Charging by induction and the gold-leaf electroscope sim
The student rubs a rod, charges two spheres (or one, by earthing it) without touching them, and tests the charges with a gold-leaf electroscope.
Static electricity · covers the O-Level (IGCSE/GCSE support) topic
What the sim shows
Rubbing moves electrons: polythene rubbed with wool gains them (−), acetate rubbed with silk loses them (+). In charging by induction the rod never touches. Its charge pushes or pulls the free electrons in the metal, while the positive charges stay fixed. Experiment A: the electrons are shared between two touching spheres; separating them while the rod is still near leaves equal and opposite charges (total zero), the nearer sphere opposite to the rod. Experiment B: touching the sphere earths it, so electrons flow to or from earth through your finger, arm and body; remove the finger first, then the rod, and the sphere keeps the charge opposite to the rod; remove the rod first and the electrons flow back, leaving it uncharged. The electroscope's leaf rises when it gains charge of either sign. A charged rod brought near makes it rise further if the charges are the same sign and fall if they are opposite: watch the first movement, because close up an opposite charge can make it rise again. An uncharged electroscope also rises a little, so only a charged one gives the test. Earthing the cap makes the leaf fall. Model: charge separated = rod charge × min(0.6, 9.6/d²) for a sphere (min(0.3, 4.8/d²) for the cap), d in cm from 30 cm down to 2 cm; an earthed conductor settles at minus that; touching shares charge equally (equal sizes); leaf angle = 5° per nC, up to 70°. Limits: the nC values are illustrative; dry air, so no leakage and no sparks; time is not to scale (each drag or button is one step, and charges settle in about half a second). Syllabus: Singapore O-Level 6091 13(g) asks for these experiments; Cambridge IGCSE 0625 and AQA 8463 do not list charging by induction.
- + fixed positive charge (never moves)
- − free electron · 1 sign ≈ 1 nC ≈ 6.2 billion electrons
- The rod: drag it near (30–2 cm); it never touches
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