Electric Charge, Conductors, Insulators and Conservation

Rub a comb on your hair and it lifts tiny paper bits — a hint that matter carries a hidden property called charge. This lesson fixes the vocabulary and the three unbreakable rules of charge.

Definition — Electric charge: an intrinsic property of matter, carried by protons (+) and electrons (−), that produces electric effects. SI unit: coulomb (C).
Definition — Conductor vs insulator: conductors (metals, human body, earth) have free electrons that let charge flow; insulators/dielectrics (glass, plastic, ebonite) have no free charges, so charge stays put.

Basic properties of charge

• Two kinds: like charges repel, unlike charges attract.
• Additivity: total charge is the algebraic scalar sum, q = q₁ + q₂ + q₃ + ….
• Quantisation: charge exists only in integer multiples of e: q = ne, where e = 1.6 × 10⁻¹⁹ C and n is an integer.
• Conservation: the total charge of an isolated system stays constant; charge is only transferred, never created or destroyed.

Charging a body

• Friction: electrons transfer between two rubbed insulators; one becomes + and the other − by equal amounts.
• Conduction: touching a charged body to a conductor shares charge of the same sign.
• Induction: a nearby charged body (no contact) redistributes a conductor's charge; grounding then leaves an opposite net charge.

Exam Tricks & Tips

• 🎯 Use q = ne to find the number of electrons: n = q/e; a charge of −1 C means about 6.25 × 10¹⁸ excess electrons.
• 🎯 Charging by friction conserves charge — the two bodies gain equal and opposite amounts, so the pair's total stays zero.
• 🎯 Induction gives the opposite sign; conduction gives the same sign — a classic one-mark discriminator.
• 🎯 A body becomes positive by LOSING electrons, never by gaining protons (protons are bound in nuclei).
• 🎯 The earth is a huge reservoir: grounding neutralises a conductor by supplying or absorbing electrons.
• ❌ Common mistake: treating charge as continuous. It is quantised — a value like 2.5e is impossible for a real charge.

Expected exam pattern

Usually a 1-mark quantisation calculation (number of electrons) or a conceptual item distinguishing conduction from induction; sign prediction and conservation appear as assertion-reason questions.

Quick recap

Charge is quantised (q = ne, e = 1.6 × 10⁻¹⁹ C), additive, and conserved. Like repels, unlike attracts. Conductors carry free charge; insulators do not. Charge by friction, conduction (same sign) or induction (opposite sign).

Cover the answer, recall, then check. 11 cards on the basics of electric charge.

Q1. What is the SI unit of electric charge?
A1. The coulomb (C).

Q2. State the quantisation of charge.
A2. Charge exists only as integer multiples of e: q = ne, where e = 1.6 × 10⁻¹⁹ C.

Q3. What is the value of the elementary charge e?
A3. 1.6 × 10⁻¹⁹ C — the magnitude of the charge on a proton or electron.

Q4. State the law of conservation of charge.
A4. The total charge of an isolated system is constant; charge is only transferred, never created or destroyed.

Q5. How do like and unlike charges behave?
A5. Like charges repel; unlike charges attract.

Q6. What makes a material a conductor?
A6. It has free electrons (or ions) that can move, allowing charge to flow through it.

Q7. Name the three methods of charging a body.
A7. Friction, conduction (contact), and induction.

Q8. Which charging method leaves the OPPOSITE sign?
A8. Induction — the far conductor gains a charge opposite to the inducing body after grounding.

Q9. How does a body acquire a positive charge?
A9. By losing electrons — never by gaining protons, which are locked in nuclei.

Q10. How many excess electrons give a charge of −1 C?
A10. n = q/e = 1/(1.6 × 10⁻¹⁹) ≈ 6.25 × 10¹⁸ electrons.

Q11. Why is charge called additive?
A11. Total charge is the algebraic (signed) scalar sum of the individual charges: q = q₁ + q₂ + ….

Rub a plastic comb through dry hair and it lifts tiny bits of paper. That everyday spark is your first window into electric charge — the property of matter responsible for all electrical and magnetic phenomena, from lightning to the chip running your phone.

What is electric charge?

