Day 128

Early Attempts at Generating an Electric Current

  • Michael Faraday tried to generate an electric current from a magnetic field, on the heels of Oersted’s discovery that an electric current produced a magnetic field.
  • Joseph Henry was simultaneously experimenting and generating similar results.

Faraday’s Experimental Design

  • A primary coil was wrapped around an iron ring; a battery supplied the current and a switch was in the circuit.
  • A secondary coil was wrapped on the other side of the iron ring and was attached to a galvanometer.

Faraday’s Data

  • When the switch is closed (primary coil), a momentary deflection is seen on the galvanometer attached to the secondary coil.
  • When the switch is opened, a momentary deflection is seen in the galvanometer in the opposite direction.
  • When there is a steady current in the primary circuit, the galvanometer reads zero.

Faraday’s Conclusion

  • An electric current can be produced by a changing magnetic field; therefore, an induced emf (voltage) is produced in the secondary circuit by the changing magnetic field.

Magnetic Flux

  • The emf produced by the changing magnetic field is actually induced by a change in the magnetic flux Magnetic flux – the product of the cross sectional area of the wire loop and the strength of the magnetic field perpendicular to the wire loop – designated by the Greek letter Φ.

Magnetic Flux Equation

Φ = BA cos θ

  • Unit of flux is the Weber (Wb)
  • B (magnetic field) is in Teslas (Wb/)
  • A (area of wire) is in 
  • If B is  to the wire loop, Θ = , Φ is maximum
  • If B is || to the wire loop, Θ = 90°, Φ is zero

Magnetic Flux Hints

  • θ is the angle between the magnetic field and the normal () to the loop.
  • The value of Φ is proportional to the number of magnetic field lines passing through the loop.
  • The more lines that pass through (cut through) the loop when it is  to the magnetic field, the greater the flux!

Faraday’s Law of Magnetic Induction (Conceptual)

  • A current is set up in a circuit as long as there is relative motion between the magnet and the loop.
  • The instantaneous emf induced in a circuit = the rate of change of magnetic flux through the circuit.

Faraday’s Law of Magnetic Induction (Equation)

ε= -N (ΔΦ/Δt)

ε is the induced emf in Volts

  • N is the # of loops in the circuit
  • ΔΦ is the change in mag flux (– ), in Wb
  • Δt is the change in time, in sec
  • (-) is the polarity of the induced emf

Try This to Simulate Flux

  • If you place a loop of masking tape and a pencil at right angles () to each other, this simulates an angle of 0° with the normal and therefore max flux.
  • If you turn the pencil so that it is || to the tape, it cannot cut through the loop, the angle between the normal to the loop and the pencil is 90° = min flux.

Lenz’s Law

  • States that the polarity (sign) of the induced emf is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loop.
  • “GFI” – ground fault indicator – is a good example of an application of Lenz’s Law


  • The principle of electromagnetic induction is the basis for a generator. Generators converts mechanical to electrical energy.
  • Rotating a loop of wire on an axle in a magnetic field generates a current in the loop.