AC vs DC Explained: Impedance, Phase and Power Factor
Direct current (DC) is electricity that always flows the same way. Alternating current (AC) changes its direction and size in a regular wave. In Japan, for example, wall outlets repeat this back-and-forth 50 or 60 times a second. With AC, capacitors and coils let current through more or less easily, depending on how fast it changes. The total opposition to AC, including resistance, is called impedance.
What you'll learn
- How DC and AC differ, and why wall outlets supply AC
- Why a "100-volt" outlet (as in Japan) is rated by its RMS value, not an average, and peaks at about 141 volts
- How resistors, capacitors and coils behave with AC, and what impedance means
- The basics of the timing shift between current and voltage (phase), radio tuning (resonance) and power factor
- Where this difference matters inside electronics, such as chargers and filters
Prerequisites: a rough idea of voltage (the push on electricity), current (the flow of electricity) and resistance (how hard it is to flow). There are almost no equations.
In everyday life
Look closely at a phone charger and you will find small print. It may say something like "Input: AC 100-240V 50/60Hz" and "Output: DC 5V." AC stands for alternating current, and DC for direct current. The charger takes AC from the wall outlet. It turns it into DC that the phone can use, and sends it out.
Electricity from dry-cell batteries and phone batteries, on the other hand, is DC from the start. The circuits inside a remote control or a phone also run on DC. So a home has two kinds of electricity, and they are converted where they meet.
This article first looks at how the two kinds differ. Next, it explains how capacitors and coils behave, which is special to AC. Finally, it shows how this helps in chargers and filters.
How it works 1: DC keeps one direction, AC keeps switching
Connect a small bulb to a battery, and current always flows the same way. It leaves the plus terminal, goes around the circuit and returns to the minus terminal. This electricity, steady in both direction and size, is DC. On a graph, it is a flat line whose height does not change over time.
Electricity from a wall outlet is different. The voltage rises smoothly to plus, returns to 0, falls to minus and comes back again. Electricity that repeats this back-and-forth is AC. Its shape is a wave called a sine wave. Because the voltage changes direction, the current also flows back and forth.
The number of back-and-forth cycles per second is called the frequency. Its unit is the hertz (Hz), and 50 Hz means 50 cycles per second. Japan, for example, uses two frequencies. The boundary runs roughly from the Fuji River in Shizuoka to Itoigawa in Niigata, with 50 Hz to the east and 60 Hz to the west. In the Meiji era (1868–1912), Tokyo brought in 50 Hz generators made in Germany. Osaka brought in 60 Hz generators made in the US, and the split remains today (Panasonic).
How it works 2: Why wall outlets use AC
The main reason is that AC voltage is easy to raise and lower. Wind two coils (wire wound round and round) on an iron core. A change in the current in one coil then creates a voltage in the other. This device is called a transformer. The voltage ratio is set by the ratio of turns. With one-tenth the turns, you get one-tenth the voltage.
A transformer works only while the current is changing. AC is always changing, so it passes through with its voltage changed. DC is steady, so nothing happens in the other coil, except at the moment you connect it. To change the voltage in many steps between a power plant and a home, AC was the convenient choice.
Sending power at high voltage cuts losses
So why change the voltage at all? Power lines have a little resistance. When current flows, part of the energy turns into heat and is lost. The lost power equals the current, times the current again, times the wire's resistance. As a formula, it is "current² × resistance," also written I²R.
The power you send is voltage times current. Send the same power at 10 times the voltage, and you need only one-tenth the current. The loss is proportional to the current squared, so it drops to (1/10)², or one-hundredth. In Japan, for example, large power plants raise the voltage to about 275,000–500,000 volts. Substations then lower it in steps. Transformers on utility poles make the last step, to 100 or 200 volts (Nippon Steel's company magazine).
How it works 3: An outlet's "100 volts" is an RMS value
AC voltage changes all the time. So which moment does an outlet rating, such as Japan's "100 volts," refer to? If you take the average, plus and minus cancel out, and you get 0. So a different definition is used.
Think of a device that turns electricity into heat, such as an electric heater. Find the DC voltage that makes it exactly as hot as the AC does. That DC voltage is the AC's "effective value," usually called its RMS (root mean square) value. A 100-volt outlet is 100 volts RMS. It warms a heater as much as 100 volts of DC would.
