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Magnetic Flux & Current Loops

Instructor: Matthew Bergstresser
Magnetic flux is a magnetic field that penetrates a specific area. In this lesson, we will learn how a conductor in the presence of a changing magnetic flux induces an EMF causing current to flow through the conductor generating another magnetic field.

The Microphone is On

The next time you are listening to someone sing into a microphone, think of this. Microphones turn sound into electric current. You may think there must be a mistake with that last sentence, but it's true. Microphones don't turn the sound they absorb into the sound that comes out of a speaker. That's the job of an amplifier. By the end of the lesson, you will know how the components of a microphone turn sound energy into an electrical signal.

Magnetic Induction

Michael Faraday discovered the phenomenon known as magnetic induction around 1830. He knew electric current in wires induced a magnetic field around the wire and was determined to figure out the reverse of that phenomenon, which is magnetic induction. In other words, he wanted to figure out how a magnetic field can induce an electromotive force (EMF) in a wire, which is what causes an electric current to flow. He eventually figured it out. He determined the wire has to be in a changing magnetic field in order for an EMF to develop and electric current to flow.

A Faraday disk. Moving a conductor through a magnetic field induces an EMF and current, which is represented by the dotted lines.
Faraday_disk

Magnetic Flux

Recall that magnetic field lines move out from the North end of the magnet and wrap around to the South end as shown in Diagram 1.


Diagram 1
B_field


Now, imagine these field lines going through a defined area. Faraday's law involves magnetic flux, which is the amount of magnetic field perpendicular to a defined area. Diagram 2 shows magnetic flux.


Diagram 2. Magnetic flux is a magnetic field (B) penetrating a specific area (A).
Flux


Faraday's Law.

Faraday's law is represented by Equation 1.


Equation 1
Eq1


  • ΔV is the electromotive force in volts.
  • N is the number of turns in a coil of wire.
  • B is the magnetic field strength in tesla.
  • A is the area the magnetic field is permeating in square meters.
  • Δt is the change in time in seconds.

There are two Δ symbols or ''changes in'' in this equation. The one in the numerator tells us that either the magnetic field strength changes and/or the area through which the magnetic field penetrates changes. Either way, this indicates a change in magnetic flux. The rate at which this change occurs affects the magnitude of the induced EMF. The faster the change in magnetic flux, the higher the magnitude of the voltage.

The negative in front of the equation deals with the sign of the induced EMF, which is Lenz's law. If the magnetic flux is increasing, the voltage is negative. If the magnetic flux is decreasing, the voltage is positive. Recall that current flow induces a magnetic field. When magnetic flux changes in a coil of wire, the direction of current induced is to generate a new magnetic field to resist the change in magnetic flux. In other words, nature wants magnetic flux to be constant.

To determine the direction of the induced current, point your right thumb in the direction the magnetic flux, Φ, needs to increase to keep the net magnetic flux constant. Your curled fingers indicate which way the current flows in a coiled wire. Diagram 3 shows the direction of current flow induced from a magnet moving relative to a coil of wire.


Diagram 3. The magnet is moving to the left causing a weakening flux inside the coil. The induced magnetic field needs to increase to the left to compensate for the decreasing magnetic flux, so you point your right thumb to the left. Your curled fingers show the direction of the induced current.
flux_change


Let's see how a microphone works based on the principles of Faraday's law.

Microphone

A microphone is similar to Faraday's disk in that it induces an EMF and electrical current due to movement of a conductor in a magnetic field. In the case of a microphone, the conductor is a coil of wire, and it isn't rotated in a magnetic field, it moves linearly in the field. Diagram 4 shows the inside of a basic microphone.


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