Electromagnets and Work Watch

lerjj
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If I make an electromagnet with a coil of wire and an iron nail connected to a battery, how is the electrical power being used? Surely there aren't any transducers so the wire should just melt? Or does creating magnetic fields require work? What about if I use it to push about a bar magnet, where's the extra energy coming from to move the magnet as well as whatever electrical power is being used?

I'm kinda confused here so any explanation would be great, although I don't strictly need to know the answers for my exams I guess.
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uberteknik
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(Original post by lerjj)
If I make an electromagnet with a coil of wire and an iron nail connected to a battery, how is the electrical power being used? Surely there aren't any transducers so the wire should just melt? Or does creating magnetic fields require work? What about if I use it to push about a bar magnet, where's the extra energy coming from to move the magnet as well as whatever electrical power is being used?

I'm kinda confused here so any explanation would be great, although I don't strictly need to know the answers for my exams I guess.
The wire coil will have a resistance: it will be small, but a resistance nonetheless. Also, the battery has a small internal resistance.

Both resistances sum to limit the current in the wire (I = V/R).

Voltage is defined as Joules/Coulomb i.e. V = E/Q and tells us that work must be done in order to create the current that flows in the coil. In essence work is done because the battery is nothing more than a controlled exothermic ionic chemical reaction which uses an external conduction path to transport excess charge as part of the reaction between the cathode and anode.

The magnetic force is a manifestation of electric charge, generated by the movement of charge (current). No charge movement = no magnetic force.

So yes, to create the magnetic field there must be a flow of charge produced by the work done in the ionic chemical reaction of the battery.

If YOU push the bar magnet, you just answered your own question!
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lerjj
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What about if I switched it on in the presence of a bar magnet? The resistance hasn't changed so the power of the circuit hasn't increased either, but it would repel/attract the bar magnet. In comparison to the case where there is no magnet present, haven't I got more energy out in this case (kinetic energy of the stationary bar magnet increased)?
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uberteknik
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(Original post by lerjj)
What about if I switched it on in the presence of a bar magnet? The resistance hasn't changed so the power of the circuit hasn't increased either, but it would repel/attract the bar magnet. In comparison to the case where there is no magnet present, haven't I got more energy out in this case (kinetic energy of the stationary bar magnet increased)?
Energy conservation laws cannot be broken: there is no such thing as a 'free energy-lunch' in physics!

This is neatly described by Lenz's law which shows how electromagnetic-induction complies with Newtons laws for the conservation of energy and action/reaction forces.

a) Lenz said that a changing current in a conductor will generate a magnetic flux.

b) Additionally, a changing magnetic flux will induce a current in a conductor exposed to the flux. Moreover, the induced current will always oppose the change of flux that created it.

The first part describes how the solenoid generates a magnetic field. The second part describes how a changing magnetic field generates a current. Together these laws are known as electromagnetic induction and the principle is exploited in the modern and ubiquitous electrical transformer.

The current in the solenoid coil generates a magnetic field whose strength (flux) is proportional to the current and dimensions of the coil. The current in that coil will be determined by the conductor resistance and the voltage applied to the coil.

So far so good. In the absence of any ferro-magnetic object placed within the induced magnetic field, or, interacting with another magnetic field, then the only thing using energy is the self resistance of the coil and internal resistance of the battery which converts the voltage potential to heat in the coil. (i.e. the voltage potential energy which is Joules per Coulomb of charge, is converted to heat by the interaction of the coils resistance.)

If the current to the solenoid is switched on within the presence of the external static bar-magnetic field, two things will occur:

1) The solenoid magnetic field will interact with the atoms of the bar magnet and create an attraction or repulsion force with it depending on the orientation and position of the bar magnet within the solenoid magnetic field.

2) The electron current in the solenoid coil will interact with the bar-magnet field which (again depending on orientation and position) will either try and increase or impede the current. Note though, that whatever happens, the induced currents and fields will always be set up to oppose the currents and fields that created it.

Whichever way you try and thwart the rules, as Scotty always said, "ye cannae break the laws of physics cap'n".
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