Final answer:
Inductance in DC circuits occurs when the DC voltage is switched on or off, due to the property of conductors to induce voltage when there is a change in current, described by Faraday's law of induction.
Step-by-step explanation:
When considering inductance in DC circuits, it primarily occurs at the moment a DC voltage is switched on or off. This is because inductance is the property of an electrical conductor by which a change in current in one conductor induces a voltage in itself and in any nearby conductors by electromagnetic induction. Faraday's law of induction describes how an emf (electromotive force) is generated due to a changing magnetic flux. Although inductance is usually associated with AC circuits, where an inductor opposes changes in current resulting in sinusoidal voltage and current shifts, there is also a brief inductance effect observed in DC circuits exactly at the moments of turning on and off the DC supply.
The inductance in DC circuits manifests as a transient event when the switch is flipped. This transient inductance is relatively short-lived compared to AC circuits, where it is continuous due to the sinusoidal nature of AC voltage. In summary, inductance in DC circuits occurs at the moment the circuit is energized or de-energized - right when the continuous flow of DC current is either established or interrupted.