For sine-wave AC, the peak is about 1.41 times the RMS value (the square root of 2). So a 100-volt outlet actually swings between +141 and −141 volts. The heat produced is proportional to the voltage squared. Averaged over one cycle, this square is exactly half the square of the peak. Take the square root, and you get the peak divided by 1.41 (we skip the derivation).
Outlet voltage is not always exactly 100 volts, either. In Japan, the Electricity Business Act and its enforcement rules set a target for grid companies. They must try to keep the voltage within 6 volts above or below 101 volts. That is 95 to 107 volts.
How it works 4: Resistors, capacitors and coils
Here is the big difference between AC and DC. With AC, how easily a part lets current through depends on how fast the current changes. This section looks at three basic parts of electronic circuits. To keep things simple, we treat them all as ideal parts.
A resistor opposes current in the same way for DC and AC. Its opposition does not change with frequency. The electrical energy that flows through it leaves as heat.
Capacitors: the faster the change, the more they pass
A capacitor is two metal plates facing each other, with a thin insulating film between them. Apply a voltage, and plus charge collects on one plate and minus charge on the other. With DC, the current stops once the plates are full. How long filling takes is explained in "RC Time Constant Explained."
With AC, each time the voltage reverses, the stored charge flows out and builds up again the other way. As charge moves in and out, current flows back and forth in the circuit. The faster the direction reverses, the faster the charge moves, so more current flows. A capacitor blocks DC and passes fast changes more easily.
Coils (inductors): the faster the change, the less they pass
A coil is a part made of wire wound round and round. When current flows, it creates a magnetic force around it. When the current tries to change, that force changes too. This creates a voltage in the coil that works against the change. This voltage is called back EMF.
With DC, the current does not change, so a coil acts like a plain wire and passes it easily. With AC, the faster the change, the bigger the opposing voltage, and the harder it is for current to flow. A coil passes DC easily and resists fast changes. It is exactly the opposite of a capacitor.
Reactance and impedance
The opposition that capacitors and coils give to AC is called reactance. Its unit is the ohm (Ω), the same as for resistance. But unlike a resistor, it does not use up power as heat. Within each cycle, it stores energy and then returns it to the circuit, again and again.
A capacitor's reactance gets smaller as frequency rises. The formula is "1 ÷ (2 × pi × frequency × capacitance)." Capacitance is a measure of how much charge the part can store. For a 1 microfarad capacitor, it is about 3.2 kilohms at 50 Hz. At 1 megahertz (a million cycles per second), it drops to about 0.16 ohm. A coil works the other way. Its reactance is "2 × pi × frequency × the coil's strength," so it grows in proportion to frequency.
The total opposition to AC, resistance and reactance together, is called impedance. It gives one number for "how many ohms this circuit is at this frequency." But resistance and reactance do not simply add up. The reason is a "timing shift," which we look at next.
How it works 5: Current and voltage fall out of step (phase)
In a circuit with only a resistor, current peaks when voltage peaks, so the two waves line up. With a capacitor, they do not. The current into a capacitor is largest when the voltage is changing fastest. For a sine wave, that is the moment the voltage crosses 0.
As a result, in a capacitor the current peaks a quarter cycle before the voltage does. The timing within a wave is called its phase. Counting one cycle as 360 degrees, we say "the current leads by 90 degrees." The current does not foresee the future. It only means the peaks sit at different points in a repeating wave. In a coil, the opposite happens: the current lags the voltage by 90 degrees.
Current in a resistor is in step with the voltage, while current in a capacitor or coil is shifted by 90 degrees. Shifted waves peak at different times, so their sizes cannot simply be added. That is why impedance is not "resistance + reactance." Calculations use a method that handles size and shift angle together (we won't go into it here).
How it works 6: Resonance and radio tuning
Coils and capacitors respond to frequency in opposite ways. So when you combine them, there is exactly one frequency where their effects are equal. There, their opposite shifts cancel out, and the circuit responds strongly to that frequency. This is called resonance (the "Resonance" point in the figure above).
Radio tuning uses this. Signals from many stations reach the antenna together, each at a different frequency. In Japan, for example, AM radio uses 526.5 to 1606.5 kilohertz (as of 2026, Ministry of Internal Affairs and Communications). Turning the tuning knob changes the capacitance. That moves the resonant frequency, so only that station's signal is picked out strongly.
What we described is the classic design with a tuning knob. Instead of moving parts with a knob, most radios today pick a station by controlling frequency finely with electronic circuits.
