2026年7月2日 / Last updated : 2026年7月2日 桑子 研 Science

See the Invisible: The Mysterious Phase Shift Hiding in Coils

I’m Ken Kuwako, the Science Trainer. Every day is an experiment.

Did You Know That Current and Voltage Can Actually Fall Out of Step?

Have you ever imagined that the electricity flowing from a wall outlet might have voltage and current that aren’t perfectly synchronized? We use electricity every day without giving it much thought, but what powers our homes is alternating current (AC), where voltage and current constantly rise and fall like waves. Depending on the electrical component, those waves can line up perfectly—or drift apart in time.

In this experiment, we used an EasySense sensor to make this normally invisible behavior visible.

We measured the phase difference between voltage and current when AC voltage was applied to a coil. The recording lasted 500 ms with a sampling rate of 1000, giving us a highly detailed waveform. (Looking back, a 100 ms recording might actually have made the wave patterns easier to see.) Before we dive in, take a look at the video below.

In the graph, the red line represents voltage, while the blue line represents current.

First, the Basics: With a Resistor, They Move Perfectly Together

Let’s begin by looking at what happens when AC flows through a resistor.

Notice how the red voltage wave and the blue current wave overlap almost perfectly, like identical twins. Every peak and every valley occur at exactly the same moment.

That’s because a resistor has a very simple job: it merely limits the flow of electricity. As soon as the voltage rises, the current rises with it. Their relationship is direct and immediate, with no delay between them.

Everything Changes with a Coil: Voltage Leads, Current Follows

Now let’s replace the resistor with a coil while applying the same AC voltage.

This graph tells a completely different story. The voltage reaches its peak first, and the current arrives a little later. It’s almost like a game of tag, where the voltage takes off first and the current chases after it.

The delay isn’t random—it measures exactly 90 degrees of phase difference. In other words, the current reaches its maximum one-quarter of a cycle after the voltage does.

Why Does the Current Lag Behind in a Coil?

The answer lies in a unique property of coils.

Whenever current flows through a coil, it creates a magnetic field around it. With alternating current, that magnetic field is constantly changing as the current changes.

A changing magnetic field causes the coil to produce a voltage that opposes the change in current. This phenomenon is called self-induction.

You can think of it as the coil applying its own brakes. Even when the applied voltage is saying, “Come on, current—speed up!”, the coil generates a back electromotive force (back EMF) that resists that change. As a result, the current cannot respond instantly and lags behind.

That slight hesitation is exactly what appears on the graph as a 90-degree phase difference.

This Hidden Property Powers Many Everyday Devices

The behavior of coils isn’t just something you see in physics labs. It’s essential to countless devices we use every day.

Fluorescent lamp ballasts, transformers, electric motors, speakers, and even wireless smartphone chargers all rely on coils.

Engineers either take advantage of the phase difference between voltage and current or work to minimize the energy losses it can create. Either way, understanding this timing difference is a key part of making modern electrical devices work efficiently.

Electricity is usually invisible, but by plotting it with sensors, we can watch voltage and current perform an elegant dance—sometimes moving perfectly in sync, and sometimes intentionally stepping out of rhythm.

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