Question

How does charging by conduction occur?

A charged object is brought near a neutral object without touching it.
A neutral object touches another neutral object.
A charged object touches a neutral object.
A neutral object is brought near another neutral object without touching it.

Answer 1

Charging by conduction (also called charging by contact) means charge is transferred between objects by direct contact so that electrons actually move from one object to the other. Requirements:
- There must be a net charge difference between the two objects (a driving force).
- There must be a conducting path (they must be conductors or allow charge flow).
- When they touch, charges flow until the two conductors reach the same electric potential; total charge is conserved and is redistributed between them.

Apply that to the four situations you listed:
1. A charged object is brought near a neutral object without touching it.
- This is charging by induction (polarization), not conduction. No contact → no direct charge transfer (unless you then ground or touch the neutral object).

2. A neutral object touches another neutral object.
- No net charge to drive a transfer, so in ideal electrostatics nothing happens (still neutral). Not conduction in the sense of charging by contact.

3. A charged object touches a neutral object.
- This is charging by conduction. Charge flows on contact and the two share the net charge until they reach the same potential. Both end up charged (same sign as the original net charge).

4. A neutral object is brought near another neutral object without touching it.
- Nothing happens (both remain neutral); not conduction.

(If one of the neutrals initially has a tiny net charge or if contact involves insulating materials and friction, other effects like triboelectric charging can occur, but under the usual electrostatics assumptions only case 3 is charging by conduction.)