This video was made to demonstrate the phenomenon known as back EMF in a simple circuit, employing a lightbulb, a DC power supply, and a large coil. The light bulb voltage is monitored using an oscilloscope. What is noticed is very subtle, but there is a small negative voltage spike that arises immediately after the circuit is open (no longer connected to the battery). If you freezeframe it on one of those downward spikes, you can see that it is behaving as an negative inverse exponential curve, as theory predicts. It goes negative because the EMF that is generated tries to oppose any changes. This is Lenz’s Law (that minus sign in Faraday’s law).

This may not be that significant with only a 3 V power supply as seen here, but when dealing with highvoltage power supplies, sometimes the back EMF is so strong that the electric field across a simple open switch can pull the switch closed again—which can be extremely dangerous.