Physics Help Forum A basic question about magnetism

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Aug 9th 2017, 07:07 AM   #11
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 Originally Posted by AndrewS I'm pleased to see you are not saying that a separate magnetic field exists. The answer to your question depends on the design of the electromagnet, but the maths would give the same answer. The difference being that any reference to magnetic fields is replaced by changes to the electric field.
Separate ? Separate from what ? and why is that an issue ?

You are the one making extravagant claims, not I.

I simply asked you to post how you would carry out, what to me is an elementary 3 to 5 line calculation in classical magnetic field theory.

Aug 9th 2017, 07:10 AM   #12
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 Originally Posted by studiot Here is a simple (in engineering terms) question In a scrapyard a crane with a large electromagnet lifts cars to the crusher. How much current is required to lift a car of mass m? I would be interested to see your improved solution over applying the theorem of virtual work to the magnetic field.
Just a side issue .... it's a permanent magnet in that lifter ... the current is applied to a coil around this magnet to cancel it out to drop the car ....not only does this save power ,since the current only flows to drop the metal , but also for safety ,if power should fail no one gets killed by falling metal.

 Aug 9th 2017, 07:22 AM #13 Member   Join Date: Jul 2015 Posts: 84 Thanks Oz.
 Aug 9th 2017, 10:19 AM #14 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 756 So you can't do this simple calculation and therefore just wish to mock.
Aug 9th 2017, 03:29 PM   #15
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 Originally Posted by AndrewS As no one has provided any evidence that traditional magnetic fields exist, nor even that they qualify as a scientific hypothesis, it looks like we can agree they do not exist.
Do you deny that a magnet can pick up something it is not touching? That "experiment" has been done many times. And it is simple to calculate exactly what the magnetic force is at a specific distance from the magnet. That is precisely what "magnetic field" means!

