Most recent answer: 10/22/2007
Why do you say that magnets ’spend energy’ attracting magnetic materials? If the material is attracted to the magnet and moves closer to it, the magnetic potential energy goes down. That’s just like when something falls toward the Earth and the gravitational potential energy goes down. Now if the object is stopped somehow (say by friction) that energy turns into thermal energy, just like when a ball gradually stops bouncing.
If something (e.g. you) then pulls the object away from the magnet, then that’s what supplies some more energy to the system- the same as how you have to supply energy to lift something up in a gravitational field.
I guess I’m missing what the problem is supposed to be.
(published on 10/22/2007)
Follow-Up #1: magnetic energy
- Dave Cline (age 45)
Lake Oswego, OR
There’s energy in the magnetic field in either case. With the steel ball, the system has a way to reduce that field energy by having the ball approach the magnet. A lot of the energy goes into kinetic energy of the ball. For the golf ball, all the field energy just stays in the initial form.
(published on 10/22/2007)
Follow-Up #2: where does field energy come from?
- Gayle (age 46)
Great question. I’ll skip the warmup and go right to your key point: where does the field energy come from?
Typical permanent magnets are made deliberately. Some magnetic material is heated up, a magnetic field applied, and then the material cools in the field. Lots of energy is wasted in this process, but let’s follow just where the field energy is made. Initially the material has magnetic domains pointing in every direction, so their fields approximately cancel, leaving essentially no field energy outside the magnet. When a field is applied (usually via an electromagnet) to the warm material, it lines up the domains. It takes energy input to that electromagnet to do work lining up the magnetic domains. So the energy came from whatever supplied the electrical power.
Yes, the same sort of question could be asked about gravity or any field. Tracing the various energies in the universe back to shortly after the Big Bang, I guess you’d say that the big forms of energy then were the rest mass (energy) of particles (many of which then decayed) and the kinetic energy of their relative motions, There are other ways of expressing and thinking about those energies, e.g. as field energies, however. As for how it all got started in some state, nobody knows. (Admittedly that’s pretty much a non-answer.)
On your other question, you can tell when a lot of the field energy is gone because that’s the same as when the field is much weaker. You’d notice that the next ball was not pulled in very much.
The total energy in the system also depends on the configuration of the balls. The energy in the field is directly proportional to the integral of the square of the field strength over the whole volume, plus terms from the interaction with the material. If you can arrange the balls to divert field lines farter away from the magnet and cause them to take longer paths around, then you have increased the energy of the system. An example of this -- a bar magnet with a bunch of iron balls will likely have its lowest energy content when a chain of balls lines up from one pole to the other, allowing the fringe field of the magnet to follow the chain of balls, reducing the field outside the region containing magnets and balls. If you move the balls around, say stack them up end to end all on the North pole of the bar magnet, then the energy of that configuration will be higher than the one I described above. You’d have to hold the balls in place with some other force, or they will eventually end up in their lower-energy configuration.
Some actions involve repulsive forces, where to bring in an an additional magnet, you must add energy.
(published on 10/22/2007)
Follow-Up #3: Is force energy?
- Mitelin (age 28)
Force is not energy. They aren't even quantities with the same units. Force is expressed as distance*mass/time2, while energy is distance2*mass/time2. That extra factor of distance in energy should serve as a reminder that work (transfer of mechanical energy) is done only when a force acts on something which moves some distance.
(published on 03/01/2014)
Follow-Up #4: how permanent are magnets?
- Lucas (age 32)
Yes, for ordinary-shaped magnets, the magnetized state has higher energy than states with much less magnetism. Those lower energy states have a mixture of domains with magnetism pointing different directions. In the long run, the magnetic domains will rotate around toward one of the lower energy, less magnetized states. That isn't required at all by energy conservation (the First Law) but rather by entropy maximization (the Second Law).
That thermal demagnetization process can take an extremely long time. For example, geologists used magnetization patterns left in rocks for millions of years as a tool for studying their history. The typical time scales for the magnetization decay depend strongly on the type of magnetic material and the temperature, as well as other variables. So there's no simple equation.
For special shapes, e.g. thin wires magnetized along the wire direction, the single-magnetic domain states are actually the ones with lowest energy. There are two such states, with the magnetic field pointing one way or the other along the wire. The time for thermal energy to randomly flip the magnet between these two states, at ordinary temperatures, can be longer than the age of the universe.
(published on 02/18/2015)
Follow-Up #5: ways to lose magnetic energy
Chicago, IL USA
On your first question, about how long permanent magnets last, it depends on the material and the temperature. Refrigerator magnets are close to your description, and they stay stuck for many years.
With regard to the frictionless oscillating magnet, yes there will be gradual conversion of the energy to heat. Even if somehow contact friction is removed and even friction with air is removed, there's still a way for energy to get out. The oscillating magnet will radiate electromagnetic waves. Their energy will get absorbed somewhere, heating something up.
Coiling a wire in a closed circuit around the magnet will cause a much more rapid loss of energy. The current induced in the wire by the oscillating field will create heat in the circuit. If the circuit isn't closed, the wire won't matter much.
(published on 05/10/2015)