Basic Properties of Electric Charge

Additivity of Charge

Electric charge is a scalar quantity. The total charge of a system is the algebraic sum of all the individual charges present.

If a system contains $n$ charges $q_1, q_2, q_3, \dots, q_n$, the net charge $Q$ is:

• Sign matters: Positive and negative charges cancel each other in the sum.
• Example: A system with $+5 \, \text{C}$ and $-3 \, \text{C}$ has a net charge of $+2 \, \text{C}$.

Conservation of Charge

Charge is conserved in any isolated system. It can neither be created nor destroyed — only transferred from one body to another.

• The total charge of an isolated system remains constant over time.
• Example: When a glass rod is rubbed with silk, the rod gains positive charge and the silk gains an equal amount of negative charge. The net charge of the rod–silk system remains zero.

Quantization of Charge

Charge exists in discrete packets. The smallest possible free charge is the magnitude of the charge on an electron or proton, denoted by $e$.

Any observable charge $q$ is an integer multiple of $e$:

• Why quantization occurs: Charge is carried by electrons and protons, which are indivisible particles.
• Implication: You cannot have a charge of $0.5e$ or $1.7e$ in isolation.

Invariance of Charge

The amount of charge on a particle does not depend on its speed. Charge is a relativistic invariant — it remains the same in all frames of reference.

• Example: An electron moving at high speed still carries the same charge $e$ as a stationary electron.

Point Charges …