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Electrostatics Foundation

Properties of Electric Charge

A premium full-width Physics resource for CBSE, NEET, IIT-JEE, JEE Main, JEE Advanced, IB, ICSE and IGCSE students.

  • Quantization
  • Conservation
  • Additivity
  • Invariance
  • Exam Questions
  • Case Studies
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Electric charge diagram showing attraction and field lines + - e Q = ±ne | Charge is conserved

Concept foundation

Meaning of Electric Charge

Electric charge is an intrinsic property of matter because of which particles and bodies participate in electromagnetic interaction. It explains attraction, repulsion, electric fields, current, sparks, atomic binding and the behavior of conductors and insulators.

What students must remember

An electron carries charge -e and a proton carries charge +e, where e = 1.6 × 10-19 C. Matter is normally neutral because total positive charge balances total negative charge. A charged body has either excess electrons or deficient electrons.

  • Negative charge usually means excess electrons.
  • Positive charge usually means deficiency of electrons.
  • Charge is algebraic, so signs must be retained.
  • The SI unit is coulomb, symbol C.

Exam-focused interpretation

Most electric charge questions are not about memorizing a definition. They test whether you can decide the sign, count electrons, apply conservation to the correct system and reject impossible fractional values of charge.

Key line: Electric charge is transferred, redistributed or separated; it is not created from nothing in ordinary charging.

Core properties

Additivity, Conservation, Quantization and Invariance

These four properties build the logic of electrostatics. They also appear repeatedly in NEET, JEE Main, JEE Advanced, IB, ICSE and IGCSE questions.

Additivity of charge

Qtotal = q1 + q2 + q3 + ... = Σq

The total charge of a system is the algebraic sum of all charges. For example, +8 µC, -3 µC and -7 µC give net charge -2 µC. The sign is part of the value.

Conservation of charge

Σqbefore = Σqafter

Total charge of an isolated system remains constant. Individual objects may gain or lose charge, but one part gains what another part loses.

Quantization of charge

Q = ±ne,   n = 0, 1, 2, 3, ...

A body cannot gain half an electron. Therefore exact charge values must be integral multiples of e. This is why 4.8 × 10-19 C is possible but 4.0 × 10-19 C is not.

Invariance of charge

qrest = qmoving

Electric charge does not depend on speed or frame of reference. A fast electron still has charge -e, and a fast proton still has charge +e.

Charging methods

Friction, Conduction, Induction, Pair Production and Annihilation

All charging processes must obey conservation of charge. The difference lies in how charges move, separate or appear as equal opposite pairs.

Charging by friction

Rubbing two materials transfers electrons. One body becomes negative and the other becomes equally positive if the pair is isolated.

Charging by conduction

Charge is transferred through contact. Identical conductors often share total charge equally after contact.

Charging by induction

A nearby charged body causes charge separation without contact. Grounding can make the induced charge permanent.

Pair production

A neutral photon can produce an electron and positron. The final charge is -e + e = 0.

Annihilation

An electron and positron can annihilate into photons. Initial and final total charge are both zero.

System boundary

If charge crosses your chosen boundary, the charge of that chosen system may change. The larger isolated system still conserves charge.

Signs and comparison

Interaction Between Charges and Charge vs Mass

Like charges repel; unlike charges attract. Charge behaves very differently from mass because charge has sign and can cancel algebraically.

Positive charges repel + + Like charges repel
Negative charges repel - - Same sign repulsion
Opposite charges attract + - Unlike charges attract
PointElectric ChargeMass
SignPositive, negative or zeroPositive in school-level mechanics
AdditionAlgebraic addition with signsArithmetic addition
InteractionAttraction or repulsionGravitational attraction
QuantizationQ = ±neNot treated as quantized in this chapter
InvarianceCharge is frame independentRest mass is invariant; energy changes with speed

Question bank 1

NEET Electric Charge Questions

Fresh single-correct MCQs focused on quantization, electron count, conservation, charge transfer, attraction-repulsion and common conceptual traps.

NEET original MCQ

NEET MCQ 01: Allowed Elementary Charge

Question: Which charge can exist on an isolated body?

  1. +2.5e
  2. -7e
  3. +0.75e
  4. -3.2e
Correct answer: B. -7e
Short explanation: Charge on an isolated body must be Q = ±ne, where n is an integer. Only -7e satisfies this condition.
NEET original MCQ

NEET MCQ 02: Impossible Coulomb Value

Question: Which value is not possible as the exact charge of an isolated oil drop?

  1. 3.2 × 10-19 C
  2. 4.8 × 10-19 C
  3. 5.6 × 10-19 C
  4. 8.0 × 10-19 C
Correct answer: C. 5.6 × 10-19 C
Short explanation: Dividing by e = 1.6 × 10-19 C gives n = 3.5, not an integer.
NEET original MCQ

NEET MCQ 03: Excess Electrons From Charge

Question: A sphere has charge -6.4 × 10-19 C. How many excess electrons are present?

  1. 2
  2. 3
  3. 4
  4. 8
Correct answer: C. 4
Short explanation: n = |Q|/e = 6.4/1.6 = 4. Negative charge means excess electrons.
NEET original MCQ

NEET MCQ 04: Electron Deficiency

Question: A conductor has charge +9.6 × 10-19 C. The conductor is deficient by how many electrons?

  1. 3
  2. 4
  3. 6
  4. 9
Correct answer: C. 6
Short explanation: n = Q/e = 9.6/1.6 = 6. Positive charge means missing electrons.
NEET original MCQ

NEET MCQ 05: Net Charge Addition

Question: Three small bodies carry +2e, -5e and +4e. The net charge is

  1. +e
  2. -e
  3. +11e
  4. -7e
Correct answer: A. +e
Short explanation: Add algebraically: Q = +2e - 5e + 4e = +e.
NEET original MCQ

NEET MCQ 06: Friction Charging Pair

Question: A plastic rod becomes -12 nC after being rubbed with fur. The charge on the fur is

  1. -12 nC
  2. +12 nC
  3. 0 nC
  4. +6 nC
Correct answer: B. +12 nC
Short explanation: In rubbing, electrons transfer. The isolated rod-fur system keeps total charge zero, so the fur gets equal positive charge.
NEET original MCQ

NEET MCQ 07: Conservation In Contact

Question: Two conductors with charges +7 nC and -2 nC are touched and separated. What must be the total charge after separation?

