Electric Charge

Electric charge is a fundamental property of matter that causes it to experience a force in the presence of other charges. It is as basic to physics as mass — but instead of gravity, charge governs electric and magnetic phenomena.

Real-Life Analogy: When you rub a balloon on your hair and it sticks to a wall, you are witnessing electric charge transfer — the same principle that governs lightning, photocopiers, and static shocks from a car door.

Electric charge is an intrinsic property of certain fundamental particles (like electrons and protons) that gives rise to electric and magnetic forces, causing them to experience a force when placed in an electromagnetic field.

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Electric charge is the physical property of matter that causes it to experience a force when placed in an electric field.

Key facts:

• SI Unit: Coulomb (C)
• Dimensional Formula: [A T] (Ampere × Time)
• Two types exist: positive and negative

Golden Rule: Like charges repel; unlike charges attract.

• Charge is a scalar quantity (adds algebraically, not vectorially)
• Charge is conserved — total charge of an isolated system remains constant
• Charge is quantized — it exists only in integral multiples of the elementary charge
• Charge is additive — net charge of a system is the algebraic sum of all individual charges
• Charge is invariant — does not change with the speed of the charged particle (unlike mass)

• The smallest amount of charge that can exist freely is called the elementary charge (e).
• Its value is 1.602 × 10⁻¹⁹ C — this is the charge on each proton and electron.
• Protons carry positive elementary charge (+e), and electrons carry negative elementary charge (−e).
• Since protons and electrons are equal in number, an atom is electrically neutral.

• Every atom has a nucleus with positively charged protons and neutral neutrons, surrounded by negatively charged electrons.
• If an electron is removed from an atom, it becomes a positive ion.
• If an electron is added to an atom, it becomes a negative ion.
• When a plastic comb is rubbed on dry hair, electrons transfer from hair to the comb — hair becomes positive, the comb becomes negative.

• The unit of charge is coulomb (C); the charge of an electron is e = 1.6 × 10⁻¹⁹ C.
• Any charge q must be an integer multiple of e: q = ne, where nn is a whole number.
• One coulomb contains 6.25 × 10¹⁸ electrons.
• Practical sub-units: 1 µC = 10⁻⁶ C, 1 nC = 10⁻⁹ C, 1 pC = 10⁻¹² C.

• When two non-conducting bodies are rubbed, electrons transfer — the body gaining electrons becomes negatively charged, the body losing electrons becomes positively charged.
• The SI unit of charge is coulomb (C); smaller units are milli-coulomb (mC = 10⁻³ C), micro-coulomb (µC = 10⁻⁶ C), nano-coulomb (nC = 10⁻⁹ C).
• Charge on a body is always q = ±ne, an integer multiple of the electron charge.
• In an insulator, charge stays static at the point of excess or deficit of electrons — it does not flow.
• In a conductor, free electrons can move and flow throughout the entire volume.

• When an uncharged conductor is brought near a charged body (without contact), the nearer side develops opposite charge and the far side develops similar charge — this is called induction.
• In induction, no charge is transferred between the charged body and the conductor.
• When the charged body is removed, the charges in the conductor redistribute freely again.

• Term "electricity" derived from Greek word elektron (amber), as amber attracted light objects when rubbed
• Benjamin Franklin introduced the convention of naming charges "positive" and "negative"
• Charles Coulomb later quantified the force between charges (Coulomb's Law)