ECE 201 Circuit Theory I1 Self - Inductance Faraday postulated that a magnetic field consists of lines of force surrounding the current-carrying conductor. Think of these lines as elastic bands that close on themselves. As the current increases and decreases, the bands expand and collapse about the conductor.

ECE 201 Circuit Theory I2 Self – Inductance continued Expand for increasing magnetic field (due to increasing current). Compress for decreasing magnetic field (due to decreasing current). Faraday’s Law : the voltage induced into the conductor is proportional to the number of lines that cut the conductor.

ECE 201 Circuit Theory I3 Faraday’s Law λ is called the “flux linkage” and is measured in weber-turns

ECE 201 Circuit Theory I4 Magnetic field linking an N-turn coil

ECE 201 Circuit Theory I5 Permeance, P Permeance describes the magnetic properties of the space occupied by the flux. Linear for non-magnetic materials Units are webers/ampere-turn.

ECE 201 Circuit Theory I6 Self - Inductance

ECE 201 Circuit Theory I7 Voltage Polarity When i increases, di/dt is + and v is +. –Energy produces the magnetic field

ECE 201 Circuit Theory I8 Voltage Polarity When i decreases, di/dt is - and v is -. –i is still + –Magnetic field collapses - +