  1. +9 nC
  2. +5 nC
  3. -5 nC
  4. 0 nC
Correct answer: B. +5 nC
Short explanation: Touching may redistribute charge, but total charge remains +7 nC - 2 nC = +5 nC.
NEET original MCQ

NEET MCQ 08: Sure Test Of Charge

Question: The sure test that two objects are charged is

  1. attraction between them
  2. repulsion between them
  3. heating on rubbing
  4. change in mass
Correct answer: B. repulsion between them
Short explanation: Attraction can occur between a charged and a neutral body due to induction. Repulsion requires both bodies to be charged with the same sign.
NEET original MCQ

NEET MCQ 09: Neutral Matter

Question: A neutral atom is neutral because

  1. it has no particles
  2. it has no protons
  3. total positive charge equals total negative charge
  4. electrons have no charge
Correct answer: C. total positive charge equals total negative charge
Short explanation: The positive charge of protons is exactly balanced by the negative charge of electrons.
NEET original MCQ

NEET MCQ 10: Losing Electrons

Question: A neutral body loses 5 electrons. Its charge becomes

  1. -5e
  2. +5e
  3. 0
  4. -10e
Correct answer: B. +5e
Short explanation: Loss of electrons leaves an electron deficiency, so the body becomes positively charged.
NEET original MCQ

NEET MCQ 11: Gaining Electrons

Question: If a neutral body gains 8 electrons, its charge is

  1. +8e
  2. -8e
  3. +4e
  4. zero
Correct answer: B. -8e
Short explanation: Electrons are negatively charged. Gaining 8 electrons gives Q = -8e.
NEET original MCQ

NEET MCQ 12: Charge Of One Microcoulomb

Question: The number of elementary charges in 1 µC is approximately

  1. 6.25 × 106
  2. 6.25 × 1012
  3. 1.6 × 1013
  4. 1.6 × 10-13
Correct answer: B. 6.25 × 1012
Short explanation: n = 10-6/(1.6 × 10-19) = 6.25 × 1012.
NEET original MCQ

NEET MCQ 13: Algebraic Nature

Question: The additivity of charge means total charge is found by

  1. multiplying magnitudes
  2. adding masses first
  3. algebraic addition of signed charges
  4. ignoring negative charges
Correct answer: C. algebraic addition of signed charges
Short explanation: Charge has sign, so +q and -q can cancel in the total.
NEET original MCQ

NEET MCQ 14: Positron Pair

Question: In pair production, a photon produces an electron and a positron. The total charge after production is

  1. -e
  2. +e
  3. 0
  4. -2e
Correct answer: C. 0
Short explanation: Electron charge is -e and positron charge is +e, so the final total is zero.
NEET original MCQ

NEET MCQ 15: Annihilation Charge

Question: An electron and a positron annihilate to produce photons. Which statement is correct?

  1. Charge changes from -e to zero
  2. Charge changes from +e to zero
  3. Net charge remains zero
  4. Photons carry +e and -e
Correct answer: C. Net charge remains zero
Short explanation: Before annihilation, -e + e = 0. Photons are neutral, so total charge remains zero.
NEET original MCQ

NEET MCQ 16: Invariance

Question: A fast moving electron has charge

  1. less than -e
  2. more than -e
  3. zero
  4. exactly -e
Correct answer: D. exactly -e
Short explanation: Electric charge is invariant; it does not depend on the speed of the particle.
NEET original MCQ

NEET MCQ 17: Unlike Charges

Question: A +3 nC body and a -2 nC body are brought near each other. The force between them is

  1. attractive
  2. repulsive
  3. always zero
  4. gravitational only
Correct answer: A. attractive
Short explanation: Unlike electric charges attract each other.
NEET original MCQ

NEET MCQ 18: Like Charges

Question: Two bodies repel each other. Which pair is possible?

  1. +q and -q
  2. +q and neutral
  3. -q and -q
  4. neutral and neutral
Correct answer: C. -q and -q
Short explanation: Like charges repel. A charged and neutral body usually attract due to induction.
NEET original MCQ

NEET MCQ 19: Quantization Trap

Question: A body is said to have charge 2.4 × 10-19 C. This is

  1. possible because charge is continuous
  2. possible because n = 2
  3. impossible as an exact isolated charge
  4. equal to 4e
Correct answer: C. impossible as an exact isolated charge
Short explanation: n = 2.4/1.6 = 1.5, which is not an integer.
NEET original MCQ

NEET MCQ 20: Charge Transfer Direction

Question: In ordinary charging of a metal sphere, the mobile charge carriers are mainly

  1. protons
  2. neutrons
  3. electrons
  4. nuclei
Correct answer: C. electrons
Short explanation: In metals and most solid charging examples, electrons move while protons remain bound in nuclei.
NEET original MCQ

NEET MCQ 21: Contact With Earth

Question: A positively charged conductor is touched by hand and becomes neutral because

  1. protons leave the conductor
  2. electrons flow from earth to the conductor
  3. neutrons enter the conductor
  4. charge is destroyed
Correct answer: B. electrons flow from earth to the conductor
Short explanation: Positive charge is electron deficiency. Grounding supplies electrons and neutralizes the conductor.
NEET original MCQ

NEET MCQ 22: System Boundary

Question: If charge enters a chosen system from outside, then the charge of that chosen system

  1. must remain unchanged
  2. may change
  3. must become zero
  4. violates conservation of charge
Correct answer: B. may change
Short explanation: Conservation applies to isolated systems. A system exchanging charge with surroundings is not isolated.
NEET original MCQ

NEET MCQ 23: Charge Unit

Question: The SI unit of electric charge is

  1. ampere
  2. volt
  3. coulomb
  4. tesla
Correct answer: C. coulomb
Short explanation: Electric charge is measured in coulomb, symbol C.
NEET original MCQ

NEET MCQ 24: Net Neutral System

Question: A system contains +6e, -2e, -3e and -e. Its net charge is

  1. 0
  2. +2e
  3. -2e
  4. +12e
Correct answer: A. 0
Short explanation: Q = +6e - 2e - 3e - e = 0.
NEET original MCQ

NEET MCQ 25: Smallest Freely Observed Charge

Question: For school-level electrostatics, the smallest freely observed unit of charge is

  1. 1 C
  2. e
  3. e/2
  4. 2e
Correct answer: B. e
Short explanation: The elementary charge e = 1.6 × 10-19 C is the smallest freely observed charge unit in ordinary school physics.

