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Q & A: energy and fields

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Most recent answer: 10/22/2007
Q:
The first law of Thermodynamics states that energy cannot be created or destroyed only transformed. However i do not think this makes sense relative to ideas on attraction and repulsion. A permant magnetic can attract a magnetic object. The object moving is kinetic energy, but where did the energy come from?? The magnet diidn’t lose any energy, therefore none was transfered, but that means you’ve created energy which surely is not possible. so can you create energy?
- catherine (age 16)
england
A:
When two magnets pull together, increasing their kinetic energy, some magnetic potential energy is lost. That's not really any different from the case where you drop a ball. It picks up kinetic energy, and gravitational potential energy is lost.

Perhaps you're wondering where that magnetic potential energy is located. The standard assignment is to say that it is distributed throughout space, with a density proportional to the square of the magnetic field. When two attracting magnets get closer (e.g. letting their north and south poles approach each other) their fields partly cancel, reducing the magnetic potential energy.

Mike W.

(published on 10/22/2007)

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