Why is electricity divided into AC and DC?

Have you ever wondered why a mobile phone's charging plug is bigger than usual?

In fact, strictly speaking, this "plug" is called a switching power adapter, and its purpose is to convert alternating current from mains to direct current and transmit it to electronic equipment. The laptop's power adapter is larger, and it's more convenient to warm your hands in winter~

Why change AC to DC then? Let's start from beginning

Electricity is a natural phenomenon. Static or moving charges can cause a lot of interesting physical phenomena, such as lightning you see during a thunderstorm and crackle of sparks when you take off your sweater in winter. Later, scientists discovered laws of various electrical effects, so they invented batteries, generators, and electric motors.

Use DC or AC first?

Alternating current 220 volts / 50 Hz is power supply standard in our country, so many people think that all household appliances use alternating current. As far as DC is concerned, I can't think of anything other than "DC inverter air conditioner" to be lauded in advertising. In fact, direct current is widely used, and you have probably seen and used it. For example:

Definition

Direct current is an electric current whose direction does not change with time.

Alternating current, refers to a current that periodically changes in magnitude and direction.

When viewed with an oscilloscope, one is straight and other is curved...

It's electricity anyway, so it doesn't make much of a difference?

No~ No~ No~

Let's go back to history and tell some gossip!

In 1884, with a letter of recommendation from his former employer, a young man named Tesla hurried aboard a ship bound for Holy Land during gold rush in United States. He was going to find his idol, Thomas Edison, hoping that he would help him complete invention of alternating current system.

Thomas Edison

The letter reads:"Dear Edison! I know two great people, one of whom is you and other is this young man."At that time, Edison was selling himself to world. The DC system is not at all optimistic about AC. But with this letter, Tesla entered Edison team, as he wanted.

Edison promised to pay him equivalent of a million dollars in today's money if he solved "the problem of DC generator and motor."

But when Tesla fixed Edison's machine and asked, "What about agreed million?" American humor."The idol in his heart collapsed, and Tesla, who was teased, left in anger and became an independent business. Since then, he has devoted himself to his AC system.

Tesla and AC motor

At time, Edison's DC system required a power station per square mile, and transmission distance was very limited due to excessive losses in transmission process, while Tesla's AC system used more wires. Thin, higher voltage, small transmission loss, long transmission distance. Obviously, AC is more beneficial. What did loser Edison do? To "black" alternating current, Edison had a big hole in his brain. Bribe government officials to change death penalty from hanging to electrocution.

AC chair

They even hired children to shock stray cats and dogs with alternating current to promote that alternating current was "very dangerous." Fortunately, Tesla firmly believed that "regardless of whether it is dark or not, alternating current exists." Through lighting project at Columbia Expo, company demonstrated reliability and safety of AC, and finally won "battle of electric currents" and regained its innocence.

Aerial view of World's Fair in Colombia

Since then, alternating current has been recognized, replacing direct current, and has become main source of power.

Which is better, AC or DC?

As grid voltage continues to rise, transmission power and transmission distance continue to increase, and engineers are once again favoring DC. Since direct current does not require rectification and filtering, there is no phase difference and it is relatively stable. How is direct current amplified?

In simple terms, step-up work is done by AC, and AC can be converted to DC again ~ Moreover, from an economic point of view, although a DC converter station is more expensive than AC transmission. substation, DC line should only be positive, negative two wires, three wires are required for three phases of AC lines, and cost of DC lines is lower, so longer distance, more suitable for DC transmission.

Currently, highest voltage level transmission project in world is DC ±1100kV Zhongdong-South Anhui project. The total length of project line is 3,324 km, which is more than straight-line distance from Harbin to Haikou.

In field of power transmission, more than 30 kilometers of submarine cables and asynchronous connections between two AC systems also use DC.

Alternating current and direct current have their own characteristics. Some say AC is like a high-speed railroad and DC is like an airplane in air. Exactly. Which one to use depends on specific analysis of specific issues.

Why do household appliances use direct current?

Going back to home appliances, IT products (eg mobile phones, computers) and most home appliances use DC power internally. Since laptops and mobile phones need to be lightweight, rectifier is placed outside, while desktop computers, refrigerators, air conditioners, etc. place rectifier inside appliance.

Why? Back to point: direct current flows continuously from plus to minus, while alternating current oscillates. Electronic components work by detecting high and low potentials.

For example, potential of a computer is 1 and potential is 0, and alternating current itself will have a zero crossing potential, so electronic components cannot draw correct logical conclusions.

Electricity is an ever-evolving and complex topic, and it is obviously impossible to cover every situation in detail in this article. However, if your girlfriend asks, "Did you bring a plug to charge your phone with you?", you can safely get this article and correct it: "Honey, it's called a power adapter."