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Homework Help: How do i make a motor move? (i know very little)

  1. Oct 29, 2016 #1
    1. The problem statement, all variables and given/known data
    I have created multiple electromagnets with copper wire, nuts bolts, 9v batteries, and some duck tape. My intent was to make an electromagnet then hit it with a weak emp, so i can see how if the emp pushes the electromagnet away. Next i was going to mount the two on a cart to see if the emp will move the cart with both the electromagnet and the emp generator. The end goal would be to make this repeate itself without my interferience so that the cart will go straight. I would then change the speed of this repeating process to see if it pushes the cart faster. And try to make it so i can slow it down or speed it up with a switch. My only problem is... I have no clue how to make an emp. Or what percations(if any) i should take for my safty and my personal electronics security. I would like someone to point me in a direction so i can read on this. This home experiment is purily out of curiosity.

    I apologize if i put this under the wrong catergorie. But im not sure where it belongs.
    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 29, 2016 #2


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    I think perhaps you need to tell us what you mean by an electromagnetic pulse. A weak emp could be as simple as switching an electric circuit on or off. Strong emps come from nuclear bombs, lightning strikes, solar storms, but I don't think you have access to these! If you were generating your own weak emp, it's probably a very inefficient way of sending power from one device to another: most energy will get lost, radiated in other directions.

    If you use a battery to power your emp generator, why not just connect the battery to the electromagnet, so that all the power gets to it?

    Your solenoid: if you supply current, it can generate a magnetic field. If you supply a changing magnetic field, it can generate a current. Either way, how does that result in movement?
    A solenoid can behave like a magnet and be repelled by another fixed magnet, or attracted by iron or another magnet. But it will only move a small distance before it reaches the other magnet or moves too far from it.

    Your better bet is to use an electric motor rather than a solenoid. In that it has been arranged for solenoids and magnets to push each other sideways around a circle, so that they stay in range and can be made to keep moving. A battery connected directly to a motor is a good way to make a cart move.

    If you want to try something with wire and magnets, how about the world's simplest electric train?
  4. Oct 29, 2016 #3
    I will explain what exaclty im trying to figure out in this experiment because i never was able to take physics in highschool. This experiment was simply the first consept for finding our the awnser i wanted.

    Im going to start with the observation that led me to the question

    2 magnets replling eachother but are fixed to eachother wont be able to move.

    So how could i get one magnet to move in one direction while keeping everything connected?

    The idea is to hit one magnet field with another magnetic field thats not actually attached to anything.

    So first i had to make a magnetic field.(electro magnet)

    Then i had to some how hit it with another field thats not attached to a magnet {electro magnetic pulse( basically a magnetic field pulse)}

    If and only if this worked

    I was going to then start seeing how effective this was... If at all. Its where the cart came in. I could weigh a cart down to see how much it was able to move.

    I want to see how effectivly i can maintain any movement by creating intervels between the magnetic fields actually hitting eachother, and how fast i can get it to go.

    An electric motor came to mind but its not what i wanted to do. I dont want any magnets to actually move away from eachother. Just the fields generated by them.

    As far as i know about electric motors is that the shaft is being pushed by the magnets on it by the magnets generated outside of it. (These magnets being the coils of copper on it and around it. Generated by the current of electricity)

    In that example the magnets do physically move away from eachother, albiet in a circle, just to return to where it was.

    I was asking how to create an emp that would help me see my experiment work or not work. I saw a design using a disposable camera but that was contained in a ring and only had a 1-3" range. This seems to weak to me.

    But i have a limited understanding of electro magnetic pulses. And what they actually are...
  5. Oct 29, 2016 #4


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    Thread closed temporarily for Moderation...

    EDIT -- the use of the term EMP set off alarm bells, but it looks like the OP's question is different, and the term EMP is incorrectly used. We will try to clean this up and clarify soon so we can re-open the thread. Thanks.
  6. Oct 29, 2016 #5


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    As I've discussed with @James lajoie jr via PM, his question is not about EMP, it is just about how to make electric motors and motion work. Thread is reopened. :smile:
  7. Oct 29, 2016 #6
    Thank you. Sorry about the confusion.
  8. Oct 30, 2016 #7
    Switching an array of electromagnets on and off creates a moving a magnetic field.
  9. Oct 30, 2016 #8


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    No worries, do you understand how a DC brushed motor creates movement? :smile:
  10. Oct 31, 2016 #9


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    First, thanks for your explanation. It really helps to see where you're "coming from".
    You're quite right about the two magnets joined to each other. Any force between them is pushing one forward and the other backwards, with no net result.

    But this observation is even more generally true. We have conservation of momentum and Newtons 3rd law. If something gains forward momentum, something else must gain backward momentum or lose forward momentum, And if an object experiences a force in one direction, some other object experiences an equal force in the opposite direction.

    This can get a bit confusing with fields, because we don't actually see objects touching each other, but it still applies. Whatever causes the magnetic field will experience the opposite force.
    You certainly could generate a magnetic field with another fixed coil or solenoid and power it continuously or in pulses. The cart magnet (coil, solenoid or permanent magnet) could be repelled by this, but as it moved away, the effect would weaken. Maybe you could have a sequence of coils, so that the effect could be extended.
    The Youtube video of the electric "train" is something like this - an extended coil of wire, where it is powered only in the region around the "train".

    Since you understand that problem, I guess that's why you thought about electromagnetic waves. These can be beamed in a way that magnetic fields can't. So perhaps you could have, say, a beam of microwaves directed at the cart, pushing it along. We're getting into water that is a bit deep for me, but I think you are right. I believe people (sensible, real scientists!) have thought about powering spacecraft with the pressure of radiation from the sun. But the forces are very small - I think for photons (lumps of electromagnetic radiation) momentum is energy divided by the speed of light, so you need something like 100 MJ worth of radiation to provide 1kg m/sec of momentum - so its only useful for long space satellite missions, where there is very little "friction", a lot of time to accumulate momentum and, in our part of the solar system, over a kilowatt per square metre of free energy available from the sun. (WikiP says the plan was to use a 640,000m2 reflector, utilising 800MW of solar radiation.)

    Here, if you wanted to use electromagnetic waves to power a cart, you'd be better off using the energy rather than the momentum. Then using the power received to drive an ordinary electric motor pushing against the ground to get momentum.
    This is where I lose the plot a bit. I don't see why you want to pulse the power? If you can get some sort of magnetic repulsion drive, why not run it continuously? What advantage do you want to get by switching it off intermittently?

    What I suspect is that you are thinking of an analogy like throwing rocks at your cart. Each rock hitting it imparts a bit of momentum. But this effect can also be obtained by squirting a jet of water at it, or firing a continuous stream of small rocks (like sand grains) from an air jet. The quantum nature of throwing rocks is not an essential nor a useful part of the method. It's just that it's convenient to throw rocks by hand one at a time. The essential feature is, that moving mass has momentum and can lose some of it to the cart.
    Magnetic fields don't have mass or momentum. They simply create a force. The force starts when the field is created and continues as long as the field is maintained. You don't have to (and can't) throw them at something (nor at each other.)

    You can create a magnetic force between an electromagnet and a conductor, by switching the magnetic field on and off, because a changing magnetic field will induce a current in the conductor, which effectively makes it become an electromagnet and try to repel the causative changing magnetic field. This is a reason for using an AC (or pulsing) electromagnet, but the range and effectiveness would not be different from simply having two permanent magnets.

    Again, I'm a bit out of my depth here. AFAIK a magnetic field doesn't move: it just is. The field from a permanent magnet or solenoid is everywhere at the same time. If it changes, the changes propagate at the speed of light (as electromagnetic photons, aka radio waves.) These changes carry energy, but relatively very little momentum.
    I don't know much about what emps actually mean either. If it is simply a sudden change in electrical current, then I suppose the camera was used for its flash. Here a capacitor is charged up to a few hundred volts, then suddenly discharged through a glass tube containing Xenon gas at low pressure. The essence of this for emp is, that by charging the capacitor to a high voltage, when the discharge starts, the current will be large. So the current will change very quickly from zero to some large value. The sudden large change in current is what generates the emp.
    If you want to increase the size of the emp here, you can charge the capacitor (not the same one - one with a higher voltage rating) to a higher voltage. The bigger the voltage, the bigger the current when it starts to discharge. You might find something other than a Xenon tube to discharge through, which would allow a bigger current to flow. Just a big switch would probably be good (though it would probably be damaged by arcing quite quickly.)
    Any system like this is quite wasteful of energy. Some will be radiated electromagnetically, but much (maybe most) will just end up as heat in the circuit.

    If you know what sort of photons you want emit, you should be able to increase the efficiency by making your circuit resonant at the required frequency and adding a suitable resonant radiating element. You can also get away from the pulse emp of this spark generator and move to a more effective, continuously radiating radio transmitter.

    But let me reiterate, electromagnetic photons are not much good for transmitting momentum. If they are any use at all here, it is for transmitting energy, which you convert back to electric current to drive some sort of (conventional) motor to get movement.

    1-3" range may sound weak to you, but I find it amazing that a camera can cause a mechanical effect even that far! (Though I suppose two Nd magnets would repel at that range.) I would be interested to see details of that experiment.
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