3.3 Conservation of Electric Charge

3.3 Conservation of Electric Charge

Conservation of Electric Charge

The conservation of electric charge is a fundamental principle in physics that states:

The total electric charge in a closed system remains constant over time.

This means that while charge can be transferred between objects, it cannot be created or destroyed. For example, if two spheres with charges of and touch and then separate, each sphere will have , maintaining a net charge of throughout the process.

Key Concepts:

1. Electric Charge:

   • A property of particles like electrons (C) and protons (C).

   • Charge can exist in positive or negative forms.

2. Closed System:

   • A system where no charge enters or exits the boundaries.

   • Ensures the net charge remains unchanged.

3. Charge Transfer:

   • Common in phenomena like charging by conduction, induction, or grounding.

CollegeBoard Essential Knowledge:

• Charging by Conduction: Electric charge is conserved when transferred between objects in contact.

• Induced Charge Separation: External charged objects can redistribute charges within a neutral system.

• Grounding: Excess charge is transferred to a larger system (e.g., Earth).

Conductors and Insulators

Charge movement depends on the material’s properties. Materials are classified as:

1. Conductors:

   • Allow electric charge to flow freely.

   • Examples: Metals (copper, silver, gold) and salt water.

   • Applications: Electrical wiring, circuits.

2. Insulators:

   • Restrict the flow of electric charge.

   • Examples: Rubber, plastic, glass.

   • Applications: Insulating wires, preventing short circuits.

Example Problem 1:

Q: A copper wire is 1 meter long with a cross-sectional area of 0.1 mm and a resistance of 1 ohm. What is its resistivity?

Solution:

Result: The resistivity is .

Example Problem 2:

Q: A copper wire is 2 meters long with a cross-sectional area of 0.2 mm and a resistivity of . What is its resistance?

Solution:

Result: The resistance is .

Example Problem 3:

Q: What is a device called that transfers electric charge between objects, and what are its characteristics?

Solution:

• The device is called a conductor.

Characteristics:

1. Low resistance.

2. High electron mobility.

3. Small or zero band gap.

4. Excellent conductivity.

Understanding the conservation of electric charge and the properties of materials helps us design efficient circuits, prevent electrical hazards, and explore new technologies. Keep practicing to strengthen your grasp of these essential principles!