Grades 3–5 · Systems
How can a hand-crank flashlight shine after the handle stops?
Turning the handle on a hand-crank flashlight supplies mechanical energy. Gears can turn a small generator faster than the handle. Inside the generator, motion between magnetic parts and coils produces an electrical output.
In the diagramHand work can be stored for later lightcrankgeneratorchargerbatteryLEDmechanical → electrical → stored chemical energyThe battery can supply the light after the handle stops.
In the illustrated design, a charging circuit sends some of that energy into a rechargeable battery. Changes inside the battery store energy chemically. The handle therefore does more than keep the lamp connected for the instant it is moving.
After the handle stops, the battery can continue supplying a circuit that powers the light. An LED converts part of the electrical energy into light and releases other energy as heat. Once the stored energy is used up, turning the handle again can replenish some of it.
This device does not make free energy. The person turning the handle does work, and each conversion has losses. How long the light lasts depends on the energy stored, the lamp’s power and the particular design, rather than on a fixed rule for every crank flashlight.
About this explanation
Starting knowledge: Know that a circuit transfers energy and distinguish storing energy from supplying it directly.
Sources
- Goal Zero — Torch 250 Flashlight: hand-crank charging and battery storage