Question bank 2

JEE Main Electric Charge Questions

Fresh numerical and conceptual questions with formula, step-by-step solution and final answer.

JEE Main original

JEE Main 01: Electrons For Nanocoulomb Charge

Question: A metal sphere has charge -3.2 nC. Find the number of excess electrons.

Formula used: n = |Q|/e
Step-by-step solution:
  1. Convert charge: |Q| = 3.2 × 10-9 C.
  2. Use e = 1.6 × 10-19 C.
  3. n = (3.2 × 10-9)/(1.6 × 10-19) = 2.0 × 1010.
Answer: 2.0 × 1010 excess electrons.
JEE Main original

JEE Main 02: Charge After Electron Loss

Question: A neutral body loses 4.0 × 1012 electrons. Find its charge in microcoulomb.

Formula used: Q = +ne
Step-by-step solution:
  1. Losing electrons produces positive charge.
  2. Q = (4.0 × 1012)(1.6 × 10-19) C.
  3. Q = 6.4 × 10-7 C = 0.64 µC.
Answer: +0.64 µC.
JEE Main original

JEE Main 03: Total Charge After Internal Transfer

Question: Bodies A, B and C have charges +5 µC, -8 µC and +1 µC. If 2 µC of charge is transferred internally from C to B, find the final total charge of A+B+C.

Formula used: Qtotal = Σq
Step-by-step solution:
  1. Initial total = +5 - 8 + 1 = -2 µC.
  2. The transfer is internal to the three-body system.
  3. Internal transfers do not change total charge.
Answer: -2 µC.
JEE Main original

JEE Main 04: Possible Charge Check

Question: Check whether 7.2 × 10-19 C can be the exact charge of an isolated particle cluster.

Formula used: n = |Q|/e
Step-by-step solution:
  1. n = (7.2 × 10-19)/(1.6 × 10-19).
  2. n = 4.5.
  3. Since n is not an integer, this exact charge is not allowed.
Answer: Not possible as an exact isolated charge.
JEE Main original

JEE Main 05: Microcoulomb To Electron Count

Question: How many electrons must be added to a neutral conductor to give it -4.8 µC?

Formula used: n = |Q|/e
Step-by-step solution:
  1. |Q| = 4.8 × 10-6 C.
  2. n = (4.8 × 10-6)/(1.6 × 10-19).
  3. n = 3.0 × 1013.
Answer: 3.0 × 1013 electrons must be added.
JEE Main original

JEE Main 06: Millisecond Current Charge

Question: A current of 2 mA flows for 5 ms. How much charge passes a cross-section?

Formula used: Q = It
Step-by-step solution:
  1. I = 2 × 10-3 A.
  2. t = 5 × 10-3 s.
  3. Q = (2 × 10-3)(5 × 10-3) = 1.0 × 10-5 C.
Answer: 1.0 × 10-5 C, or 10 µC.
JEE Main original

JEE Main 07: Net Charge In Mixed Units

Question: Find the net charge of +2 mC, -300 µC and +0.5 mC.

Formula used: Qtotal = Σq with common units
Step-by-step solution:
  1. Convert to mC: -300 µC = -0.300 mC.
  2. Add: Q = 2.0 - 0.300 + 0.5 mC.
  3. Q = 2.2 mC.
Answer: +2.2 mC.
JEE Main original

JEE Main 08: Nanocoulomb To Coulomb

Question: A charge of -64 nC is equal to how many coulomb?

Formula used: 1 nC = 10-9 C
Step-by-step solution:
  1. -64 nC = -64 × 10-9 C.
  2. Write in scientific notation.
  3. Q = -6.4 × 10-8 C.
Answer: -6.4 × 10-8 C.
JEE Main original

JEE Main 09: Charge Entering System

Question: A chosen system initially has net charge +10 nC. A particle of charge -3 nC enters the system. Find the new charge of the chosen system.

Formula used: Qnew = Qinitial + qentering
Step-by-step solution:
  1. The chosen system is not isolated during entry.
  2. Add the incoming charge: +10 nC - 3 nC.
  3. Qnew = +7 nC.
Answer: +7 nC.
JEE Main original

JEE Main 10: Least Charge Above A Limit

Question: Find the least positive charge greater than 2.0 × 10-18 C that an isolated body can have.

Formula used: Q = ne, n is an integer
Step-by-step solution:
  1. Need ne > 2.0 × 10-18.
  2. n > (2.0 × 10-18)/(1.6 × 10-19) = 12.5.
  3. Least integer n is 13, so Q = 13e = 2.08 × 10-18 C.
Answer: 2.08 × 10-18 C.
JEE Main original

JEE Main 11: Two Neutral Spheres

Question: A neutral sphere A transfers 2.5 × 1011 electrons to neutral sphere B. Find charges on A and B.

Formula used: Q = ne with sign from electron transfer
Step-by-step solution:
  1. A loses electrons, so A becomes positive.
  2. Magnitude = (2.5 × 1011)(1.6 × 10-19) = 4.0 × 10-8 C.
  3. B gains the same electrons, so it has equal negative charge.
Answer: A = +4.0 × 10-8 C, B = -4.0 × 10-8 C.
JEE Main original

JEE Main 12: Algebraic Sum With Elementary Charges

Question: A system contains charges -3e, +8e, -2e and -6e. Find the net charge.

