Core Practical: Investigating Charging by Friction
Electron Transfer: During friction, work is done against the surface forces, providing energy for electrons to migrate. The material with a higher affinity for electrons becomes negative, while the other becomes positive.
Conservation of Charge: The total amount of charge in the system remains constant; the positive charge gained by one material is exactly equal to the negative charge lost by the other. This principle is mathematically expressed as .
Force Interaction: Charged objects exert non-contact forces on one another. Like charges repel, causing objects to move apart, while opposite charges attract, drawing objects together. The magnitude of this force increases as the distance between the charges decreases.
| Material Pair | Direction of Electron Flow | Final Charge on Rod | Interaction with Polythene |
|---|---|---|---|
| Polythene & Cloth | From Cloth to Rod | Negative (-) | Repulsion (if like charge) |
| Acetate & Cloth | From Rod to Cloth | Positive (+) | Attraction |
Focus on Electrons: In any exam explanation involving static charging, you must explicitly mention the movement of electrons. Never state that 'positive charge moves,' as protons are bound within the nucleus and do not transfer during friction.
Force Identification: If a rod is repelled by a known negative charge (like polythene), it must be negatively charged. If it is attracted, it could be positively charged or potentially neutral (due to induction), though in this specific practical context, we usually look for positive charge.
Mathematical Intuition: Remember the 'loss of negative' logic. If an object starts at neutral () and loses one electron (charge ), the remaining charge is .
The 'Positive Flow' Error: Students often incorrectly assume that positive charges (protons) are transferred to make an object positive. Always correct this by explaining that positivity is the result of an electron deficit.
Discharging by Touch: Touching the charged ends of the rods with your hands will 'earth' the rod, allowing the excess electrons to flow through your body to the ground. This neutralizes the rod and causes the experiment to fail.
Conductive Rods: Attempting to use a metal rod (conductor) without an insulating handle will result in no charge build-up, as the charge will immediately flow through the metal to the holder.