5j10.23  Back emf

PURPOSE: To demonstrate the back emf caused by interrupting the current of  a coil.

DESCRIPTION: An AC neon lamp, part number NE-34 is connected in parallel to a large coil.  Note that this lamp has two metal plates.  One plate will glow neon orange for a positive 90 volts.  The other plate will glow neon orange for a negative 90 volts.  Thus it is can be operated as an AC lamp.   A power supply of  positive 90 volts DC is connected through a knife switch to the power the parallel combination of coil and lamp. 

When the knife switched is closed the voltage is applied.  The magnetic field of the coil builds increases and as it does the coil slowly lets a steady state current flow and one half of the AC lamp glows orange.  The AC lamp shows the polarity of the steady state current through the coil.

When the knife switch is opened the magnetic field coil will collapse.  The collapsing magnetic field produces a back emf that will try to keep the current from dropping to zero.  The energy from the collapsing magnetic field is deposited through the lamp in the form of an electric current, flashing it orange.  The reverse polarity of the back emf is observed as the opposite half of the lamp flashes, as shown in the photograph above.

It can also be shown that the amplitude of the back emf is greater than the applied voltage.  By lowering the power supply voltage below 70, to 10 volts, the voltage is not sufficient to light the positive half of the neon lamp while the coil maintains a small steady state forward current.  But when the knife switch is opened the large back emf is sufficient to flash the negative half of the lamp, showing that the back emf is greater than the forward voltage.
 

EQUIPMENT: Neon lamp and base, large coil with iron core, knife switch, two C clamps, video camera, DC power supply.

SETUP TIME: 1/2 hour.

Below are photos of another arrangement for showing the back emf without the NE-34 neon lamp.  It uses a small wattage lamp in parallel with the coil and a high wattage lamp in series with the knife switch.