The ignition system in a car uses a transformer whose primary has 200 turns and carries 10 A; its secondary, with 20,000 turns, is connected to the spark plug. An electronic switch suddenly stops the current in the primary, creating a brief, 40,000 V pulse in the secondary that makes a spark in the plug, which ignites the fuel mixture. Why does the voltage pulse occur only as the primary current is stopped? Why is it important that the primary current be stopped suddenly?