Charging by friction transfers electrons when different materials are rubbed together; charging by contact transfers charge through direct touching; charging by electrostatic induction charges an object without the charged body touching it. This is SL and HL content in IB Physics topic D.2, Electric and magnetic fields.
All three methods obey conservation of electric charge: charge is transferred or redistributed, not created. In solids, electrons usually move while protons remain bound inside nuclei.
| Method | Mechanism | Typical final result |
|---|---|---|
| Friction | Two initially neutral, different materials are rubbed together. Electrons transfer because the materials attract electrons differently. | One object becomes negative and the other becomes positive, with equal and opposite transferred charge in an isolated system. |
| Contact | A charged object touches another object, allowing electrons to move between them. For conductors, charge redistributes until electrostatic equilibrium is reached. | The initially neutral object normally acquires the same sign as the original charged object. |
| Induction | A charged object brought nearby causes charge separation in a conductor. The conductor is grounded, allowing electrons to enter or leave; the ground is removed before the inducing object. | The conductor gains a net charge opposite to the inducing object's charge, without touching it. |
For example, bring a negatively charged rod near a neutral metal sphere. Electrons in the sphere are repelled. If the sphere is grounded, some electrons leave through Earth; disconnecting the ground and then removing the rod leaves the sphere positively charged.
A common misconception is that merely bringing a charged object near a conductor charges it permanently. This produces only polarization, meaning charge separation with zero net charge. Grounding is required for permanent charging by induction.
Exam technique: For an IB Explain question, identify whether contact occurs, describe electron movement, and state the final charge. Never say that positive charge “moves” through an ordinary solid or that rubbing creates charge.