How it works 7: Current that works and current that just sloshes (power factor)
When current and voltage are out of step, power is affected too. Capacitors and coils only store energy and give it back. That part of the current just travels back and forth between the power source and the device. It does no work, such as making heat or motion.
The share of the current that actually does work is called the power factor. It is a value between 0 and 1, and it is 1 when there is no shift. For example, a device using 800 watts at 100 volts needs only 8 amps at a power factor of 1. At a power factor of 0.8, it needs 10 amps. Line loss goes with the current squared, so it grows about 1.56 times.
Devices with many coils, such as motors, tend to have lagging current and a lower power factor. In power supplies such as those of PCs, a distorted current waveform also lowers the power factor. So many devices include a power factor correction circuit. It brings the input current closer to the voltage in shape and timing.
Inside electronics
Chargers: turning AC into DC
A charger first makes AC flow in one direction, using a part that lets current through only one way (a diode). This is called rectification, explained in "What Is a Diode?" The direction is now the same, but the voltage still ripples, so a capacitor smooths it out. A capacitor used for this is a smoothing capacitor.
Most chargers today then switch that DC on and off tens of thousands of times a second or faster. They pass it through a small transformer to lower the voltage, then rectify and smooth it again (ROHM). The faster the switching, the fewer turns and the smaller the core needed for the same voltage. This uses the coil's habit of "reacting more strongly to faster changes" to make chargers small and light (this is a simplified picture).
Filters: sorting by frequency
How easily current flows through capacitors and coils depends on frequency. With this, you can build a circuit that passes only the frequencies you need and weakens the rest. This is called a filter. The point where it starts weakening signals is given by the cutoff frequency (see "Decibels Explained").
A familiar example is the small capacitor placed right next to a chip's power pins. It connects between the power line and the voltage reference (ground). It does not pass the DC supply. It sends only fast wobbles (noise) to ground, which keeps the voltage reaching the chip steady. Filters are also basic parts of analog circuits that handle sound and radio waves (see "What Is an Analog Chip?").
Common misconceptions
Myth: A 100-volt outlet is always at 100 volts
In fact, it swings between about +141 volts and about −141 volts, 50 or 60 times a second. 100 volts is the RMS value, the DC voltage that would give off the same heat. Testers and other meters usually show the RMS value too, which makes this myth easy to believe.
Myth: A capacitor passes AC because electrons get through the insulating film
In fact, electrons do not cross the film. When charge builds up on one plate, the same amount of charge is pushed out of the other plate. This repeats as the direction flips, so from outside it looks as if current keeps flowing. "Passes" describes the circuit as a whole.
Myth: DC is old technology, and all power lines use AC
In fact, there are now devices that convert large amounts of power between AC and DC (today they mostly use semiconductors). Facilities that send power as DC are also in use. For example, since March 2021, a facility in Japan has linked Gifu and Nagano prefectures with a DC line (TEPCO Power Grid). It lets the 50 Hz and 60 Hz regions share power. AC was chosen in the 19th century because transformers made its voltage easy to change (see "What Is a Power Semiconductor?").
Key numbers
| Item | Number |
|---|---|
| Outlet frequency in Japan | East 50 Hz / West 60 Hz |
| Outlet voltage in Japan (RMS) | 100 V (kept within 95–107 V where possible) |
| Same, peak | About 141 V |
| Example transmission voltage (Japan) | 275,000–500,000 V |
| Transmission loss at 10 times the voltage | One-hundredth (for the same power) |
| Opposition of a 1 µF capacitor | About 3.2 kΩ at 50 Hz, about 0.16 Ω at 1 MHz |
| AM radio frequencies in Japan | 526.5–1606.5 kHz |
Take the same 1 microfarad capacitor. When the frequency rises 20,000 times, its opposition falls to one 20,000th. It barely passes outlet AC, yet lets changes as fast as radio waves almost straight through. A part's "how many ohms" only makes sense once you fix the frequency. That is a key point in the world of AC.
Learn more
- Terms: transformer / rectifier circuit / power factor / back EMF / characteristic impedance (the impedance that matters in wiring for high-speed signals)
- Try the numbers: RC time constant calculator / series and parallel resistor and capacitor calculator / Bode plot calculator
- Compare in a table: converter vs. inverter / series vs. parallel circuits / LDO vs. switching regulator
- Related explainers: "RC Time Constant Explained" / "Decibels Explained" / "What Is a Diode?" / "What Is an Analog Chip?"