Electric charge is an intrinsic property of certain elementary particles that gives rise to electric force between them. There are two kinds — positive (carried by the proton) and negative (carried by the electron). Benjamin Franklin fixed the sign convention: like charges repel, unlike charges attract.

A body is normally neutral because it has equal numbers of protons and electrons. Charging simply means creating an imbalance — almost always by transferring electrons, because they are light and loosely bound, while protons sit locked in the nucleus.

Beginner: how bodies get charged

• Friction: rubbing glass with silk pulls electrons off the glass (glass becomes +, silk becomes −).
• Conduction: touching a charged body to a neutral one shares charge.
• Induction: a charged body brought near a conductor redistributes its charges without contact; earthing then leaves a net charge.

Intermediate: conductors vs insulators

Conductors (metals, human body, earth, ionic solutions) have free electrons that move easily, so charge spreads over the surface and can flow away. Insulators (glass, plastic, rubber, dry wood) hold charge exactly where it is placed because electrons stay bound. This is why a metal sphere must sit on an insulating stand to stay charged.

Advanced: the three governing principles

Charge obeys three deep laws you must know verbatim:

1. Additivity — total charge is the algebraic sum: q = q₁ + q₂ + q₃ + … (signs included). Charge is a scalar.
2. Quantisation — charge exists only in integer multiples of the elementary charge e: q = n·e, where n is an integer and e = 1.6 × 10⁻¹⁹ C. You can never isolate half an electron's charge. (Quarks carry ±e/3 but are never free.)
3. Conservation — the total charge of an isolated system is constant. Charges are neither created nor destroyed, only transferred. In pair production a photon becomes an electron (−e) and a positron (+e): net charge before and after is zero.

The SI unit is the coulomb (C). One coulomb is enormous — it is the charge of about 6.25 × 10¹⁸ electrons.

Worked example

A body has a charge of −2 μC. How many excess electrons does it carry?

Using quantisation, n = q / e = (2 × 10⁻⁶) / (1.6 × 10⁻¹⁹) = 1.25 × 10¹³ electrons. The negative sign tells us these are excess electrons (the body gained them).

Real-world / exam application

Conservation and quantisation underpin every numerical: charge sharing between identical spheres (each ends with the average charge), photoelectric and nuclear reactions (check charge balances on both sides), and why a charged conductor grounds instantly when you touch it. Aircraft and fuel trucks are bonded by conducting wires so friction-generated charge does not spark.

Exam tricks & shortcuts

• If two identical conducting spheres carrying q₁ and q₂ touch and separate, each carries (q₁ + q₂)/2.
• Any given charge must be a whole-number multiple of 1.6 × 10⁻¹⁹ C — a value like 3.2 × 10⁻²⁰ C is impossible.
• Mnemonic "QCA" — Quantised, Conserved, Additive — the three properties of charge in one word.

Key formulas

• Quantisation of charge: q = ne (e = 1.6 × 10⁻¹⁹ C, n = integer).
• Conservation of charge: total charge of an isolated system is constant.
• Additivity: q_total = q₁ + q₂ + … (algebraic sum, with sign).
• Charging by induction/conduction redistributes charge without creating it.
• 1 coulomb = charge of 6.25 × 10¹⁸ electrons.

Conductors have free electrons (charge moves freely); insulators do not. Charge is always an integer multiple of e.

Worked Example

Problem: A body carries a net charge of −1 microcoulomb (−1 μC). How many excess electrons does it possess? (Charge of an electron e = 1.6 × 10⁻¹⁹ C.)

Solution:
Step 1 — Recall that charge is quantised: any charge is an integer multiple of the elementary charge e, so Q = n·e.

Step 2 — Rearrange to find the number of electrons n:
n = |Q| / e.

Step 3 — Substitute Q = 1 × 10⁻⁶ C and e = 1.6 × 10⁻¹⁹ C:
n = (1 × 10⁻⁶) / (1.6 × 10⁻¹⁹).

Step 4 — Evaluate:
n = 6.25 × 10¹² electrons.

Step 5 — The negative sign of the charge indicates an EXCESS of electrons (a gain of electrons).

Answer: The body has 6.25 × 10¹² excess electrons.