SAT Subject Physics Practice Question 602: Answer and Explanation
13. When a voltage source that is inducing voltage into a large number of coils is disconnected, and a switch that is in series with the coils of wire is also opened, a spark is observed to jump across the switch terminals as the switch begins to open up. What is the cause of this spark?
A. Free electrons from the voltage source
B. Free electrons from the coils of wire
C. Collapse of the magnetic field in the coils of wire
D. Secondary electron flow from the source
E. Stored voltage in the coils of wire
Correct Answer: C
The correct answer is (C). When the voltage is disconnected there is a small lag time between the disconnection of the current and the ceasing of the flow because the magnetic field, which is induced into the wires, collapses. This produces a current flow that causes the spark.