There are two main types of electricity used in homes. Let us learn their characteristics and how electricity is used in homes and industry.
Basic Knowledge
Direct Current (DC: Direct Current): Electric current with a constant direction and magnitude. Dry-cell batteries, batteries, solar cells, and USB devices.
Alternating Current (AC: Alternating Current): Electric current whose direction and magnitude change periodically. Household outlets and transmission from power plants.
AC frequency: The number of times the direction changes per second, expressed in Hz. In Japan, 50Hz in eastern Japan・60Hz in western Japan.
Household voltage:
・Japan: 100V(ordinary households), 200V(air conditioners・IH)
・United States: 120V、Europe: around 230V
A transformer changes these values and can raise or lower voltage according to the ratio of the numbers of turns in its coils: V₁ / V₂ = N₁ / N₂
Direct Current (DC)（Electric current with a constant direction. Batteries・battery packs）
Alternating Current (AC)（Electric current whose direction changes periodically. Outlets）
Frequency（Number of AC oscillations per second. Hz）
Transformer（Device that converts AC voltage using electromagnetic induction）
Rectification（Changing AC into DC. Uses diodes）
Adapter（Device that changes outlet voltage for electronic equipment. It also performs rectification）
Further Study (Background・Significance)
At the end of the 19th century, Edison promoted DC transmission, while Tesla and Westinghouse promoted AC transmission, resulting in the “War of the Currents”. AC eventually prevailed. This was because transformers can easily change voltage, reducing I²R losses during long-distance transmission.
In the 21st century, the importance of DC has been reevaluated for solar power generation, batteries, EVs (electric vehicles), and data centers. DC transmission technology (HVDC: High Voltage Direct Current) has also developed and is used for long-distance transmission in the North Sea region and China.
Flow of electricity used in homes:
Power plant(AC) → Transmission at high voltage → Step-down voltage successively at substations → Home(100V AC) → Rectification by an AC adapter → Smartphone・PC(DC)
Therefore, both DC and AC are indispensable in modern society.
High-school topics such as reactive power・active power, power factor, three-phase AC, and reactance will also be needed in the future. In middle school, make sure you understand the basic concepts.
- Direct current(DC): Constant. Batteries・battery packs
- Alternating current(AC): Periodic variation. Outlets・transmission
- Frequency in Japan: East 50Hz, West 60Hz
- Household voltage: Japan 100V, United States 120V, Europe around 230V
- Transformers are effective for AC only
- Transmission at high voltage = reduced loss ( I²R )
- Rectification: AC → DC(uses diodes)
Points to Note (Common Confusions)
① Transformers are for AC only. They cannot be used with DC because electromagnetic induction does not occur. ② Be aware of the difference between 50Hz and 60Hz. Some appliances (such as microwave ovens) may break if used in a region with a different frequency. ③ The 100V supplied by an outlet is an RMS value, and its instantaneous maximum value is about 141V(√2 times). ④ The danger of electric shock is determined more by the current and the path through the body than by the voltage. It is extremely dangerous when the current passes through the heart.
Practice
- Is the electricity from a household outlet direct current or alternating current?
- What is the frequency of alternating current in eastern Japan?
- Briefly explain why a transformer cannot be used with direct current.