What is electromagnetic induction?

・In Japanese
＜Premise knowledge＞
・Magnet
・Magnetic flux

■What is electromagnetic induction?

When a magnet is brought close to or moved away from a coil, magnetic flux is generated within the coil to cancel out the magnetic flux of the magnet, and when magnetic flux is generated, an electric current is generated. This phenomenon is called electromagnetic induction, and the generated current/voltage is called induced current and induced electromotive force. According to Lenz's law, current flows in the direction of the index finger of the right hand when the thumb grips the coil in the same direction as the magnetic field lines.



＜Differences depending on how the coil is wound＞
The direction of the current changes depending on how the coil is wound. (If you consider that the coil is only viewed from the opposite side, it is natural that the direction of the current appears to have changed.)

　

■Self-induction

When current flows through the coil, a magnetic field is generated, but when the current begins to flow, a back electromotive force is generated to prevent the generation of the magnetic field. This is called self-induction, and the back electromotive force at this time is called self-induced electromotive force. However, this does not mean that a back electromotive force is generated until the current reverses direction. Also, when a current is flowing and a magnetic field is generated, the coil has the properties of a magnet.



＜Examples of using self-induction＞
The coil is used as a low-pass filter to remove circuit noise by taking advantage of its slow response to changes in current. Below is the RL circuit diagram using a coil. In this case, we do not know how the coil is wound and the direction of the magnetic flux, but the direction of the current does not change.



Other usage examples include making use of its property of becoming a magnet when current is passed through it, and using it as motors and solenoid valves.

■Mutual induction

With the coil wrapped around the iron rod as shown below, a current is applied to the primary coil. Then, magnetic flux is generated from the primary coil, but magnetic flux is generated from the secondary coil to cancel that magnetic flux. When magnetic flux is generated, current also flows to the secondary coil. This is called mutual induction. However, since magnetic flux is generated in the secondary coil only when the magnetic flux in the primary coil changes, current does not flow all the time.

Note that if the secondary coil is wound in the opposite direction, the direction of the current flowing will also be reversed.



The circuit diagram is shown below. The black dots represent the direction in which the coil is wound. A black dot is placed at the beginning of winding the coil, but I think it varies depending on the person as to which side to start winding. However, if the winding relationship with the secondary coil is appropriate, you can define either side as the winding start.



An example of the use of mutual induction is a transformer that converts alternating current.

List of related articles TOP MENU Electromagnetism ・Electromagnetic induction