AC vs DC Voltage-What’s the Difference?

Voltage is the “push” that causes current to flow in an electric circuit. This push drives the flow of electrons in the circuit, which is called current and measured in Amperes (Amps) or Coulombs per second. 

DC stands for Direct Current, which is what you get from a battery or solar panel. The voltage associated with direct current is constant, meaning it doesn’t fluctuate over time. 

AC stands for Alternating Current, which is what you get from a wall outlet or other high-voltage source. As the name suggests, the voltage associated with alternating current fluctuates between upper and lower values, usually alternating between positive and negative and centered around zero volts. 

DC voltage is constant while an electrical circuit is functioning (i.e., connected or plugged in). In contrast, AC voltage fluctuates back and forth between high and low values. If you think of the current flow as water in a pipe, it would appear as though the water is rushing back and forth rapidly. If you were to chart perfect AC voltage versus time, it would follow a sinusoidal pattern. The frequency of the AC current and voltage—how often it fluctuates back and forth—is measured in Hertz (Hz), where one Hertz represents one cycle per second. In most of the world, household AC voltage is provided at either 50 Hz or 60 Hz, depending on the country. At 60 Hertz, the current and voltage change direction every 1/60th of a second. The time it takes for one sinusoidal wave to complete is 1/60th of a second. 

A DC circuit doesn’t have a frequency because there is no change in voltage or current over time. In the U.S., AC voltage has a frequency of 60 Hz and an ideal voltage of 120V. 

In Europe, AC voltage has a frequency of 50 Hz and an ideal voltage of 230V. In both cases, the voltage may fluctuate by as much as ±5% from the ideal value. However, the frequency is controlled more tightly, with variations of no more than ±0.5%. 

Around the world, AC voltage ranges from 100V to 127V and 220V to 240V. The frequencies are usually 50 Hz or 60 Hz, though there are exceptions. For example, some countries use 16 2/3 Hz for certain railway power networks, like in Germany, Austria, Switzerland, Sweden, and Norway. Commercial airplanes use 400 Hz both on the ground and in the air. 

Transmitting electricity via AC is preferred because it’s less expensive to produce and more efficient for long-distance transmission, as there are fewer energy losses compared to DC power. 

As a side note, power is transmitted at high voltage because this further maintains efficiency and reduces losses through heat. These transmission voltages can be as high as 1.5 million volts! If you’re familiar with Ohm’s law (see a previous article), this helps keep the amperage low, and lower amperage means less heat production. The AC voltage is stepped down to a more usable level before reaching homes or factories. 

Higher voltage is also less expensive to transmit, as it reduces the current, allowing for thinner wires to be used. Thinner wires lower the cost of transmission. This is true for house wiring as well—wires can be thinner at higher voltages because less current flows through them (which determines wire size) while delivering the same power as lower-voltage, higher-current systems. 

Lower voltage, like in the U.S., is preferred because it’s generally safer, particularly in cases of electrocution. 

When it comes to electric heating elements, they don’t “care” if they are subjected to AC or DC voltage, and with AC voltage, they don’t “care” about the frequency either. It’s all the same to them. A heater designed to operate at 230V AC and 60Hz in a U.S. factory will work just as well in Europe at 50Hz. If you could find a source, the heater would also work equally well at 120V DC. 

A minuscule amount of inductance is created when an AC source is used, but it’s so small that it can generally be ignored. 

Many of the different units in science are named after people. Here’s a list from today’s article. 

The Volt (the pushing force in a circuit) 
 - Alessandro Giuseppe Antonio Anastasio Volta (1745 – 1827) - Italian physicist and chemist 

The Amp or Ampere (the unit of current)
 - André-Marie Ampère (1775–1836) - French physicist and mathematician 

 The Coulomb (the unit of electrical charge)
 - Charles-Augustin de Coulomb (1736 – 1806)
 - French officer, engineer and physicist The Hertz (used to measure frequency)
 - Heinrich Rudolf Hertz (1857 – 1894) - German physicist