Measuring the Force Required to Separate Two Magnets at a Distance

[Dion]

Hi, i have a question.

I need to know how much Newton it takes to separate 2 magnets or a magnet from a from a piece of metal.

I've bought a spring scale that measures max 50 gram, so very accurate.

Now what if i attach the scale to the magnet and i pull it horizontally just enough to separate the magnets and the meter reads 10 gram, does it then mean it took 0.001 Newton to separate the magnets? And i can use that to convert it into watthours to have a mechanim separate the magnet with that amount of watthours?

Really appreciate the answers

 

[jbriggs444]

I've bought a spring scale that measures max 50 gram, so very accurate.

The maximum scale reading on a measurement device is only weak evidence for the accuracy or precision of the device. But yes, we can plausibly assume that this scale is good to at least plus or minus one gram.

Now what if i attach the scale to the magnet and i pull it horizontally just enough to separate the magnets and the meter reads 10 gram, does it then mean it took 0.001 Newton to separate the magnets?

Hmmm. If 1 kg is 10 Newtons then 0.1 kg is 1 Newton. 0.01 kg (10 grams) is 0.1 Newton. I think you divided by g when you should have multiplied.

And i can use that to convert it into watthours to have a mechanim separate the magnet with that amount of watthours?

You've measured the force required to initially separate the magnet from the piece of metal. You have not measured the separation that you obtained as a result.

If you want to take the measured force and convert it to an amount of energy you could use the fact that Work = Force * Distance.

But the force is not constant (it drops off rapidly with increasing separation). So you would have to integrate instantaneous force times incremental distance along a path from the initial state (no separation) to the final state (large or infinite separation).

 

[Dion]

The maximum scale reading on a measurement device is only weak evidence for the accuracy or precision of the device. But yes, we can plausibly assume that this scale is good to at least plus or minus one gram.

Hmmm. If 1 kg is 10 Newtons then 0.1 kg is 1 Newton. 0.01 kg (10 grams) is 0.1 Newton. I think you divided by g when you should have multiplied.

You've measured the force required to initially separate the magnet from the piece of metal. You have not measured the separation that you obtained as a result.

If you want to take the measured force and convert it to an amount of energy you could use the fact that Work = Force * Distance.

But the force is not constant (it drops off rapidly with increasing separation). So you would have to integrate instantaneous force times incremental distance along a path from the initial state (no separation) to the final state (large or infinite separation).

I think i used Newton-meter for the equation, 10 gram is +- 0.001 Newton meter. But is this not correct to measure the force to separate the magnets then? How else would be a correct way to calculate it? And if Newton meters per second is correct shouldn't i need to divide 0.001 by 100 since i only move it +- 1cm instead of 1 meter?
Thanks for the help

 

[jbriggs444]

I think i used Newton-meter for the equation, 10 gram is +- 0.001 Newton meter.

grams measure mass. Newton-meters measure energy (or torque). As is, the above equation is not dimensionally consistent. It is nonsense.

The first step is to take that 10 grams and convert to 0.1 Newtons of force. This is justified because the scale is intended for use on the surface of the Earth where the acceleration of gravity is approximately 10 meters/second². A 10 gram reading is indicative of the presence of 0.1 Newtons of force.

The second step would be to take that 0.1 Newtons of force and multiply it by the distance over which the force acts. 0.1 Newtons applied over 0.01 meters is indeed 0.001 Newton meters of work energy = 0.001 Joules = 0.001 Watt-second.

But that assumes that the force is uniform from direct contact out to 1 cm separation. Is it?

 

[russ_watters]

I think i used Newton-meter for the equation, 10 gram is +- 0.001 Newton meter.

Grams and Newton-meters (and watt-hours) are not directly related units. That conversion is wrong/meaningless.

But is this not correct to measure the force to separate the magnets then? How else would be a correct way to calculate it? And if Newton meters per second is correct shouldn't i need to divide 0.001 by 100 since i only move it +- 1cm instead of 1 meter?
Thanks for the help

If you want the force, measure it and then you are done. If you want energy, there is no easy way to measure that. You will probably need to build some sort of test rig.

 

[Dion]

grams measure mass. Newton-meters measure energy (or torque). As is, the above equation is not dimensionally consistent. It is nonsense.

The first step is to take that 10 grams and convert to 0.1 Newtons of force. This is justified because the scale is intended for use on the surface of the Earth where the acceleration of gravity is approximately 10 meters/second². A 10 gram reading is indicative of the presence of 0.1 Newtons of force.

The second step would be to take that 0.1 Newtons of force and multiply it by the distance over which the force acts. 0.1 Newtons applied over 0.01 meters is indeed 0.001 Newton meters of work energy = 0.001 Joules = 0.001 Watt-second.

But that assumes that the force is uniform from direct contact out to 1 cm separation. Is it?

Thanks for the reply,

Yes it is +- 1cm, in my setup the magnet doesn't touch the metal, the magnet is attached to an arm and the metal it attracts is +- 3cm away from the magnet holding it just in place against an plastic wall. When i move the magnet 1cm away from from the metal the metal drops and its separated. Thats why the force to move the magnet is so low.