Calculating magnetic force between two bar magnets

In summary, two students completed a term project for their physics II lab where they calculated the repulsive forces between two bar magnets. However, they are lacking an equation to calculate the force and only have data such as the mass of the magnets, distance of repulsion, and magnetic field at the point of repulsion. They were unable to find a relevant formula online and their professor suggested they search for one. One of the students also attempted to modify a formula for magnets placed end-to-end, but could not find a way to convert it to the parallel case. They will need to gather more data in the lab in order to use this formula.
  • #1
Ascendant78
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Homework Statement



Me and my physics II lab partner did a term project where we are looking to calculate the repulsive forces of two bar magnets on each other. We are lacking an equation that would let us calculate the force. All we did was have one magnet sit on a flat surface while we brought the other one closer to it in a parallel fashion until the magnetic force overcame the frictional force of the magnet. While we have all the pertinent data, we simply don't know what to do with that data to find the force at this point.

Homework Equations



We are unsure. We calculated the friction coefficient of the magnet on the surface it was repelled on top of, but we do not have a formula for the force between the two magnets.

The Attempt at a Solution



Here is all the information we have:
mass of magnet 1 (repelled) - 69g
mass of magnet 2 (repeller) - 68g
Distance at which the magnetic repulsion overcame the frictional force - 9mm
Friction coefficient (mu) of repelled magnet on top of aluminum surface - 0.277
Magnetic field at point of repulsion (repelled) - 0.439mT
Magnetic field at point of repulsion (repeller) - 0.439mT

They are both narrow, long bar magnets. I am thinking that their exact size will probably play a significant role here too, but I'm thinking I can guesstimate that or check tomorrow in class. However, if anyone has a relevant formula for magnetic forces that can work for us here, I'd greatly appreciate it.
 
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  • #2
Forget about the magnetism for a moment. You have an object of known mass resting on a horizontal surface, and you know the coefficient of friction (static, I trust). What force is needed to overcome the friction?
 
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  • #3
haruspex said:
Forget about the magnetism for a moment. You have an object of known mass resting on a horizontal surface, and you know the coefficient of friction (static, I trust). What force is needed to overcome the friction?

We found the force to be 6.21x10^-4N based on that, but our professor also wanted us to try to find a way to calculate the force based on the the magnetic repulsion as well. That way, we could compare the two. He wasn't sure of a formula to do it off the top of his head, so he suggested we search the web. Nothing I found so far is feasible off the data we have nor off data we could obtain with our lab resources. Even if we found something that at least let us take a rough estimate, it would be better than nothing.
 
  • #4
Ascendant78 said:
We found the force to be 6.21x10^-4N based on that, but our professor also wanted us to try to find a way to calculate the force based on the the magnetic repulsion as well. That way, we could compare the two. He wasn't sure of a formula to do it off the top of his head, so he suggested we search the web. Nothing I found so far is feasible off the data we have nor off data we could obtain with our lab resources. Even if we found something that at least let us take a rough estimate, it would be better than nothing.
Take a look at http://en.wikipedia.org/wiki/Force_between_magnets#Force_between_two_bar_magnets. This is for magnets placed end-to-end, and uses the somewhat doubtful technique of treating the ends as magnetic monopoles. See if you can modify the terms in the second bracket to convert it to the parallel case. Don't forget to take into account vector addition.
 
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  • #5
haruspex said:
Take a look at http://en.wikipedia.org/wiki/Force_between_magnets#Force_between_two_bar_magnets. This is for magnets placed end-to-end, and uses the somewhat doubtful technique of treating the ends as magnetic monopoles. See if you can modify the terms in the second bracket to convert it to the parallel case. Don't forget to take into account vector addition.

Thank you. I did find this formula online a while back, but was hoping for something that didn't require magnetic flux density close to each of the poles. I'm thinking we will probably have to get back into the lab to find that as it seems to be the only equation that we could really work with at this point.
 

1. How do you calculate the magnetic force between two bar magnets?

The magnetic force between two bar magnets can be calculated using the formula F = (μ0 * m1 * m2) / (4π * r^2), where μ0 is the permeability of free space, m1 and m2 are the magnetic moments of the two magnets, and r is the distance between them.

2. What is the unit of measurement for magnetic force?

The unit of measurement for magnetic force is Newtons (N).

3. Can the magnetic force between two bar magnets be attractive or repulsive?

The magnetic force between two bar magnets can be either attractive or repulsive, depending on the orientation of the magnets. Like poles (North-North or South-South) will repel each other, while opposite poles (North-South or South-North) will attract each other.

4. How does the distance between two bar magnets affect the magnetic force?

The magnetic force between two bar magnets is inversely proportional to the square of the distance between them. This means that as the distance increases, the force decreases, and vice versa.

5. Can the magnetic force between two bar magnets be increased?

Yes, the magnetic force between two bar magnets can be increased by either increasing the magnetic moment of the magnets or decreasing the distance between them. Alternatively, using stronger magnets can also increase the force.

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