Generation of Electricity
Ways to make electricity
Electricity is generated by converting other forms of energy: friction (static), pressure (piezoelectric crystals), heat (thermocouple, Seebeck effect), light (photovoltaic cell), chemical action (cells) and magnetism (electromagnetic induction, the source of virtually all aircraft power).
Electromagnetic induction
Faraday's law: an emf is induced in a conductor whenever it experiences a changing magnetic flux. e = −N dΦ/dt. The minus sign is Lenz's law: the induced emf drives a current whose flux opposes the change that produced it. Three things are needed: a magnetic field, a conductor, and relative motion (or a changing field).
Motional form: a straight conductor of length l moving at velocity v perpendicular to flux density B generates e = Blv. A coil rotating in a uniform field produces a sinusoidal emf, e = BANω sin ωt, peaking when the coil plane is parallel to the flux.
Traps
- No change, no emf: a stationary conductor in a steady field induces nothing, and a steady primary current induces nothing in a secondary after the transient.
- Lenz's law questions ask for the direction of induced current - use opposition, not attraction.
- The rate of change matters more than the magnitude of flux.
- Piezoelectric means pressure, thermocouple means heat, photovoltaic means light; questions mismatch these.
Worked example
A 0.4 m conductor moves at 50 m/s through a 0.8 T field perpendicular to the flux: e = Blv = 0.8 x 0.4 x 50 = 16 V.