This article is a tutorial on electromagnets and variable inductors. The purpose of this article is to present the theory of magnetic circuit in an easy to understand style.
Magnetic Flux
“If we wanted to demonstrate that a magnetic field existed around a magnet, we would place a strong magnet under a piece of cardboard. Then we would sprinkle iron filings on top of the cardboard. The iron filings will form numerous paths from the north pole of the magnet to the south pole. These paths represent the magnetic flux or the magnetic lines of force.” Reference: Quoted from my article ‘Electricity and Magnetism’ published on Associated Content.
Magnetic Flux and The Wire Carrying Current.
To find out the direction of the magnetic flux in an insulated wire carrying current use your right hand in the following manner. Let your thumb point in the direction of conventional current flow and wrap your fingers around the insulated wire. Your fingers point in the direction of the magnetic flux which forms rings around the wire.
Right Hand Rule For A Coil
You can also determine the direction of magnetic flux in a coil. Wrap your fingers around the coil in the direction of conventional current flow. Your thumb will point in the direction of the magnetic flux. If confused, turn the coil so you are looking down at once end of the coil with the positive end of the coil closest to you. Then with your right hand, wrap your fingers around the coil in the direction of conventional current flow. Your thumb will point in the direction of the flux.
The Coil
When current changes in a coil, the magnetic field around each loop cut across the adjacent loops creating a voltage that opposes the voltage of the source of that current. The voltage across the coil is equal to the number of turns multiplied by the change in magnetic flux with respect to time. Let’s consider a coil of insulated wire wrapped around an iron core. Each loop of the coil is adjacent to the next loop. There is no space between loops. With the loops of wire touching each other, any change in current in a loop will result in a change in magnetic flux across the next loop hence creating an opposing current. When a current flows thru the coil, magnetic lines of force exist around the coil. The magnetic lines of force travel through the iron core magnetizing it. The effect of the iron core is the contract the magnetic lines of force. Due to the contracted magnetic lines of force, the inductance of the coil is increased because more lines of flux cross the coil with an increase in current.
Electromagnet
An electromagnet consists of a coil of insulated wire that is wrapped around an iron core. If you want to make an electromagnet, you need a battery, wire and the electromagnet. Because you are using DC current, opposition to the flow of current only occurs when power is provided and when power is removed. Hence there is no opposition to the flow of current and the coil may burn up if you are not mindful of the heat produced by the circuit. Hence, don’t use a large powerful battery unless you have a variable resistor in series with the coil to limit the current.
Variable Inductor
A variable inductor is an inductor with an adjustable core. In other words you can change the inductance by adjusting how much of the core in inside the coil. In the adjustable inductors I’ve seen, the core has threads and a slot at the top. So you can use a small screw driver to adjust the core. I recall using a plastic screwdriver so that the screwdriver would not affect the inductance.
References:
Understanding Physics
Isaac Asimov
Barnes & Noble Inc.
ISBN 0-88029-251-2
Introductory Circuit Analysis
Third Edition
Robert L Boylestad
Charles E Merill Publishing Company
ISBN 0-675-8559-4
My knowledge: I have a Bachelor of Science in Electrical Engineering.