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Doubling voltage. Halving amps. Quadrupling wire length and ohms. More magnetic flux?

  1. Dec 14, 2007 #1
    So what produces a solenoid's magnetic field? Voltage or Power?

    Voltage = Current * Resistance
    Power = Current^2 * Resistance

    Magnetic field magnitude of a solenoid = magnetic constant * turns * current

    We can quadruple the turns using the four times of the same type of wire, but that will quadruple the resistance. We can then double the voltage, but that will still leave us with half the current. Power here doesn't change. However turns quadrupled and current is half. More magnetic field?

    Alternatively, with the same voltage, we can double the turns using the same type of wire, but that will double the resistance and give us half the current. Voltage would remain the same, and the ampere-turns would remain the same! The power does not!

    So why do they say that power is needed to produce a magnetic field? Why isn't it voltage that produces the magnetic field?
    Last edited: Dec 14, 2007
  2. jcsd
  3. Dec 15, 2007 #2


    Staff: Mentor

    Neither. Consider a superconducting loop, it has neither voltage nor power but can have a very strong magnetic field. Fundamentally, it is the current that produces a magnetic field.

    The simplest way to approach this is through conservation of energy. The magnetic field stores energy, any resistance dissapates energy. The energy supplied to the circuit must go either to increasing the magnetic field or to heating the resistance, so the magnetic field energy is the difference between the energy supplied and the energy lost by resistance. In the steady-state, when the magnetic field and the current are not changing, ALL of the power goes into the resistance.
    Last edited: Dec 15, 2007
  4. Dec 15, 2007 #3
    edit: typo
    When you say "current" do you mean amps, or do you mean amps driven over a distance? When you say "magnetic field" do you mean the strength of the magnetic field, or do you mean the extent of the magnetic field?

    The energy lost by resistance is Current^2 * Resistance. So where does the rest of the power come from in AC, or say, in a DC system where the circuit is repeatedly connected and disconnected? How do I maximize the torque constant of the motor?
    Last edited: Dec 15, 2007
  5. Dec 15, 2007 #4


    Staff: Mentor

    Current is just Amps, not Amps/second. But you are right, the longer distance your current covers the greater the magnetic field.

    When I say "magnetic field" in the context of energy conservation I mean both the strength and the spatial extent. A higher magnetic field stores a greater amount of energy per unit volume of space. Try this link for a start http://hyperphysics.phy-astr.gsu.edu/hbase/electric/engfie.html As you can see, you can store the same amount of energy as a large and weak magnetic field or as a small and strong magnetic field.

    I'm not sure I understand your question. The power always comes from the power source, e.g. the chemical reaction in the battery, the mechanical work done on the turbine blades, etc. Wether it is AC or DC is irrelevant.

    Also, my comments about energy are really not appropriate for the context of a motor, they are about a coil in space just setting up a magnetic field in free space and not for a magnetic field that is doing work on something. If you are asking about motors could you please re-state your question in the proper context.
    Last edited: Dec 15, 2007
  6. Dec 15, 2007 #5
    How is the torque constant maximized? Is there a theoretical limit to it?
  7. Dec 15, 2007 #6


    Staff: Mentor

    Last edited: Dec 15, 2007
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