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Q & A: two magnets together

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Most recent answer: 12/16/2013
Q:
If I take two equal strength magnets and allow then to stick together, what would there net strength be? Would it double?....or would the force be the square of the single magnets strength?..... Thanks, Jim
- Jim (age 40)
Chicago Ill. U.S.
A:
This depends on how they stick together. I guess these are bar magnets. I'll also guess that to a good approximation the magnetism is "hard", meaning that the magnetization of each magnet doesn't change in response to the field of the other one. That means that the fields are just the sum of the fields from the two magnets, but since the fields are vectors, the magnitude of the sum is at most as large as the sum of the magnitudes. That equality only happens when the vector point the same direction.

If the north end of one magnet sticks to the south end of the other and vice versa (side by side), the fields from the two magnets actually cancel at a distance. The long range pull is much weaker than either magnet separately. That's the most stable (lowest energy) way for them to stick together.  Right near the ends the result is more complicated.

If you put them in a tube so they stick end-to-end (north end of one on south end of the other, other ends far apart) at a distance the field strength is just the sum of the fields from the two magnets, double those from a single magnet. Again, the fields up closer are more complicated.

Mike W.

(published on 04/17/2011)

Follow-Up #1: Separation force of two magnets

Q:
I want to use two counter sunk disc magnets in a coupling device. They are sold as a pair with opposing fields. They are rated at 5.35 lbs. each. My question is when put together is the force needed to separate them double that of their individual rating?
- Tim Moen (age 57)
Cabool, Missouri, USA
A:

Hello Tim,

The force separating the two identical magnets should be about the same as of one magnet stuck to a steel plate.  The reason is that when one magnet approaches the plate, it effectively 'sees' its mirror image and is attracted to it with the same force as a real one.

I did a simple experiment with some bar magnets I have around the lab and just making a guess it seemed like the two separation forces were the same.

 

LeeH   (I once visited Cabool)


(published on 12/16/2013)

Follow-up on this answer.