Say Goodbye to the Electric Swing! Discover Fleming’s Left-Hand Rule with Just One Sheet of Aluminum Foil (The Force on a Current in a Magnetic Field)
I’m Ken Kuwako, your Science Trainer. Every day is an experiment.
Every time I do this experiment, I can’t help but say, “Wow!” There’s electric current that we can’t see, and there’s a magnetic field that we can’t see either. Yet when these two “invisible things” meet, something amazing happens: a real force appears.
And with the method I’m introducing today, you can recreate this fascinating phenomenon using something as familiar as aluminum foil, without having to build a large apparatus like an electric swing. It’s incredibly simple, but the result is the kind of experiment that sticks in your memory. Give it a try in class or as a science project!
What happens when electric current and a magnetic field “shake hands”?
The phenomenon in which an electric current experiences a force from a magnetic field is called the “Lorentz force.” We rarely think about it in everyday life, but this force is actually at work at the heart of machines that support our daily lives, such as motors and generators. Electric fans, trains, washing machines—in the end, all of them are powered by this invisible force.
Many people probably see the words “Fleming’s left-hand rule” in a textbook and think, “That sounds complicated.” But once you actually see a piece of aluminum foil twitch right before your eyes, the rule suddenly becomes something you can experience for yourself. Today, I’ll show you a simple way to experience this phenomenon firsthand.
What you need
- Aluminum foil (about 25 cm wide): Ordinary household aluminum foil is perfectly fine.
- Scissors: For cutting the aluminum foil.
- Ruler: Handy for cutting the aluminum foil into straight strips.
- Cellophane tape or double-sided tape: Used to secure the aluminum foil to the desk.
- DC power supply: Needed to run the current. I recommend a mini ammeter called Petit-X. It’s also a convenient safety feature that the power automatically shuts off when the current reaches 1 A.
- Lead wires (with clips): Used to connect the power supply to the aluminum foil.
- Strong magnets such as Alnico or neodymium magnets: Used to create the magnetic field. A horseshoe magnet can also be used.
How to do the experiment
1. Cut the aluminum foil: Cut the 25 cm-wide aluminum foil into long, narrow strips about 1.5 cm wide. Prepare enough strips for the number of students taking part. The narrower the strips, the lighter they are, making their movement easier to see even when the force is very small.
2. Secure it to the desk: Place the cut aluminum foil on the desk and firmly secure both ends with cellophane tape or double-sided tape. The key is to pull the foil tight so that it doesn’t sag when you attach it to the desk. If it sags, its movement will be much harder to see when the force acts on it.
3. Build the circuit: Connect a lead wire to each of the positive and negative terminals of the DC power supply (or batteries).
4. Position the magnet: Place a strong magnet, such as a neodymium magnet, directly above or below the aluminum foil. Simply changing the orientation of the magnet (N pole or S pole) will change the direction of the force. If you have time, be sure to give it a try and observe what happens.
5. Turn on the current!: Turn on the power supply and let the current flow. Be careful not to let too much current flow. When the current starts flowing, you can see the twisted part of the aluminum foil suddenly twitch. This movement is evidence of the force exerted on the current by the magnetic field—the Lorentz force.
For example, if you arrange the apparatus like this and turn on the current,
the aluminum foil strip is pushed toward you and moves.
If you arrange it like this and turn on the current,
the aluminum foil strip is pushed downward, almost as if a dog were lying down. The current is the same, and the magnet is the same, yet simply changing the orientation produces such a dramatic change in the movement. That is exactly what makes the Lorentz force so fascinating.
Here is an image-based teaching resource that you can use as a handout. Feel free to use it if you find it helpful.
Can students work their way to Fleming’s left-hand rule?
After the experiment, encourage students to think for themselves about the patterns they can find among the direction of the current, the direction of the magnetic field, and the direction of the force. What makes this phenomenon tricky is that it takes place in three-dimensional space, and it is difficult to fully explain using a flat sheet of paper. That is precisely why it is so valuable to move your hands and “learn it with your body” before trying to understand it purely in your head.
Hold your thumb, index finger, and middle finger at right angles to one another. Point your index finger in the direction of the magnetic field and your middle finger in the direction of the current, and your thumb will indicate the direction of the force. This is known as “Fleming’s left-hand rule.”
Students’ understanding can become much deeper when they first watch the aluminum foil move right before their eyes and then work their way toward the rule, rather than simply memorizing it from a textbook. First, discover the phenomenon with your own hands. Then learn that it has a name and a rule behind it. Perhaps this sequence is one of the best ways to make science feel like something that truly belongs to you.
Questions, requests, and collaborations
I want to bring the wonder and excitement of science a little closer to everyone! I share fun science experiments you can try at home, along with easy-to-understand tips and explanations. Take a look around and search for whatever catches your curiosity!
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