Formula used: Qtotal = (-3 + 8 - 2 - 6)e
Step-by-step solution:
  1. Add coefficients with sign.
  2. -3 + 8 - 2 - 6 = -3.
  3. Thus Q = -3e = -4.8 × 10-19 C.
Answer: -3e, or -4.8 × 10-19 C.
JEE Main original

JEE Main 13: Equal Sharing By Identical Spheres

Question: Two identical conducting spheres carry +12 nC and -4 nC. They are touched and separated. Find charge on each sphere.

Formula used: qeach = (q1 + q2)/2 for identical spheres
Step-by-step solution:
  1. Total charge = +12 nC - 4 nC = +8 nC.
  2. Identical spheres share equally on contact.
  3. Each gets +8/2 = +4 nC.
Answer: +4 nC on each sphere.
JEE Main original

JEE Main 14: Triple Sphere Redistribution

Question: Three identical metal spheres carry +9 nC, -3 nC and 0. They are connected together by thin wires and then separated. Find charge on each.

Formula used: qeach = Qtotal/3
Step-by-step solution:
  1. Total charge = +9 - 3 + 0 = +6 nC.
  2. Identical connected conductors share total charge equally.
  3. Each charge = +6/3 = +2 nC.
Answer: +2 nC on each sphere.
JEE Main original

JEE Main 15: Deficient Electrons From Positive Charge

Question: A conductor carries +1.28 × 10-7 C. Find the number of deficient electrons.

Formula used: n = Q/e
Step-by-step solution:
  1. Positive charge means electron deficiency.
  2. n = (1.28 × 10-7)/(1.6 × 10-19).
  3. n = 8.0 × 1011.
Answer: 8.0 × 1011 electrons are deficient.
JEE Main original

JEE Main 16: Reported Charge Validity

Question: A measured charge is 1.12 × 10-18 C. Is it consistent with quantization?

Formula used: n = |Q|/e
Step-by-step solution:
  1. n = (1.12 × 10-18)/(1.6 × 10-19).
  2. n = 7.
  3. Since n is integer, the value is consistent with quantization.
Answer: Yes, it equals 7e.
JEE Main original

JEE Main 17: Charge Removed From Negative Sphere

Question: A sphere has charge -10 nC. If 4 nC of negative charge is removed from it, find the final charge.

Formula used: Qfinal = Qinitial - qremoved
Step-by-step solution:
  1. Initial charge = -10 nC.
  2. Removed charge = -4 nC.
  3. Qfinal = -10 - (-4) = -6 nC.
Answer: -6 nC.
JEE Main original

JEE Main 18: Positive Charge Neutralization

Question: How many electrons must be supplied to neutralize a body of charge +3.2 µC?

Formula used: n = Q/e
Step-by-step solution:
  1. Positive charge means electrons are missing.
  2. n = (3.2 × 10-6)/(1.6 × 10-19).
  3. n = 2.0 × 1013 electrons.
Answer: 2.0 × 1013 electrons.
JEE Main original

JEE Main 19: Coalescing Drops

Question: Four tiny drops with charges +2 nC, -5 nC, +6 nC and -1 nC coalesce into one drop. Find the final charge.

Formula used: Qfinal = Σq
Step-by-step solution:
  1. Coalescence is internal charge combination.
  2. Add algebraically: +2 - 5 + 6 - 1 = +2 nC.
  3. The final drop carries this total charge.
Answer: +2 nC.
JEE Main original

JEE Main 20: Charge Invariant In Motion

Question: A proton moving at high speed enters a detector. What charge should the detector assign to it?

Formula used: qmoving = qrest
Step-by-step solution:
  1. Electric charge is invariant.
  2. The proton has rest charge +e.
  3. Speed changes momentum and kinetic energy, not charge.
Answer: +e = +1.6 × 10-19 C.
JEE Main original

JEE Main 21: External Electron Removal

Question: A body initially has -1.6 µC. Electrons corresponding to -0.4 µC are removed to earth. Find final charge.

Formula used: Qfinal = Qinitial - qremoved
Step-by-step solution:
  1. Initial Q = -1.6 µC.
  2. Removed charge q = -0.4 µC.
  3. Qfinal = -1.6 - (-0.4) = -1.2 µC.
Answer: -1.2 µC.
JEE Main original

JEE Main 22: Charge From Ampere Second

Question: A steady current transfers 0.15 C of charge in 3 s. Find the current.

Formula used: I = Q/t
Step-by-step solution:
  1. Use I = Q/t.
  2. I = 0.15/3.
  3. I = 0.05 A = 50 mA.
Answer: 0.05 A.
JEE Main original

JEE Main 23: Boundary Conservation Decision

Question: The total charge inside a box changes from -6 nC to -2 nC after a charged bead leaves the box. What charge did the bead carry?

Formula used: Qafter = Qbefore - qleaving
Step-by-step solution:
  1. -2 = -6 - qleaving in nC.
  2. 4 = -qleaving.
  3. qleaving = -4 nC.
Answer: The bead carried -4 nC.
JEE Main original

JEE Main 24: Converting Millicoulomb To Electrons

Question: A charge of -0.016 mC is present on a body. Find the number of excess electrons.

Formula used: n = |Q|/e
Step-by-step solution:
  1. -0.016 mC = -1.6 × 10-5 C.
  2. n = (1.6 × 10-5)/(1.6 × 10-19).
  3. n = 1.0 × 1014.
Answer: 1.0 × 1014 excess electrons.
JEE Main original

JEE Main 25: Final Charge After Two Transfers

Question: A sphere initially has +5 nC. It gains 3 nC of negative charge and then loses 2 nC of negative charge. Find its final charge.

Formula used: Qfinal = Qinitial + qgained - qlost
Step-by-step solution:
  1. Initial Q = +5 nC.
  2. Gaining negative charge: Q = +5 + (-3) = +2 nC.
  3. Losing -2 nC: Q = +2 - (-2) = +4 nC.
Answer: +4 nC.

