Van de Graaff Generator
Switch on the motor, watch charge pile up on the dome, then bring in the earthed wand and make a spark jump.
Drag the wand ↔
Motor off
Dome voltage
0 V
Spark reach
—
Sparks made
0
In this model the dome gains extra electrons, so it becomes negatively charged. Some generators are built to make the dome positive instead.
How it works
Tap a step to light up that part of the generator.
Big idea: the generator doesn't make charge. It moves electrons from one place to another, so the dome ends up with extra electrons and the bottom of the machine ends up short of them. Charge is always conserved.
Try this
- Charge the dome fully, then move the wand in slowly. How far away does the first spark jump?
- Let the dome recharge with the wand held still just outside spark range. What happens to the gap between sparks?
- Switch to a humid day. Why can't the dome reach full charge any more?
- At full charge, switch the motor off. How long does the dome hold its charge on a dry day compared with a humid one?
- Touch the wand right onto the dome. Why is there no big spark?
Did you know?
Quick check
Safety in the real lab
Teacher-led only. Anyone with a pacemaker or heart condition should not touch or stand near a charged generator. Keep phones, laptops and other electronics away from the dome. Always discharge the dome with the earthed wand before touching it.