 Aug 9th 2017, 03:32 PM #16 Junior Member   Join Date: Oct 2013 Location: Leeds, Yorkshire, England Posts: 27 Andrew, I like the fact that you question things. I haven't done a lot with magnetism, but Bernard Burchell has done a page on his website about it. It is worth taking a look. It is thought provoking. Magnetism Explained Tom.
 Aug 10th 2017, 02:52 AM #17 Member   Join Date: Jul 2015 Posts: 84 Thanks HallsofIvy for your question, and Tom for your help. I know from personal experience it can be very difficult to rearrange our long established brain connections, but please think about what I keep repeating. Motion just modifies the electric field. There seems to be no evidence for the existence of perpendicular magnetic fields. The mathematical technique of cross products does not alter the physics: forces do not act perpendicular to their field. The magnetic field is superimposed on the electric field. It acts in the same direction as the electric field. Forces are only measured in the direction of the electric field. Experiments do not detect forces along "magnetic lines of force". This is not where the forces or the field lie. Currents that circulate in a magnet (permanent or not) weaken the electrostatic repulsion between electrons that circulate in the same direction. Like currents attract and opposite currents repel. This simple rule can explain a net force of attraction at a distance, much as there is an electrostatic attraction between distant positive and negative charges. However, if you have reasons for saying that a perpendicular magnetic field is a valid, i.e. provable, scientific hypothesis, please let me know. Last edited by AndrewS; Aug 10th 2017 at 10:21 AM. Reason: Word omitted
 Aug 10th 2017, 10:20 AM #18 Member   Join Date: Jul 2015 Posts: 84 Thanks Tom for joining both threads. I've now had time to look at the site you suggested. Mr Burchell certainly has some ingenious ideas about things such as magnetism. But I haven't spent the few hours it would take me to try to understand the implications of what he's saying. I was put off by his negative views about time dilation. As you recommended the site, perhaps you sympathise with this view. In which case it would be great to start a separate thread on the subject to see if we can resolve any differences - if you have some time to spare.
 Aug 10th 2017, 02:31 PM #19 Junior Member   Join Date: Oct 2013 Location: Leeds, Yorkshire, England Posts: 27 If you were put off Bernard by his views about time dilation, you will also be put of about mine. Time dilation, length contraction and mass increase are all illusions cause by the finite speed of light not being taken account of. This is from my web page :- http://myweb.tiscali.co.uk/carmam/Hollings.html#lorentz Imagine now a space rocket, which is propelled by ejecting a small amount of matter (the rocket exhaust) at high speed from the rear, so imparting a thrust in the opposite direction. We will assume that the exhaust velocity is 3,000 m/s and the mass of the rocket is 30,000 Kg (very similar to NASA's Mercury-Redstone rockets). Now we can use the Lorentz transformation to find the new mass. The velocity between exhaust and rocket is 3000 m/s, so :- m = m0 / sqrt( 1 - ( v / c )^2) m = mass of rocket at velocity v as measured by the essential observer (Remember that Einstein's observer, properly called the essential observer, is always at rest relative to the motive force. In this example therefore, the essential observer is in the same frame as the rocket exhaust). m0 = 30,000 Kg (proper mass of rocket or rest mass when v = 0) v = 3,000 m/s - rocket's velocity relative to the exhaust c = 300,000,000 m/s m = 30000 / sqrt( 1 - (3000 / 3e8)^2) = 30000.0000015000000001125 Kg The mass increase is therefore 0.0000015 Kg or 0.0015 gram which is simply not measurable compared to 30,000 kilograms. For all intents and purposes the mass increase is zero. A further point to note here is that the mass increase is measured against the exhaust which is providing the motive force, and no matter what the velocity of the rocket when measured against its starting point (or anything else for that matter), the velocity between rocket and exhaust never changes, so the rocket mass is always 30,000.0000015 Kg (disregarding the loss of mass due to fuel used). In other words, the mass is fixed at 30,000.0000015 Kg for the values used above between rocket and exhaust, and the extra 0.0000015 Kg is an insignificant amount. As there is no significant mass increase with velocity, and certainly no accumulative mass increase, there is no theoretical upper limit to the velocity of the rocket. It therefore follows that as the mass increase is virtually zero, m approximates very closely to m0. If the acceleration is regulated to 1g for the comfort of the crew, the space ship can reach an enormous velocity, and time on this space ship will pass at exactly the same rate as back at home on earth. "The effects of gravity are indistinguishable from the effects of acceleration" [AE] (with the qualification in section 5). I know that relativists would say that the mass increase has to be measured relative to the starting point of the rocket, but why is that? Einstein used the (essential) observer against which to measure the mass increase, with the tacit assumption that the starting point was where the propulsion unit was located, as in a particle accelerator. With that assumption, it is reasonable to refer the mass increase to the starting point. If we assume the propulsion unit (rocket motor) is remote from the rocket, then it is perfectly true that the rocket cannot exceed the speed of the rocket exhaust, as a particle in a particle accelerator cannot exceed c. This is analogous to a space vehicle which uses light sails for propulsion. The sails are deployed in the vicinity of a star (the sun), and the light hitting the sails imparts a tiny acceleration away from the sun. This acceleration will propel the vehicle away from the sun, and the velocity will gradually but steadily increase. As the vehicle approaches light speed however, the energy from the light striking the sails gets less and less, and the acceleration gets less and less. Quote from "Begin The Adventure" by Homer Tilton and Florentin Smarandache. "A sailing vehicle which depends on light from the sun to accelerate it remains in that way connnected to the sun, its reference is the sun, and its speed is limited to less than the speed of light c, relative to the sun. Propulsive energy cannot reach a vehicle traveling away from the sun faster than that. It is limited to the speed of light for the same reason that a cablecar is limited to the speed of the cable pulling it." Tom.
Aug 10th 2017, 04:44 PM   #20
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 Originally Posted by oz93666 Just a side issue .... it's a permanent magnet in that lifter ... the current is applied to a coil around this magnet to cancel it out to drop the car ....not only does this save power ,since the current only flows to drop the metal , but also for safety ,if power should fail no one gets killed by falling metal.

In order to 'cancel something out' it implies that the cencelling agent is equal in magnitude but opposite in direction to that which is being cancelled.

Since the force of exerted by the lifting magnet must equal the weight of the mass, we can immediately calculate the force of the cancelling electromagnet.

So this is really an unnecessary complication.

Would one of you gentlemen like to demonstrate that you actually know some of the Physics you are attacking by performing the three line calculation I asked for.

It's so easy it's given to apprentice electricians.

Last edited by studiot; Aug 10th 2017 at 04:47 PM.

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