Question bank 3

JEE Advanced / IIT-JEE Difficult Questions

All questions in this section are written as JEE Advanced style original questions. They are not claimed as previous-year questions.

JEE Advanced style original question

Advanced 01: Multiple Correct Boundary Reasoning

Difficulty: Moderate | Type: Multiple-correct

Question: JEE Advanced style original question. A closed insulated box contains three conductors with charges +4e, -9e and +2e. During a process, two electrons move from the second conductor to the first. Which statements are correct?

  1. Charge of the first conductor becomes +2e.
  2. Charge of the second conductor becomes -7e.
  3. Net charge of the box remains -3e.
  4. Charge conservation is violated because individual charges changed.
Complete solution: Moving two electrons to the first conductor adds -2e to +4e, giving +2e. The second conductor loses -2e, so -9e becomes -7e. The total remains +2e - 7e + 2e = -3e.
Final answer: A, B and C
Key concept tested: Internal transfer changes individual charges but not total charge.
JEE Advanced style original question

Advanced 02: Integer Charge Constraint

Difficulty: Moderate | Type: Integer-type

Question: JEE Advanced style original question. The least number of electrons that must be removed from a neutral body so that its charge exceeds +1.0 × 10-18 C is N. Find N.

Complete solution: Need Ne > 1.0 × 10-18. N > 1.0/1.6 × 10 = 6.25. The least integer greater than 6.25 is 7.
Final answer: 7
Key concept tested: Quantization and strict inequality.
JEE Advanced style original question

Advanced 03: Assertion Reason On Invariance

Difficulty: Conceptual | Type: Assertion-reason

Question: JEE Advanced style original question. Assertion: A relativistic electron has the same electric charge as an electron at rest. Reason: Electric charge is invariant under change of inertial frame.

  1. Both assertion and reason are true, and reason explains assertion.
  2. Both are true, but reason does not explain assertion.
  3. Assertion is true and reason is false.
  4. Assertion is false and reason is true.
Complete solution: The charge of a particle is independent of speed and observer frame. Therefore, the reason is true and directly explains the assertion.
Final answer: A
Key concept tested: Charge invariance.
JEE Advanced style original question

Advanced 04: Matching Charging Methods

Difficulty: Moderate | Type: Matching-type

Question: JEE Advanced style original question. Match the charging process with the best description: P Friction, Q Conduction, R Induction, S Pair production.

  1. P-electron transfer by rubbing, Q-contact charge sharing, R-separation without contact, S-creation of +e and -e pair
  2. P-contact sharing, Q-rubbing, R-annihilation, S-grounding only
  3. P-neutralization, Q-no contact, R-current heating, S-two electrons appear
  4. P-mass transfer, Q-proton flow, R-charge destroyed, S-net charge +e
Complete solution: Friction transfers electrons during rubbing. Conduction needs contact. Induction separates charge without direct contact. Pair production creates equal opposite charges, conserving net charge.
Final answer: A
Key concept tested: Recognizing mechanisms of charging and conservation.
JEE Advanced style original question

Advanced 05: Paragraph On Three Spheres

Difficulty: Difficult | Type: Paragraph-based

Question: JEE Advanced style original question. Three identical conducting spheres A, B and C carry +12 nC, -6 nC and 0 respectively. A and B are touched and separated; then B and C are touched and separated. Find the final charges.

Complete solution: After A and B touch, total +6 nC is shared equally, so A = +3 nC and B = +3 nC. Then B and C touch: total +3 nC is shared equally, so B = +1.5 nC and C = +1.5 nC. A remains +3 nC.
Final answer: A = +3 nC, B = +1.5 nC, C = +1.5 nC
Key concept tested: Stepwise charge redistribution on identical conductors.
JEE Advanced style original question

Advanced 06: Multiple Correct Quantization

Difficulty: Difficult | Type: Multiple-correct

Question: JEE Advanced style original question. Which exact charges are possible for an isolated body?

  1. -4.8 × 10-19 C
  2. +6.0 × 10-19 C
  3. +1.6 × 10-18 C
  4. -2.0 × 10-19 C
Complete solution: Divide each magnitude by 1.6 × 10-19 C. The n values are 3, 3.75, 10 and 1.25. Only integer n values are possible.
Final answer: A and C
Key concept tested: Allowed and forbidden charge values.
JEE Advanced style original question

Advanced 07: System Boundary With Escaping Particle

Difficulty: Difficult | Type: Multi-step numerical

Question: JEE Advanced style original question. An isolated container initially has net charge +8 nC. A small particle of charge q escapes from an inner compartment into a region outside the chosen subsystem but still inside the container. The chosen subsystem charge becomes +11 nC. Find q and the charge of the whole container.

Complete solution: For the chosen subsystem, Qafter = Qbefore - q. Thus +11 = +8 - q, so q = -3 nC. The whole container is isolated, so its total charge remains +8 nC.
Final answer: q = -3 nC; whole container remains +8 nC
Key concept tested: Conservation depends on the system boundary chosen.
JEE Advanced style original question

Advanced 08: Pair Production Constraint

Difficulty: Conceptual | Type: Multiple-correct

Question: JEE Advanced style original question. A neutral photon produces particles inside a chamber. Which final charge combinations are allowed by charge conservation alone?

  1. electron + positron
  2. proton + antiproton
  3. electron only
  4. two positrons and one electron
Complete solution: The initial photon has zero charge. Electron plus positron gives -e + e = 0. Proton plus antiproton gives +e - e = 0. Electron only gives -e, and two positrons plus one electron gives +e.
Final answer: A and B
Key concept tested: Charge conservation in particle creation.
JEE Advanced style original question

Advanced 09: Induced Neutral Conductor

Difficulty: Moderate | Type: Assertion-reason

Question: JEE Advanced style original question. Assertion: A neutral isolated conductor placed near a positive rod may attract the rod. Reason: Charges inside the conductor redistribute, but the conductor's net charge remains zero.

