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Magnetism Makes Electricity
Faraday, induction, and electromotive force
Ørsted and Ampère had shown that electric current produces magnetic effects. Michael Faraday became determined to find the reverse relationship.
The key was not simply to place a wire beside a magnet and wait. A steady magnetic condition does not continuously drive current through an ordinary stationary loop. Something has to change.
a changing magnetic field can produce an electrical potential in a conductor.
Faraday's induction experiments
In 1831 Faraday demonstrated electromagnetic induction. In one famous arrangement, two coils were wound on an iron ring. One coil was connected to a battery and the other to a detector.
When the battery connection to the first coil was made or broken, the detector connected to the second coil showed a brief electrical response. While the first current remained steady, the second circuit did not continue producing the same response.
What mattered?
- Switch current on → magnetic field changes → induced response.
- Hold current steady → magnetic field becomes steady → no continuing induced response.
- Switch current off → magnetic field changes again → another induced response.
Move the magnet instead
Faraday also showed that moving a magnet into or out of a coil could produce current. Reverse the motion and the direction of the induced effect reverses.
What is EMF?
The historical term electromotive force, abbreviated EMF, can be confusing because it is not a mechanical force measured in newtons.
EMF is an electrical potential difference associated with a source or with induction. It is measured in volts.
Measurement trail 8 — now the Leyden jar gets its unit: the farad (F)
Back on the Leyden-jar page we postponed the unit of capacitance because we had not yet defined charge and voltage. Now we have both.
A capacitor has a capacitance of one farad if one coulomb of separated charge produces a potential difference of one volt between its terminals. The unit was named in honor of Michael Faraday and was formally defined in this relationship by the 1881 International Electrical Congress.
Faraday's law
In modern form, Faraday's law tells us that induced EMF depends on how rapidly the magnetic flux linking a circuit changes.
| Change | Effect on induced EMF |
|---|---|
| Move the magnet faster | A faster change in magnetic flux generally produces a larger induced EMF. |
| Use more coil turns | The induced effects of the turns add together. |
| Reverse the motion or field change | The induced polarity reverses. |
| No change in magnetic flux | No sustained induced EMF from induction alone. |
Measurement trail 9 — magnetic flux: the weber (Wb)
Faraday's law needs a way to describe how much magnetic field passes through a loop. That quantity is magnetic flux. Its SI unit is the weber, symbol Wb.
In induction terms, a change of one weber of flux in one second through a one-turn loop corresponds to an induced EMF of one volt. Notice how the new unit is built from two units we already know: the volt and the second.
The direction fights the change
Heinrich Lenz later summarized the direction of induced current in what we call Lenz's law: the induced effect acts in a direction that opposes the change that produced it.
That opposition is essential to conservation of energy. If the induced current reinforced the original change instead of opposing it, a machine could create energy from nothing.
The generator appears
Once induction was understood, a mechanical machine could move conductors through magnetic fields, or move magnets relative to conductors, and continuously generate electrical energy.
This is the essential principle behind electrical generators.
The transformer is already hiding in Faraday's ring
Faraday's two-coil iron-ring experiment also contained the basic idea of a transformer: a changing current in one coil creates a changing magnetic field, and that changing field induces an EMF in another coil.
Joseph Henry and inductance
The American investigator Joseph Henry independently discovered important induction effects at about the same time as Faraday and did major work with electromagnets and coils. His name was later given to the unit of inductance.
Measurement trail 10 — inductance: the henry (H)
Inductance describes how strongly a changing current in a circuit is linked to an induced EMF. The unit is the henry, symbol H.
So the henry now depends on quantities we have already established: volt, second, and ampere. In the 1946 international definitions, one henry was expressed as the inductance that produces one volt of EMF when current changes uniformly at one ampere per second.
The great symmetry
| Discovery | Relationship |
|---|---|
| Ørsted / Ampère | Electric current produces magnetism. |
| Faraday | Changing magnetism produces EMF. |
What comes next?
Once electricity could produce magnetism, and changing magnetism could produce electricity, two huge practical developments followed naturally:
- motors — electrical energy becomes mechanical motion;
- generators — mechanical motion becomes electrical energy.
At the same time, electromagnets made long-distance communication possible through telegraphs, relays, bells, and signaling systems.