In AC circuits, the EMF in a coil opposing the applied voltage is known as

• A. Inductance
• B. Resistance
• C. Inductive reactance
• D. Capacitive reactance

Answer: (C)

In an AC circuit with a coil (an inductor), the EMF that appears inside the coil and opposes the changing current is the back EMF produced by self-induction. The coil resists changes in current, and the voltage needed to produce those changes grows with frequency. This opposition to the AC is called inductive reactance, with magnitude X_L = ωL (where ω = 2πf and L is the inductance). The back EMF is given by v = L di/dt, so for a sinusoidal current i(t) = I sin(ωt), the induced EMF becomes e = -ωLI cos(ωt), which is 90 degrees out of phase with the current and acts to oppose the applied voltage. Inductance is the property of the coil, not the opposition itself; resistance describes energy loss, and capacitive reactance applies to capacitors, not inductors.