Chapter 5. Maxwell’s Fourth Law
Recommended Article : 【Physics】 Physics Table of Contents
1. Lenz’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
① 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.
② 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
⑹ RFID
① Comparison between RFID and NFC
| Category | RFID | NFC |
|---|---|---|
| Full Name | Radio Frequency Identification | Near Field Communication |
| Relationship | Superset concept | Subset technology of RFID |
| Communication Range | Few cm to several meters | Within 10 cm |
| Communication Direction | Unidirectional or bidirectional | Bidirectional |
| Example Applications | Logistics, asset management, animal identification, access control | Transport cards, mobile payments, e-passports |
| Frequency | LF (125 kHz), HF (13.56 MHz), UHF (860–960 MHz) | HF (13.56 MHz) |
Table 1. Comparison between RFID and NFC
② Access Gate Operation Principle
○ 1st. Bring the card with a built-in RFID/NFC tag close to the reader.
○ 2nd. The reader emits an electromagnetic wave at 13.56 MHz, which powers the card and establishes a communication channel.
○ 3rd. The card communicates with the reader via contactless transmission, sending its unique ID and authentication data upon request.
○ 4th. The reader sends the received data to the server or local controller.
○ 5th. The server/controller verifies the user information and sends the authentication result (OK/FAIL) back to the reader.
○ 6th. The reader transmits the OK signal to the gate control module.
○ 7th. The gate opens: flap gates, turnstiles, or sliding doors are automatically activated.
⑺ 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.
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