Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Current, Potential, and Coils
Name: Thomas G.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 7/17/2003


Question:
How is current increased when voltage is induced into a coil?


Replies:
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


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



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory