A force can exist forever and requires nothing to maintain. Energy only comes in when the force is able to do/move something. A magnet can apply a magnetic force and hold on to your hand from now until eternity if you ignore the physical materials degrading. You holding up an object is not quite the same.

Your muscles are constantly moving little strands by tightening and releasing them. This is why (in part) it takes energy and you get tired. Some creatures are able to move their body such that it “latches” into place and no longer requires any energy or effort to hold in position.