A solar-driven aluminum rotor operating at natural atmospheric pressure: a low-cost tool for physics education

Abstract A reproducible and low-cost solar-powered device is presented that converts ambient solar radiation into sustained rotational motion under natural atmospheric pressure. The apparatus consists of three components: a base made of expanded polystyrene, an axis provided by a plastic ballpoint pen refill with the metallic ball tip facing upward, and a rotor constructed from an inverted aluminum ‘Pastel de Belém’ mold. The rotor begins spontaneous rotation shortly after the first exposure to sunlight, typically within a few seconds, and operates continuously throughout the day. Rotation slows gradually in shade but persists for several seconds due to residual thermal gradients. Potential mechanisms—including differential micro-scale convection, photon momentum transfer, and thermomechanical effects—are discussed, supported by quantitative estimates. The simplicity, low cost, and sustainability of the design make it an ideal platform for physics demonstrations at the undergraduate level.