  1. Both are true, and the reason explains the assertion.
  2. Both are true, but the reason does not explain the assertion.
  3. Assertion is true and reason is false.
  4. Assertion is false and reason is true.
Complete solution: The positive rod draws electrons slightly closer inside the conductor. The near side becomes effectively negative and attraction dominates, while total charge remains zero.
Final answer: A
Key concept tested: Induction and neutrality.
JEE Advanced style original question

Advanced 10: Charge Versus Mass Comparison

Difficulty: Conceptual | Type: Multiple-correct

Question: JEE Advanced style original question. Choose correct comparisons between electric charge and mass in school-level physics.

  1. Charge can be positive or negative; mass is taken as positive.
  2. Charge adds algebraically; mass adds arithmetically.
  3. Electric force can attract or repel; gravitational force is attractive.
  4. Charge of a particle changes with its speed in the same way as kinetic energy.
Complete solution: The first three statements are standard differences. The fourth is false because charge is invariant and does not change with speed.
Final answer: A, B and C
Key concept tested: Charge and mass distinctions.
JEE Advanced style original question

Advanced 11: Integer Type From Final Neutrality

Difficulty: Moderate | Type: Integer-type

Question: JEE Advanced style original question. A body has charge -9.6 × 10-19 C. How many electrons must be removed from it to make it neutral?

Complete solution: The body has excess electrons. n = 9.6/1.6 = 6, so removing 6 electrons makes the net charge zero.
Final answer: 6
Key concept tested: Electron excess and neutralization.
JEE Advanced style original question

Advanced 12: Possible Final Configurations

Difficulty: Difficult | Type: Multiple-correct

Question: JEE Advanced style original question. An isolated system has total charge +5e. Which final configurations of three bodies are possible?

  1. +2e, +4e, -1e
  2. +3e, +3e, 0
  3. +6e, -2e, +1e
  4. +2.5e, +2.5e, 0
Complete solution: Final totals must be +5e and each body charge must be an integer multiple of e. A totals +5e. B totals +6e. C totals +5e. D uses fractional charges, so it is not allowed.
Final answer: A and C
Key concept tested: Combining conservation with quantization.
JEE Advanced style original question

Advanced 13: Two Stage Electron Movement

Difficulty: Difficult | Type: Multi-step numerical

Question: JEE Advanced style original question. Bodies A and B initially have +7e and -3e. First, 4 electrons move from B to A. Then 2 electrons move from A to B. Find final charges.

Complete solution: After 4 electrons move B to A, A gains -4e and becomes +3e; B loses -4e and becomes +1e. Then 2 electrons move A to B, so A loses -2e and becomes +5e; B gains -2e and becomes -1e.
Final answer: A = +5e, B = -1e
Key concept tested: Tracking electron movement with signs.
JEE Advanced style original question

Advanced 14: Annihilation Accounting

Difficulty: Conceptual | Type: Paragraph-based

Question: JEE Advanced style original question. A chamber contains 5 electrons and 3 positrons. All possible electron-positron pairs annihilate into photons. Find the remaining net charge and particles.

Complete solution: Three electrons annihilate with three positrons, making photons of zero charge. Two electrons are left. Net charge before was -5e + 3e = -2e, and after it is also -2e.
Final answer: Two electrons remain; net charge = -2e
Key concept tested: Annihilation and charge conservation.
JEE Advanced style original question

Advanced 15: Millikan Type Remainder

Difficulty: Difficult | Type: Integer-type

Question: JEE Advanced style original question. Oil drops are found with charges 8.0, 12.8, 19.2 and 24.0 in units of 10-19 C. Taking e = 1.6 × 10-19 C, find the smallest integer n among these drops.

Complete solution: The n values are 8.0/1.6 = 5, 12.8/1.6 = 8, 19.2/1.6 = 12 and 24.0/1.6 = 15. The smallest integer n is 5.
Final answer: 5
Key concept tested: Millikan-type quantization pattern.

Question bank 4

IB Physics Structured Questions

Structured reasoning questions for explanation, data interpretation, uncertainty, conservation and invariance.

IB structured

IB Structured 01: Charge Invariance

Question: A beam of electrons is accelerated from rest to a high speed inside a vacuum tube.

  1. State whether the charge of each electron changes.
  2. Explain why this is important for charge conservation in different inertial frames.
  3. Name one quantity of the electron that does change during acceleration.
Answer and explanation: The charge remains -e. Charge is invariant, so all inertial observers agree on the amount of charge. Kinetic energy and momentum change during acceleration.
IB structured

IB Structured 02: Data Table Of Oil Drops

Question: Oil drops have measured charges 3.2, 4.8, 6.4 and 8.0 in units of 10-19 C.

  1. Show that the data supports quantization.
  2. Find the elementary charge suggested by the data.
  3. Suggest one reason why real measurements may not be exact multiples.
Answer and explanation: All values are multiples of 1.6 × 10-19 C. The suggested elementary charge is 1.6 × 10-19 C. Experimental uncertainty can cause small deviations.
IB structured

IB Structured 03: Uncertainty In Small Charge

Question: A sensor measures Q = (4.8 ± 0.2) × 10-19 C.

  1. Find the nearest integer multiple of e.
  2. Comment on whether Q = 3e is reasonable.
  3. Explain how reducing uncertainty helps the conclusion.
Answer and explanation: 3e = 4.8 × 10-19 C, so the nearest multiple is 3e. It is reasonable because the central value exactly matches 3e. Lower uncertainty makes the quantization evidence stronger.
IB structured

IB Structured 04: Conservation In A Closed Box

Question: Inside a closed insulated box, two small bodies exchange electrons. Body A changes from +6 nC to +2 nC.

  1. Find the change in charge of A.
  2. State the change in charge of B.
  3. Explain the conservation principle used.
Answer and explanation: A changes by -4 nC. B must change by +4 nC if no charge crosses the box boundary. The algebraic total charge of an isolated system remains constant.
IB structured

IB Structured 05: Neutral Conductor Near A Rod

Question: A negatively charged rod is brought near a neutral metal sphere without contact.

