From: SFPhysics@aol.com
Date: Thu May 13 2004 - 15:21:42 PDT
From: SFPhysics@aol.com Message-ID: <149.2937eacc.2dd54ef6@aol.com> Date: Thu, 13 May 2004 18:21:42 EDT Subject: Solenoid Inductance... back EMF?
Question #1: Using a standard lab solenoid (r=~3cm,
~300turns) what type of voltages can be achieved when
the coil is disconeced?
Question #2: Is this how a coil works in a car engine
to produce the voltage required to make the spark plug
spark?
Hello Jon:
1) Your rediscovery of inductive kickback is wonderful. In Physics classes
it is called "self-inductance." All coils are inductors and store energy in
their magnetic fields. As you correctly surmised, when the magnetic field
rapidly falls with the removal of the initial current you get a larger reverse
voltage across the coil. How high the voltage is depends on the amount of energy
stored, the type of magnetic core, and other factors. An inductor of the
solenoid type can easily reach 200 volts. Try putting a small neon lamp such as
an NE-2 which fires at 90 volts across the terminals and apply the voltage to
the solenoid with the core held inside the coil. Remove the current source and
the lamp should flash if there is 90 volts present. Put two lamps in series
and see if 180 volts are generated. Try three lamps in series. This will
give you a rough idea of how much voltage results from the collapse of the field.
Repeat the procedure with the core removed. What happens?
You could use a calibrated triggered oscilloscope with sweep delay to find
the voltage peaks but not too many school labs have them. Just knowing the
turns and radius of the coil will not let you know the value of the inductive
kickback. The position of the solenoid core, the gauge of the wire in the coil,
etc., all work to give you the unit of magnetic reactive inductance called the
"henry." How many units of inductance and the initially applied current are
required to be able to get a value of the peak reverse voltage from formula.
Maybe there is an easier way but I am not aware of it.
2) You are close on how the ignition coil in an automobile works. They have
two coil windings. The "primary winding" is a low voltage high current
winding of few coil turns that has the 12 Volts applied. When the magnetic field
has built up, the primary circuit is opened for the field to collapse. The
"secondary winding" has many more turns of wire that will deliver a high voltage
spark at a reduced current. This is the "transformer principle" where you take
low voltage at high current and end up with high voltage at low current.
Your standard television/monitor has a high voltage coil that does the same
function to provide a high voltage for the cathode ray tube (CRT).
Hope that helps and answers your questions,
Al Sefl
Who thought a solenoid was a person who had one adenoid removed...
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