Definition: Electrical charge & electric force

1. What is Electrostatics?

• Electrostatics is the study of stationary (static) electric charges.
• Unlike current electricity (which flows), static electricity remains in one place.
• Example: Rubbing a balloon on your hair causes it to attract small paper pieces due to static charge.

2. Electrical Charge

a) Basic Properties of Charge

• Charge is associated with protons and electrons:
  • Protons have a positive charge (+).
  • Electrons have a negative charge (-).
  • Neutrons have no charge.
• Like charges repel (e.g., two positives or two negatives).
• Opposite charges attract (e.g., positive attracts negative).
• Objects with equal amounts of positive and negative charge are neutral.

b) Unit of Charge

• Charge is measured in Coulombs (C).
• 1 electron has a charge of $1.6 × 10⁻¹⁹ C.$
• 1 Coulomb is equal to the charge of $6.25 × 10¹⁸$ electrons.

3. Electric Force

a) Coulomb's Law

• The force between two charged objects is given by: $F = k \frac{Q_1 Q_2}{r^2}$

where:

• $F =$ Electric force $(N)$
• $k =$ Coulomb's constant $(9.0 \times 10^9 N·m²/C²)$
• $Q_1, Q_2 =$ Charges $(C)$
• $r =$ Distance between charges $(m)$

b) Properties of Electric Force

✔ Directly proportional to the product of the two charges.

✔ Inversely proportional to the square of the distance between them.

✔ Stronger when charges are closer, weaker when further apart.

✔ Similar to Newton's Law of Gravitation, but acts on charge instead of mass.

4. Key Takeaways

✔ Charge is measured in Coulombs $(C).$

✔ Like charges repel, opposite charges attract.

✔ Electric force follows Coulomb's Law.

✔ Force increases with greater charge and decreases with distance.

Exam Tip

💡 Use the correct signs for charge values when calculating force using Coulomb's Law!