Chapter revision

Revision summary

Key ideas, equations and common mistakes. Open any topic below for the full explanation.

Signs, units and interactions: Charge q or Q is measured in coulombs, C. Like charges repel; unlike charges attract. Neutral means equal total positive and negative charge. Attraction alone can also involve a neutral object.
Rubbing: Electrons transfer between materials. Gaining electrons makes an initially neutral object negative; losing them makes it positive. Protons do not transfer in these processes. An initially neutral isolated pair gains equal opposite net charges.
Induction with Earth: Bring the rod near without contact, connect Earth while holding the rod in place, disconnect Earth first, then remove the rod. A negative rod drives electrons out and leaves a positive conductor; a positive rod draws electrons in and leaves a negative conductor.
Electric field: A region where an electric charge experiences a force. Field arrows give force direction for a positive test charge. A negative charge has force opposite the arrow; the existing field does not reverse when the test sign changes.

Distinguish three changes

Charge movement and what remains afterwards
Process	What to explain
Transfer by rubbing	Electrons pass from one material to another. Include every object that gains or loses charge.
Redistribution in an isolated conductor	Electrons move within it while the rod is nearby. Total charge stays unchanged; removing the rod alone does not leave an initially neutral conductor charged.
Retained induction charge	Disconnect Earth, or separate touching conductors, while the inducing rod remains. Removing the rod afterwards redistributes the retained charge without removing its net amount.

With two touching neutral spheres and a negative rod near A, electrons move from A to B. Separate them before removing the rod: A remains positive and B negative. Removing the rod while they still touch allows the imbalance to disappear.

A gold leaf rises when it and the stem have like charge. Retained divergence after the inducing rod has gone indicates remaining charge; it does not alone establish the sign. Keep observations separate from the electron explanation.

Draw field patterns with their arrows

Positive isolated point charge: radial arrows outwards. Negative point charge: radial arrows inwards.
Unlike pair: lines run from positive to negative. Like pair: lines do not join like signs; they curve away from the region between the charges.
For a pair of negative charges, reverse the arrows of the corresponding positive-pair pattern so they enter the charges.
Lines do not cross. A field line represents a force-direction convention, not a guaranteed particle trajectory.

Follow the application chain

Hazard: charge separation and accumulation can cause a discharge spark, which may ignite a suitable vapour-air or combustible-dust mixture. A conducting path can reduce unwanted accumulation.

Precipitator: ionised gas transfers charge to suspended particles; the electric field drives them towards collectors; particles deposit; mechanical shaking removes the deposit. Gas flow, field direction and force on a negative particle are different directions or quantities.

New separator: use the stated field and charge sign before predicting deflection. A positive particle is forced along the field arrow; a negative particle is forced opposite it. Its previous motion does not vanish instantly.

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