Ask A Scientist Physics Archive

name         Thomas G.
status       educator
age          60s

Question -   How is current increased when voltage is induced into a coil?
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Thomas,

Why does a house lamp dim while turning on an inductive load (coil)?  Why do these lights dim when 
turning on applications such as fans, compressors, etc...???

The reason behind this is that they have a transient response time to overcome.  This is a very 
broad description and covers more than your question, for that I apologize, but I believe it needs 
to be said first before explaining current - voltage - time behavior of magnetic fields in coils.

When you "apply" a direct current (DC) source to a coil, there is a relative huge current inrush 
from your source (say a 9 volt battery for example).  Why is this?  It is very simple.  Any good 
conducting wire by Maxwell's equations must generate a magnetic field with strength proportional 
to the voltage (really the current) which is applied.  The time rate of change of the current vs. 
time is, of course, rather large in the beginning since you have no magnetic field in the 
beginning.  The current continues to rush into the
coil to build up the field strength to its best possible strength based on the coil parameters 
(mainly DC resistance, which determines I, current).
Once the field is saturated and can no longer get any stronger the current will stabilize on a 
non-zero value based upon the non - zero DC resistance of the wire and the voltage that is 
continued to be applied.

WARNING:  BE CAUTIOUS WHILE DISCHARGING ANY COIL THAT HAS BEEN ENERGIZED.  I PROMISE YOU THAT YOU 
CAN GET A LITTLE BABY JOLT EVEN FROM A 9V BATTERY AND THE RIGHT COIL IF YOUR TOUCHING THE WRONG 
THINGS AT THE WRONG TIME....ie DO NOT TOUCH BOTH TERMINALS OF THE COIL DURING ELECTRICAL 
DISCONNECT OF THE 9V FROM THE COIL.  I DID IT ONE TIME.  IT WAS QUITE AN 'EDUCATION'.

Regards,
Darin Wagner
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Whenever the magnetic flux through a closed loop changes, an emf (voltage) is induced in that loop 
which is given exactly by the time rate of change of that flux.  Magnetic flux is the magnitude of 
the magnetic field component perpendicular to an area enclosed by a loop times the area of that 
loop.  If the loop is a conductor, a current will be produced by that induced voltage. This is 
Faraday's Law.

It is a most useful law and is used in electrical generators where a coil rotating in a constant 
magnetic field continuously changes the component of the magnetic field perpendicular to the coil 
thereby producing alternating current.

A more prosaic example is provided by electrical guitars.  Each string is magnetized and, as it 
vibrates, induces an alternating voltage in a small coil placed near the string.  This voltage 
is then amplified (generally much to much, in my opinion) and fed to loudspeakers to produce 
audible sounds characteristic of the springs oscillations.

An interesting extension is Lenz's Law, which says that the induced voltage is in a direction 
to oppose the change producing it.  This is why it takes power to turn a generator when it is 
producing current and why motors running without a load take much less power to run then when 
they are doing a lot of work.

If this is not clear or you would like further information, please let me know.

Best, Dick Plano
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