The difference between impedance and resistance

Bob: It goes back to basic em fields. As to inductance the motion of a charge (which is current) causes a magnetic field. If the current is varying with time the magnetic field will vary wtih time. A time varying magnetic field causes a voltage in any conductor that is linked by that field. "Inductance" is by definition the circuit paramater that relates the voltage just described with the current that caused the voltage. There is a somewhat analogous explanation for capacitance, with the difference being that field is not a magnetic field but rather an electric field, the cause of which is a separation of charge. Remember that a "circuit" can consist of a wire. We construct specific elements to control the relationship which is inductance or capacitance.

Hi Bob,

In the case of a capacitor, the voltage between the two plates or conductors is proportional to the charge that has accumulated on them, the charges being equal but of opposite polarity on the two plates. Charge accumulates as a result of current flow between each plate and the circuit the capacitor is connected to. Therefore the charge and hence the voltage across the capacitor are proportional to the integral of current. Therefore (assuming an idealized capacitor model) an infinitely fast step increase in current will result in a gradual linear ramp-up of voltage.

In the case of an inductor, a change in current results in a change of magnetic flux, which results in a voltage being induced that opposes the change in current. See these writeups on Faraday's Law and Lenz's Law. Therefore, voltage is proportional to the rate of change (the derivative) of current; therefore current is proportional to the integral of voltage, and so a step increase in applied voltage will result in a gradual linear ramp-up of current.

Best regards,
-- Al