Wait—Your Hand Crank Generator Is Actually a Motor!? The Experiment That Reveals the Secret Double Life of Electricity
I’m Ken Kuwako, the Science Trainer. Every day is an experiment.
“Where does electricity come from?” “Why do motors spin?”
Questions like these sound simple, but textbook diagrams and explanations don’t always make the concepts feel real. Sometimes, understanding arrives not through reading, but through experience.
One tool makes that possible: the hand-crank generator.
Most people think of it as a device that simply produces electricity. In reality, it’s much more than that. With a few clever experiments, a hand-crank generator can reveal the true nature of electricity, demonstrate how motors work, and even help us experience energy conversion firsthand.
Today, I’d like to share two fascinating experiments that never fail to spark curiosity and surprise in students. Before we begin, take a look at this video and see how many mysteries you can spot.
Experiment 1: A Hand-Crank Generator Can Also Be a Motor!
When I ask middle school students, “Have you ever used a hand-crank generator?” most answer, “Of course!” But surprisingly, around 10–20% have never even seen one before.
That’s why hands-on experiences matter. Letting students touch, turn, and explore the device themselves is a powerful way to ignite scientific curiosity.
First, connect the hand-crank generator to an LED and start turning the handle. The LED lights up immediately.
At this point, the device is acting as a generator, converting motion into electricity.
But what happens if we connect it to a battery instead?
Something remarkable happens: the handle begins to spin by itself.
In other words, the hand-crank generator is not only a generator—it can also function as a motor.
The secret lies in the coil and magnet inside the device. When electric current from the battery flows through the coil, the magnetic field exerts a force on it, causing the coil to rotate. That rotation drives the handle.
This is the fundamental operating principle of a motor.
The beauty of this experiment is that it reveals the dual nature of the device. The same machine that normally creates electricity when you turn the handle can become a motor when electricity is supplied to it.
A generator converts mechanical energy into electrical energy through electromagnetic induction, while a motor converts electrical energy into mechanical energy through magnetic forces acting on electric currents. Despite appearing different, both rely on essentially the same internal structure.
Experiment 2: Connect Two Generators and One Starts Turning the Other!
Next, prepare a second hand-crank generator and connect the two devices with wires.
What do you think will happen?
The result is surprising.
When you turn the handle of one generator, the handle of the other generator starts spinning on its own.
This happens because a chain of energy conversions is taking place.
The mechanical energy from your hand is converted into electrical energy inside the first generator. That electrical energy travels through the connecting wires to the second generator. Once there, it is converted back into mechanical energy, causing the second handle to rotate as a motor.
It’s a beautiful demonstration of energy transfer in action.
This experiment naturally leads to deeper questions:
“What did the electricity become?”
“Where did the energy move?”
Questions like these encourage students to think beyond what they see and begin tracing the flow of energy through a system.
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This website is filled with fun science experiments that can be performed at home, along with clear explanations and practical tips. Feel free to explore and discover something new.
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Science Ideas Channel features videos of exciting experiments!
６月のイチオシ実験！
レモンやオレンジで風船を割ろう！インパクトが抜群のリモネン風船の実験
テレビ番組監修・イベント等のお知らせ
- ６月３日（水）20:30〜 「
バカリズムのちょっとバカりハカってみた！」（テレビ東京）を科学監修・出演します。テーマは「 そばの出前は何人前まで運べるのか、限界を測ってみた」です。 - ６月４日(木) 7:00〜 「THE突破ファイル」（日本テレビ）について科学監修しました。
- 6月14日（日） 千葉大学インスタレーション「探究」にて講師を務めます
- 6月26日（金） 公開研究会「脱作業化！デジタル化と段階的指導で実現する オームの法則の探究」
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- 『大人のための高校物理復習帳』（講談社）…一般向けに日常の物理について公式を元に紐解きました。特設サイトでは実験を多数紹介しています。※増刷がかかり６刷となりました（2026/02/01）
- 『きめる!共通テスト 物理基礎 改訂版』（学研）… 高校物理の参考書です。イラストを多くしてイメージが持てるように描きました。授業についていけない、物理が苦手、そんな生徒におすすめです。特設サイトはこちら。
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