NEET Electric Charge Basics: Notes and Important Questions by PW Lakshya

Class 12 Physics introduces concepts that form the foundation for understanding electricity, magnetism and their applications in medical technology. For NEET aspirants, learning the basic properties of electric charge is important before moving on to more advanced topics in Electrostatics.

The PW Lakshya introduces electric charge, its types, quantisation and other properties, along with study habits that support consistent preparation. Use these notes to revise the main concepts and practise the worked questions before moving on to the next topics in Electric Charges and Fields.

Electric Charge

Electric charge is a fundamental property of matter that enables it to interact with other charged bodies through electric forces. Moving charges are also associated with magnetic effects.

Charge is a scalar quantity, and its SI unit is the coulomb (C). It has magnitude, while its positive or negative sign indicates the type of charge.

Charge differs from mass:

• Mass is associated with gravitational attraction.

• Electric charges can attract or repel each other.

The charge of an electron is:

qₑ = −1.6 × 10⁻¹⁹ C

The magnitude of the electronic charge is:

e = 1.6 × 10⁻¹⁹ C

Electric Field and Motion of Charge

An electric charge produces an electric field around it. Another charge placed in this field can experience an electric force.

Key point: A charge produces an electric field, and an accelerating charge emits electromagnetic radiation.

Electrostatics and Types of Charge

Electrostatics is the study of electric charges at rest.

There are two types of electric charge:

1. Positive charge

2. Negative charge

The basic rule of interaction is:

• Like charges repel each other.

• Unlike charges attract each other.

Positive and negative charges have opposite signs. When equal amounts of opposite charge are combined, their net charge is zero.

Quantisation of Charge

The charge on a body is quantised, which means it exists in integral multiples of the elementary charge.

q = ne

Here, n is an integer and e is the magnitude of the electronic charge. The sign of q depends on whether the body has a net positive or negative charge.

A body becomes:

• Positively charged when electrons are removed.

• Negatively charged when electrons are added.

Removing electrons also decreases the body's mass slightly, while adding electrons increases its mass slightly.

To check whether a given charge is possible, calculate:

n = q / e

The value of n must be an integer for an ordinary body's net charge under the quantisation rule.

For example, a net charge of +0.5e is not possible for an ordinary body because it would require a non-integer number of elementary charges. Quarks have fractional electric charges, but they are not observed as isolated free particles under ordinary conditions.

Additivity and Conservation of Charge

Charge is additive. The net charge of a system is the algebraic sum of the individual charges, including their signs.

Qₙₑₜ = q₁ + q₂ + q₃ + ...

For example, if a system contains charges of +5 C, −2 C and +1 C, its net charge is:

Qₙₑₜ = 5 − 2 + 1 = +4 C

The law of conservation of charge states that the total charge of an isolated system remains constant. Charge can be transferred from one body to another or redistributed within a system, but the total charge remains unchanged.

Important Worked Questions on Electric Charge

Q1. What is the SI unit of electric charge?

Answer: The SI unit of electric charge is the coulomb (C).

Q2. What is the charge of an electron?

Answer: The charge of an electron is −1.6 × 10⁻¹⁹ C.

Q3. What is the charge of a proton?

Answer: The charge of a proton is +1.6 × 10⁻¹⁹ C.

Q4. A body loses 5 × 10¹² electrons. Calculate its net charge.

Answer:

q = ne

q = (5 × 10¹²) × (1.6 × 10⁻¹⁹)

q = +8 × 10⁻⁷ C

The body becomes positively charged.

Q5. A body gains 2 × 10¹⁰ electrons. Find its net charge.

Answer:

q = −ne

q = −(2 × 10¹⁰) × (1.6 × 10⁻¹⁹)

q = −3.2 × 10⁻⁹ C

Q6. A body acquires a charge of −4.8 × 10⁻¹⁸ C. How many electrons has it gained?

Answer:

n = |q| / e

n = (4.8 × 10⁻¹⁸) / (1.6 × 10⁻¹⁹)

n = 30 electrons

Q7. Can an ordinary body have a net charge of +4.8 × 10⁻¹⁹ C?

Answer:

n = q / e

n = (4.8 × 10⁻¹⁹) / (1.6 × 10⁻¹⁹)

n = 3

Since n is an integer, the charge is possible under the quantisation rule.

Q8. Can an ordinary body have a net charge of +2.4 × 10⁻¹⁹ C?

Answer:

n = q / e

n = (2.4 × 10⁻¹⁹) / (1.6 × 10⁻¹⁹)

n = 1.5

Since n is not an integer, this charge is not possible for an ordinary body's net charge under the quantisation rule.

Q9. Three bodies have charges of +6 C, −4 C and +3 C. Find the total charge.

Answer:

Qₙₑₜ = q₁ + q₂ + q₃

Qₙₑₜ = 6 − 4 + 3

Qₙₑₜ = +5 C

Q10. A system contains charges of +8 μC and −8 μC. Find its net charge.

Answer:

Qₙₑₜ = +8 μC − 8 μC

Qₙₑₜ = 0 C

Q11. A body has fewer electrons than protons. Is its net charge positive or negative?

Answer: The body has a positive net charge because it has fewer electrons than protons.

Q12. What happens when two positive charges are brought near each other? What happens when a positive and a negative charge are brought near each other?

Answer:

• Two positive charges repel each other.

• A positive and a negative charge attract each other.

Q13. What kind of field is associated with an isolated charge at rest?

Answer: A charge at rest produces an electric field around it.

Q14. Two bodies initially have charges of +7 C and −2 C. They exchange charge, but no charge enters or leaves the isolated system. What is the total charge after the exchange?

Answer:

Initial total charge = +7 C − 2 C

Initial total charge = +5 C

By the law of conservation of charge, the total charge after the exchange remains +5 C.

Q15. A neutral body gains electrons. What happens to its net charge and mass?

Answer:

• Its net charge becomes negative because it gains electrons.

• Its mass increases slightly because electrons have mass.

Watch 1st FREE Physics Class by MR Sir | Lakshya NEET 2027 by PW to revise the basics of electric charge, quarks, electrostatics, types of charge and the properties of charge. Use the class alongside these notes and practise the worked questions to strengthen your Physics preparation for NEET.

 Electric charge is a fundamental concept in Class 12 Physics and the starting point for understanding electrostatics. Revise the types and properties of charge, quantisation, additivity and conservation, and practise numerical questions to improve your understanding. The PW Lakshya NEET supports your revision with concept explanations and question practice as you prepare for the topics that follow in Electric Charges and Field.

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