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Chapter 5. Maxwell’s Fourth Law

Recommended Article : 【Physics】 Physics Table of Contents


1. Lenz’s Law

2. Faraday’s Law

3. Applications of Maxwell’s Fourth Law



1. Lenz’s Law

⑴ Definition : Induced electromotive force and induced current act in a direction to oppose the change in magnetic field.

Figure. 1. Lenz’s Law



2. Faraday’s Law

⑴ Overview

① Definition : Voltage is induced corresponding to the rate of change of the magnetic flux within a magnetic field.

② In 1820, Oersted discovered that a magnetic field is produced by an electric current.

③ In 1831, British scientist Faraday discovered Faraday’s law.

④ After Faraday proposed Faraday’s law, German physicist Lenz introduced Lenz’s law.

⑵ Formulation : It has a negative sign as the rate of change of the magnetic flux passing through the circuit with respect to time.

⑷ Induced electromotive force in a constant external magnetic field with a moving rectangular loop at a constant velocity.

⑸ Self-induced electromotive force

① Coil-induced electromotive force

② Magnetic energy of the coil

③ Series-connected composite self-inductance coefficient

④ Parallel-connected composite self-inductance coefficient



3. Applications of Maxwell’s Fourth Law

⑴ Generator

① 0° → 90° : Increase in magnetic flux density, induced current from a to b direction.

② 90° → 180° : Decrease in magnetic flux density, induced current from b to a direction.

③ 180° → 270° : Decrease in magnetic flux density, induced current from b to a direction.

④ 270° → 360° : Increase in magnetic flux density, induced current from a to b direction.

Figure. 2. Principle of a Generator

⑵ Electric Guitar

① 1st. The pickup lamp of an electric guitar has a structure in which a coil is wound around a cylindrical magnet.

② 2nd. When guitar strings are plucked, the magnetized guitar strings under the strings vibrate due to the magnet beneath.

③ 3rd. The magnetic flux passing through the coil changes due to the vibration of the strings.

④ 4th. Current flows due to the induced electromotive force generated in the coil.

Figure. 3. Principle of an Electric Guitar

⑶ Microphone

① 1st. A permanent magnet wound with a coil is connected to the diaphragm.

② 2nd. The diaphragm vibrates due to sound vibrations.

③ 3rd. The coil attached to the diaphragm moves around the permanent magnet, inducing a changing magnetic flux, resulting in induced current.

④ 4th. The greater the sound, the stronger the vibration of the coil, leading to a higher current intensity.

Figure. 4. Principle of a Microphone

⑷ Hard Disk

Component 1. Platter : Part that records information.

○ Made by coating an aluminum alloy or glass with a layer of ferromagnetic material, typically iron oxide.

○ External magnetic fields align the ferromagnetic material to store information.

○ Even after the external magnetic field is removed, the alignment remains, allowing continuous data storage.

Component 2. Head’s Core : Small electromagnet used to record information on the platter.

③ Maxwell’s third law is used when recording information on the hard disk.

④ Maxwell’s fourth law is used when reading information from the hard disk.

⑸ Magnetic Tape

① Strongly magnetic powder applied to thin plastic tape.

② Takes advantage of the property that the magnetic powder becomes magnetized by an external magnetic field to store information.

③ Information on the magnetic tape can be read using a head mounted on a magnetic tape player.

④ Example: Cards, banknotes

Transformer

⑹ Transportation Card

Wireless Charging for Electric Toothbrushes

⑻ Metal Detector

⑼ Theft Prevention Device

⑽ Illumination Device for Kickboards : When the wheel rotates, the coil around the permanent magnet generates induced current in the LED.

⑾ Magnetic Brake Device : Used in amusement rides, exercise bikes, and subways.

Induction Cooktop

⒀ Hybrid Car : Converts some of the car’s kinetic energy into electrical energy and stores it in the battery when decelerating.

⒁ Speedometer and Speed Control Device



Input : 2019.07.28 19:23

Modified : 2020.04.11 18:27

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