  1. Describe the redistribution of electrons in the sphere.
  2. State the net charge on the sphere before grounding.
  3. Explain why attraction may occur.
Answer and explanation: Electrons in the sphere move slightly away from the rod. The net charge remains zero. The nearer induced positive side is attracted more strongly than the farther negative side is repelled.
IB structured

IB Structured 06: Electron Transfer Calculation

Question: A neutral object receives 1.5 × 1013 electrons.

  1. Calculate its charge.
  2. State whether it has excess or deficient electrons.
  3. Write the result in microcoulomb.
Answer and explanation: Q = -(1.5 × 1013)(1.6 × 10-19) = -2.4 × 10-6 C = -2.4 µC. It has excess electrons.
IB structured

IB Structured 07: Charge And Mass

Question: A student compares electric charge and mass.

  1. Give one similarity.
  2. Give two differences.
  3. Explain why charge can cancel but mass normally cannot.
Answer and explanation: Both are properties of matter. Charge may be positive or negative and can produce attraction or repulsion; mass is positive in school physics and gravity is attractive. Charge cancels because it is algebraic.
IB structured

IB Structured 08: Pair Production Explanation

Question: A high-energy photon produces an electron and a positron.

  1. State the charge before production.
  2. State the total charge after production.
  3. Explain why an electron alone cannot be produced from a neutral photon by charge conservation.
Answer and explanation: Before production the photon has zero charge. After production -e + e = 0. An electron alone would give final charge -e, so charge would not be conserved.
IB structured

IB Structured 09: Charge Sign Interpretation

Question: A metal object has measured charge +0.80 µC.

  1. Does the object have excess or deficient electrons?
  2. Calculate the number of missing electrons.
  3. Explain why protons are not normally transferred through the metal.
Answer and explanation: It is deficient in electrons. n = 0.80 × 10-6/(1.6 × 10-19) = 5.0 × 1012. Protons are bound inside nuclei.
IB structured

IB Structured 10: Evaluating A Claim

Question: A student says: rubbing two neutral objects creates positive charge on one and negative charge on the other from nothing.

  1. Identify the error in the statement.
  2. Rewrite the statement correctly.
  3. State the total charge of the two-object system after rubbing.
Answer and explanation: The error is saying charge is created from nothing. Rubbing transfers electrons between the objects. If the two objects form an isolated system, total charge remains zero.

Question bank 5

ICSE and IGCSE Electric Charge Questions

Simple but conceptually clean questions on friction, charge signs, neutrality, electron transfer and attraction-repulsion.

ICSE-level questions

ICSE

ICSE 01: Define Electric Charge

Question: Define electric charge in simple words.

Answer: Electric charge is a property of matter because of which a body can exert or experience electric force.
ICSE

ICSE 02: Unit Of Charge

Question: Write the SI unit and symbol of electric charge.

Answer: The SI unit is coulomb, symbol C.
ICSE

ICSE 03: Neutral Matter

Question: Why is ordinary matter usually neutral?

Answer: It contains equal total positive charge of protons and negative charge of electrons, so the net charge is zero.
ICSE

ICSE 04: Charging By Friction

Question: A glass rod rubbed with silk becomes positive. What happens to the silk?

Answer: The silk becomes negatively charged because it gains electrons from the glass rod.
ICSE

ICSE 05: Attraction And Repulsion

Question: What happens when two like charges are brought near each other?

Answer: They repel each other.
ICSE

ICSE 06: Unlike Charges

Question: What happens when a positive charge is brought near a negative charge?

Answer: They attract each other.
ICSE

ICSE 07: Simple Electron Count

Question: A body gains 3 electrons. Write its charge in terms of e.

Answer: Its charge is -3e because electrons are negatively charged.
ICSE

ICSE 08: Diagram Reasoning

Question: In a diagram, two small spheres have arrows pointing away from each other. What does this suggest about their charges?

Answer: They are repelling, so both are charged and have the same sign.
ICSE

ICSE 09: Positive Body Meaning

Question: A body is positively charged. Does it usually mean it has gained protons?

Answer: No. In ordinary charging it usually means the body has lost electrons or is deficient in electrons.
ICSE

ICSE 10: Conservation Statement

Question: State the law of conservation of electric charge.

Answer: The total charge of an isolated system remains constant.

IGCSE-level questions

IGCSE

IGCSE 01: Static Charge Meaning

Question: What is static electricity?

Answer: Static electricity is the build-up of electric charge on an object, usually due to electron transfer.
IGCSE

IGCSE 02: Polythene Rod

Question: A polythene rod gains electrons when rubbed. What is its final charge?

Answer: It becomes negatively charged.
IGCSE

IGCSE 03: Cloth Charge

Question: If a rod gains -5 nC by rubbing, what charge does the cloth get?

Answer: The cloth gets +5 nC, assuming the rod-cloth system is isolated.
IGCSE

IGCSE 04: Neutral Attraction

Question: Why can a charged balloon attract small neutral paper pieces?

Answer: The balloon induces charge separation in the paper, making the nearer side oppositely charged.
IGCSE

IGCSE 05: Simple Quantization

Question: Can a body have exactly half an electron of charge?

Answer: No. Electrons are transferred as whole particles, so charge is quantized.
IGCSE

IGCSE 06: Coulomb Calculation

Question: Find the charge of 2 excess electrons in coulomb.

Answer: Q = -2e = -3.2 × 10-19 C.
IGCSE

IGCSE 07: Detecting Same Charge

Question: Two suspended balls repel. What does this show?

Answer: Both balls are charged and have the same type of charge.
IGCSE

IGCSE 08: Conductor Contact

Question: What can happen when a charged conductor touches a neutral conductor?

Answer: Charge can flow through contact and redistribute between the conductors.
IGCSE

IGCSE 09: Electron Excess

Question: A body has more electrons than protons. What is its net charge?

Answer: It is negatively charged.
IGCSE

IGCSE 10: Grounding

Question: What is grounding in electrostatics?

