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Jerinsam
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What Does the Lorentz Force Depend on?

04/29/2020 6:33 PM

I recently came across a paper named "Magnetohydrodynamic Propulsion for the Classroom" by Gabriel I. Font and Scott C. Dudley. In that they explain about an experiment in which they try to propel a board using magnetohydrodynamic.

In their paper, they stated with the equation F = qv X B, and later they came to two conclusions:

Where F is the force I is current, L is the distance between two electrodes and B the magnetic field.

Where F is the force,Vbattery is the voltage between the electrodes,l and W is the length and width of the electrodes,p is the resistivity of the material and B is the magnetic field.

The question is if I connect a power supply of 200 volts and 10mA (neglecting the fact that 200 volts can start electrolysis) and other parameters are as follows:

L =1cm , B =1 Tesla ,l = 5 cm ,W =2 cm and p is 0.22 ohmm.

Substituting these in equation 1 gives me an F= 0.01 X 0.010 X 1= 0.001 N.

But when substituting values in the second equation the Force F is ((200 X0.05X0.02) X1)/0.22 =0.909 ==1 N

How can these two equations give different answers for the same situation?
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Rixter
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#1

Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/29/2020 7:47 PM

The question is if I connect a power supply of 200 volts and 10mA (neglecting the fact that 200 volts can start electrolysis) and other parameters are as follows:

L =1cm , B =1 Tesla ,l = 5 cm ,W =2 cm and p is 0.22 ohmm.

Substituting these in equation 1 gives me an F= 0.01 X 0.010 X 1= 0.001 N.

But when substituting values in the second equation the Force F is ((200 X0.05X0.02) X1)/0.22 =0.909 ==1 N

How can these two equations give different answers for the same situation?

You specify your power supply with both voltage and current. The power supply applies a voltage, the load determines the current. The power supply cannot determine both voltage and current. I think that if you run the numbers, you will find that you are drawing a lot more current than 10 ma with 200 volts.
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Jerinsam
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#5
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/30/2020 5:43 AM

Thanks for the reply

But what if I keep the current constant in my power supply and increase the voltage.

Will the force increase
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Rixter
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#7
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/30/2020 9:30 AM

You can't control more than one thing with your power supply. If you set the voltage, the load determines the current. If you set the power supply current, the load determines the voltage. That's the way it works.

Forget voltage, the force is proportional to the current assuming Length and magnetic field being constant.
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Rixter
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#8
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/30/2020 9:23 PM

This might help. You can determine the resistance of your MHD cell as follows: The resistivity of saltwater is 0.22 ohm.meters. If current flows in the top-bottom direction, you can calculate the resistance of the cell as R = (0.22 x height) / (length x width), where the dimensions are in meters.
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SolarEagle
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/29/2020 8:57 PM

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Jerinsam
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#4
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/30/2020 5:39 AM

Thanks for the reply.

So will the force increase if we keep current constant and increase the voltage (neglecting the fact that electrolysis will increase).
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PWSlack
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#6
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/30/2020 7:01 AM

With a non-varying fluid, it is impossible to <...keep current constant and increase the voltage...>, so <...neglecting the fact that electrolysis will increase...> is a non-starter and the fluid must change as a result of the <...current...> being applied to it..

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SolarEagle
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#10
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

05/01/2020 12:17 AM

The Lorentz force depends on the strength of the 2 opposing forces...

Definition of strength of current. The number of amperes flowing through a circuit.

The typical strength of the Earth's magnetic field at its surface is around a half a gauss. So those are everyday units of magnetic fields. There are 10,000 gauss in one tesla.

https://en.wikipedia.org/wiki/Gauss_(unit)

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SolarEagle
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#11
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

05/01/2020 4:14 AM

No, you have to increase the current....

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PWSlack
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Re: What does the Lorentz force depends on ? Is it the current or the voltage in case of Magnetohydrodynamic Propulsion

04/30/2020 4:39 AM

[Comment withdrawn at edit.]

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Rixter
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#9

Re: What Does the Lorentz Force Depend on?

04/30/2020 10:06 PM

Here is a neat demo you might like. I'd use Nickel Iron Boron magnet, which is really strong, or maybe two, one on top and one underneath. All you need is a plastic dish, some water, salt (to make it conducting) and pepper (to see the fluid move), a couple of pieces of metal for electrodes, and of course wires and batteries.
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HiTekRedNek
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#12

Re: What Does the Lorentz Force Depend on?

05/01/2020 6:49 AM

Here is an example of the force in action:

https://www.youtube.com/watch?v=ygHN-vplJZg

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