Properties of Electric Charge
An In-depth Analysis for Class 12 Physics & Competitive Exams
1. Additivity of Electric Charges
The total electric charge of a system is the algebraic sum of all the individual charges located at different points inside the system. Unlike mass, which is always positive, charges can be positive or negative.
Example: If a system contains charges $+2C, -5C,$ and $+4C$, the net charge is $(+2 – 5 + 4) = +1C$.
2. Conservation of Electric Charge
According to the law of conservation of charge, the total charge of an isolated system remains constant over time. Charge can neither be created nor destroyed; it can only be transferred from one body to another.
- In a chemical reaction, the sum of charges before and after the reaction is always equal.
- In Pair Production, a gamma-ray photon materializes into an electron and a positron (Net charge $0 \rightarrow 0$).
3. Quantization of Charge
Experimental evidence shows that electric charge is not a continuous quantity. It exists only in discrete packets. The smallest unit of charge that can exist independently is the charge of an electron ($e$).
Where $n = 1, 2, 3…$ (any integer)
and $e = 1.6 \times 10^{-19} C$
Why is charge quantized? Because during friction or induction, only an integral number of electrons can be transferred. You cannot transfer 0.5 or 1.2 electrons.
4. Invariance of Electric Charge
Unlike mass, which increases with velocity (as per Einstein’s Relativity), the magnitude of an electric charge is independent of its speed. This means:
$Charge_{at \ rest} = Charge_{in \ motion}$
Interaction Between Charges
Like charges repel, unlike charges attract.
Charge vs Mass
| Property | Electric Charge | Mass |
|---|---|---|
| Type | Positive or Negative | Always Positive |
| Speed Effect | Invariant | Increases with Speed |
| Force | Attractive or Repulsive | Always Attractive |
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