What is the primary effect when an open circuit occurs?

The primary effect of an open circuit is that current is interrupted. An open circuit occurs when there is a break in the path through which electric current can flow, effectively creating a situation where the circuit is incomplete. As a result, no current can move through the circuit, leading to a complete cessation of electrical flow.

In an open circuit, the potential difference (voltage) may still exist across the terminals, but without a closed loop, the current cannot travel. This interruption affects the entire circuit, meaning that any devices or components that rely on current will not operate.

The other potential impacts, such as significant voltage changes or variations in resistance, do not occur strictly due to the open circuit itself in a direct manner. The current interruption is the defining characteristic of this circuit condition, which is why it is the correct answer.