Answer: Grounding is connecting a body to Earth so electrons can flow to or from the body.

Applied learning

Case Study Section

Five original case studies connect the theory of charge with friction, induction, electron transfer, particle processes and Millikan-type quantization.

Case study

Case Study 01: Glass Rod And Silk

Passage: A dry glass rod is rubbed with silk. After rubbing, the glass rod is found to be positively charged. The experiment is performed on an insulating stand so that charge loss to the surroundings is negligible.

  1. What is the sign of charge on the silk?
    The silk is negatively charged.
  2. Which particle is mainly transferred during rubbing?
    Electrons are transferred.
  3. If the rod has +16 nC, what is the charge on silk?
    -16 nC, because the rod-silk system conserves charge.
  4. Does rubbing create charge?
    No. It transfers charge between the two materials.
Case study

Case Study 02: Neutral Conductor And Induction

Passage: A neutral metal sphere is kept on an insulating stand. A positively charged rod is brought near it without touching. The near side of the sphere becomes negatively induced and the far side becomes positively induced.

  1. What is the net charge on the sphere before grounding?
    Zero.
  2. Why does the near side become negative?
    Mobile electrons are attracted toward the positive rod.
  3. Is charge conservation violated inside the sphere?
    No. Charges redistribute but total charge remains zero.
  4. What may happen if the sphere is grounded while the rod is nearby?
    Electrons can flow from Earth, leaving the sphere negatively charged after proper removal of ground and rod.
Case study

Case Study 03: Two Metal Spheres Exchange Electrons

Passage: Two identical metal spheres A and B are initially neutral. A process transfers 6.0 × 1010 electrons from A to B. No charge escapes to the surroundings.

  1. What is the charge on A?
    +9.6 × 10-9 C.
  2. What is the charge on B?
    -9.6 × 10-9 C.
  3. What is the net charge of A+B?
    Zero.
  4. Which property is demonstrated?
    Conservation of electric charge.
Case study

Case Study 04: Pair Production And Annihilation

Passage: In a high-energy event, a neutral photon produces an electron and a positron. Later, the electron and positron meet and annihilate into photons.

  1. What is the charge before pair production?
    Zero.
  2. What is the total charge just after pair production?
    -e + e = 0.
  3. What is the charge of photons after annihilation?
    Photons are electrically neutral.
  4. What principle is obeyed in both events?
    Conservation of electric charge.
Case study

Case Study 05: Millikan Type Charge Values

Passage: Five oil drops are measured with charges 3.2, 6.4, 9.6, 12.8 and 16.0 in units of 10-19 C. The readings are assumed exact for this classroom analysis.

  1. What elementary charge is suggested?
    1.6 × 10-19 C.
  2. What are the n values for the drops?
    2, 4, 6, 8 and 10.
  3. Is 10.4 × 10-19 C allowed as an exact value?
    No. It gives n = 6.5, not an integer.
  4. What property of charge is shown?
    Quantization of charge.

Final revision

Formula Box, Exam Facts and Common Mistakes

Use this section for fast revision before NEET, JEE Main, JEE Advanced, CBSE, IB, ICSE and IGCSE tests.

Formula box

  • Qtotal = Σq
  • Q = ±ne
  • n = |Q|/e
  • e = 1.6 × 10-19 C
  • 1 µC = 10-6 C
  • 1 nC = 10-9 C
  • 1 mC = 10-3 C
  • Q = It

Common mistakes checklist

  • Do not add charge magnitudes without signs.
  • Do not treat n as decimal in Q = ne.
  • Do not say friction creates charge.
  • Do not confuse attraction with sure test of charge.
  • Do not assume protons move through metals.
  • Do not apply conservation to a non-isolated chosen system without accounting for charge crossing the boundary.

Quick NEET/JEE revision

  • Positive charge means electron deficiency.
  • Negative charge means electron excess.
  • Repulsion confirms same-sign charged bodies.
  • Macroscopic charge looks continuous because e is tiny.
  • Pair production and annihilation both conserve charge.
  • A moving charged particle has the same charge as at rest.

Top 20 exam facts

  1. Charge is an intrinsic property of matter.
  2. The SI unit of charge is coulomb.
  3. Electron charge is -1.6 × 10-19 C.
  4. Proton charge is +1.6 × 10-19 C.
  5. Matter is neutral when proton and electron charges balance.
  6. Charge is scalar but algebraic.
  7. Like charges repel.
  8. Unlike charges attract.
  9. Attraction is not a sure test of charge.
  10. Repulsion is a sure test that both bodies are charged.
  11. Total charge of an isolated system is conserved.
  12. Charging by friction is electron transfer.
  13. Charging by conduction needs contact.
  14. Charging by induction can occur without contact.
  15. Grounding allows charge exchange with Earth.
  16. Charge is quantized as Q = ±ne.
  17. n must be an integer.
  18. Charge is invariant with speed.
  19. Pair production creates equal opposite charges.
  20. Annihilation preserves net charge.

FAQ

FAQ: Properties of Electric Charge

What are the main properties of electric charge?

The main properties are additivity, conservation, quantization and invariance. Charges also interact by attraction or repulsion depending on sign.

What is quantization of charge?

Quantization means charge exists as integral multiples of the elementary charge: Q = plus or minus ne.

What is conservation of charge?

The total electric charge of an isolated system remains constant. Charge can move from one body to another, but it is not created or destroyed.

Why is positive charge called electron deficiency?

In ordinary solid charging, electrons are mobile while protons remain bound in nuclei. Losing electrons leaves a net positive charge.

Is attraction a sure test of charge?

No. A charged object can attract a neutral object due to induction. Repulsion is a sure test that both objects are charged with the same sign.

Can charge be 2.5e?

No. The exact charge of an isolated body must be an integer multiple of e.

Does charge change with speed?

No. Electric charge is invariant and remains the same in all inertial frames.

How are pair production and annihilation linked to charge conservation?

Pair production creates equal opposite charges, and annihilation removes equal opposite charges. In both cases the net charge remains conserved.

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