Alternating current flows through household outlets. In addition, electromagnetic waves, in which electric and magnetic fields propagate, are an extension of electricity and magnetism.
Basic Knowledge
Alternating current (AC): Current whose direction changes periodically.
・Japan: 50 Hz in eastern Japan and 60 Hz in western Japan; voltage 100 V (rms value)
・RMS value (rms): An equivalent value that does the same work as direct current. V_rms = V₀/√2
・Peak value (maximum value) V₀ = √2 × V_rms ≒ 141 V (for household 100V)
Electromagnetic waves: Waves that propagate through space while changing electric and magnetic fields induce one another.
・Speed in a vacuum: c = 3.0 × 10⁸ m/s
・Transverse waves (the electric and magnetic fields are perpendicular to each other and to the direction of propagation)
・Classification by wavelength (long→short): radio waves → microwaves → infrared → visible light → ultraviolet → X rays → γ rays
・All travel at the same speed c (in a vacuum)
Alternating current (alternating current, AC)（current whose direction changes periodically）
RMS value (rms)（V_rms = V₀/√2. The equivalent direct-current value of AC）
Electromagnetic wave（a transverse wave that propagates as the electric and magnetic fields induce each other）
Electromagnetic spectrum（the continuous classification of electromagnetic waves from radio waves to γ rays）
Visible light（electromagnetic waves visible to the human eye. Wavelength about 380 nm ～ 780 nm）
Speed of light c（the speed of electromagnetic waves in a vacuum. c = 3.0 × 10⁸ m/s）
Further Exploration (Background and Significance)
Why alternating current is advantageous for transmission compared with direct current: voltage can be easily changed using transformers, so power is transmitted at high voltage (several hundred thousand V) from power plants and reduced to 100 V near homes. Increasing the voltage decreases the current (because P = VI and P is constant), reducing Joule heat loss I²R in transmission lines.
History of electromagnetic waves: Maxwell theoretically predicted their existence in 1865, and Hertz confirmed them experimentally in 1887. This discovery led to wireless communication, radio, television, and Wi-Fi.
Examples of electromagnetic-wave applications:
・Radio waves: radio・TV・Wi-Fi・mobile phones
・Microwaves: microwave ovens・radar
・Infrared: remote controls・heating
・Ultraviolet: sterilization・tanning
・X rays: medical diagnosis
・γ rays: cancer treatment・radiation sterilization
- RMS value = peak value/√2
- Household 100 V has a peak value of ≒ 141 V
- Electromagnetic waves: c = 3.0 × 10⁸ m/s in a vacuum
- Electromagnetic waves are transverse waves
- In order of decreasing wavelength: radio waves→visible light→γ rays
- All travel at the same speed (in a vacuum)
- AC transmission allows voltage to be increased using transformers
Notes (Common Confusions)
① The household “100 V” is an RMS value. The peak value is about 141 V. ② Electromagnetic waves are transverse waves, not longitudinal waves (the electric and magnetic fields are perpendicular to the direction of propagation). ③ Wavelength and frequency are inversely related. A longer wavelength means a lower frequency. ④ Both X rays and γ rays are electromagnetic waves (electron beams and α rays are not electromagnetic waves).
Practice
- When the peak value of an alternating current is 200 V, find the RMS value.
- Arrange four electromagnetic waves in order of decreasing wavelength (radio waves, X rays, visible light, and infrared).
- State the frequency of the AC power supply in eastern Japan.