Energy Stored in a Capacitor

What it shows:

The electrical energy stored in a capacitor is converted to mechanical work, driving a motor and raising a weight.

How it works:

A motor 1 is mounted atop a 2.5m length of 2×4. As it turns, it raises a 1 lb mass on a string from the ground by wrapping the string around a spindle (figure 1). The motor is driven by the discharge of a 12800µF, 75V capacitor previously charged by a DC power supply. 2 A double throw switch allows a clean change-over from one circuit to the other.

figure 1. charging circuitry and motor/spindle assembly
energy in a capacitor

Setting it up:

The 2×4 can be clamped to the lecture bench (careful because it is very top-heavy). The circuitry is bench mounted, although the power supply used is very heavy so could be left on a cart. We have long leads dedicated to the motor and taped to the 2×4 so all connections can be made at bench level.


A qualitative demonstration of energy storage and conversion into work. The rise time of the mass is a couple of seconds; it will also unwind and return to earth so the demo can be repeated if necessary. Rating**

1 Robins & Myers 0.1 hp DC motor
2 Sorensen DCR150-18B power supply