Project 3 — The Spinning Motor

Text written by Claude Opus 4.8 (claude-opus-4-8).

Project 3 — The Spinning Motor

The one idea: electricity can push things and make them move — and if you flip the wires, it pushes the other way.

What you need

• 3×AA battery holder (4.5 V)
• A small DC motor
• Something to make the spin visible: a paper pinwheel, a little tape flag, or a bottle cap pushed onto the shaft
• Two alligator-clip leads

A small DC motor is low-resistance, so it draws more current than the LED did — no resistor needed, and 4.5 V is fine. Don’t hold the shaft still for long (a stalled motor draws a lot of current and warms up).

Build it

Just two wires — the simplest loop yet:

  [Battery +] --clip--> [motor terminal A]
  [Battery -] --clip--> [motor terminal B]

Stick the pinwheel/flag on the shaft so the spin is obvious. It runs the instant the loop closes.

The magic step: unclip both leads and swap them — + to B, − to A. The motor now spins the other way. Let the child do the swap themselves.

What to say to the child

“Up to now electricity made light and sound. Now watch — it can be a pusher. It’s spinning! And here’s the magic trick: if we swap the two wires, which way do you think it’ll spin? …Let’s see — the other way! Flipping the wires flips the direction.”

The reverse is the moment kids remember. “Flip it and it goes backwards” feels like real magic.

For you — the physics

A DC motor is a loop of wire in a magnetic field. Current through the loop feels a Lorentz force ($\mathbf{F} = I\,\mathbf{L}\times\mathbf{B}$) that twists it; a commutator flips the current’s direction in the loop every half-turn so the twist always pushes the same way and it keeps spinning. Reverse the battery and you reverse $I$ everywhere, so the torque reverses — hence the backwards spin.

Two more hooks, if the child (or you) wants them:

• Energy conversion. Battery chemistry → electrical energy → mechanical motion. Same “energy changes form” story you’ll see again with the capacitor.
• Back-EMF. As it spins up it generates a voltage opposing the battery, which is why current is highest at startup and at stall. You can feel this: gently load the shaft and the motor draws more.

(If you ever drive a motor from something more delicate than a raw battery, add a flyback diode across it to clamp the inductive kickback when it switches off. Straight off a battery, you don’t need to worry about it.)

Try next

• One cell vs. all three: fewer volts → slower spin. More push, more speed.
• Make it a boat or a buggy: tape the motor to a cork or a toy with the pinwheel as a propeller/fan. The point is that motion is now something the child can build with.

→ Next: Project 4 — The light that fades by itself, where electricity learns to do something slowly, over time.