emf to mmf and back

magnetic field around electric a live conductor
Fig 1. emf to mmf

This page describes the relationship between electricity and magnetism as it applies to solenoids, transformers, motors and generators.

magnetic field around electric two live conductors
Fig 2. Compound mmf

Electricity and magnetism are essential converses (equal and opposite), and mutually exclusive (one cannot be converted to the other) and mutually inclusive (one cannot exist without the other).

Electrical power comes from electrical charge, which can be released as either:
1) The transfer of electrons between the atoms in a conductor (DC); or,
2) The generation of electro-magnetic energy (AC).

Magnetic power comes from magnetic charge, which can be released as either:
1) The transfer of magnetic energy between atomic particles in a ferro-magnetic material; or,
2) The exploitation of magnetic-field energy (B).

Electricity: current = emf x resistance

Magnetism: flux = mmf x reluctance

Property: Electrical Units Magnetic Units
Flow rate Current C/s Flux Wb/s
Impediment to direct flow Resistance J.s/C² Reluctance J.s/Wb²
Force needed to overcome impediment Electro-motive force J/C Magneto-motive force J/Wb
Indirect energy transfer # Field kg/C Field kg/Wb
Capacity of field flow Permittivity C² / J.m Permeance Wb² / J.m
Field density (energy storage) Capacitance kg.m²/C² Inductance kg.m²/Wb²
Electricity and Magnetism
C = Coulomb & Wb = Weber
Note: kg.m²/C² = J/A

# the term 'Field' as described above, is actually the reciprocal of relative charge capacity (RC); B = 1/RC and it units are C/kg or Wb/kg.

The Principles

tight and loose conductor coils
Fig 3. Tight and Loose Coils

A current passing through a conductor will always generate an encircling magnetic field according to the right-hand rule (Fig 1).

a solenoid
Fig 4. A Basic Solenoid

If two conductors are run alongside each other and a current is induced in both of them (e.g. a return loop), the magnetic fields generated around them will work together (in opposite directions) increasing their combined strength (Fig 2).

A conductor wound into a tight coil with a current passing through it, will generate a magnetic field as shown in Fig 3. Note, an open coil will generate a coiled version of Fig 1 in which adjacent magnetic fields will cancel each other out as they will be flowing in the same direction (Fig 3).

Insert a bar of magnetic material into the tight coil, and the bar will become magnetised (Fig 4). It should be noted that when defining the north-pole of the resulting [electro-]magnet, the direction of the current in the conductor is immaterial. The defining factor is the direction around which the current orbits the coil.

a transformer
Fig 5. Transformer

Electrical power is the product of current and voltage; p = V.A
The same power may be achieved by; raising the current and similarly reducing the voltage (and vice-versa), as long as their product remains the same. But because current is more susceptible to resistance than voltage, the most efficient method for transmitting electrical power along a very long conductor is to maximise its voltage and minimise its current. When the power is required to do work, you reverse this process. This can be done by utilising the electro-magnetic properties of a coil (Fig 5) in the form of a transformer; the transformation of electrical power.

electricity cutting through a magnetic field
Fig 6. mmf to emf

Stepping voltage up or down can be accomplished by wrapping coils around a common ferro-magnetic core (Fig 5). The current passing through the input coil will (via its magnetic field) generate a magnetic flux in the core, which will induce an electrical current in the output coil. The relative number of turns in each coil will determine the voltage - and therefore the current - in the output conductor. Increasing the voltage requires a greater number of turns in the output coil and vice-versa.

A current will be induced in a [moving] conductor passing through a magnetic field as shown in Fig 6. The nature of the energy transfer in a transformer is the conversion and transfer of; electrical force to magnetic force and back to electrical force. The electrical force is termed voltage (electro-motive) and the magnetic force is simply referred to as magneto-motive [force].

electricity being generated by a moving magnet
Fig 7. A Simple Generator

If you move a permanent magnet within a tight [conductive] coil, a current will be induced in the conductor (Fig 7). Lenz’s law states that the magnetism and the electrical current are together trying to maintain a steady state. Induce movement in the magnet to upset this steady-state and the electrical current will oppose the movement.

All of the above is called electro-magnetic induction and was discovered by Hans Christian Ørsted in 1820 and explained by Michael Faraday in 1831. This phenomenon is the basis of all motors and generators.

Further Reading

You will find further reading on this subject in reference publications(3, 17 & 22)