Electrostatic Force Notes

Lesson 2: Electrostatic Force

Focus Question

  • How can objects be electrically charged?

New Vocabulary

  • Electroscope: A device for detecting electric charge.

  • Coulomb's Law: A fundamental principle that quantifies the electrostatic force between charged objects.

  • Charging by Conduction: Transferring charge through direct contact.

  • Coulomb: The SI unit of electric charge.

  • Charging by Induction: Charging without direct contact, by bringing a charged object near.

  • Grounding: The process of removing excess charge by connecting an object to the Earth.

  • Elementary Charge: The smallest unit of electric charge, equal to the charge of a single electron or proton.

Review Vocabulary

  • Force: A push or pull on an object that has both direction and magnitude.

    • Types of forces: Contact forces and field forces.

Forces on Charged Objects

  • Electrostatic Force: Demonstrated using a suspended charged rod.

  • Key Principles:

    • Two types of electric charge: Positive and Negative.

    • Like charges repel each other while unlike charges attract.

    • The force between charges is stronger when they are closer together.

Electroscope Functionality

  • Structure: Metal knob connected to a metal stem and two lightweight metal foil leaves.

  • Purpose: Used to determine the electric charge.

  • Behavior of Leaves:

    • They hang loosely when neutral.

    • They spread apart when charged due to induction or conduction from a charged object.

Charging Methods

  • Charging by Conduction:

    • Example: A negatively charged rod touches the electroscope, transferring electrons and causing the leaves to repel.

  • Charging by Induction:

    • Occurs when a charged object is brought near a neutral object without contact and creates a redistribution of charges, leading to polarization.

Grounding

  • Definition: The process of connecting an object to the Earth to remove excess charge.

Coulomb’s Law

  • Equation: F{AB} = K \cdot \frac{qA \cdot q_B}{r^2}

    • F: Magnitude of the electrostatic force between two point charges

    • qA and qB: Charges of the two objects

    • r: Distance between the charges

    Coulomb's Constant is expressed as:
    K = 8.99 \times 10^9 \frac{N \cdot m^2}{C^2}
    It quantifies the electrostatic force between two point charges and is a crucial element in the application of Coulomb's Law.

  • Characteristics:

    • Is a vector quantity; requires both magnitude and direction for complete definition.

    • Direction is determined through vector diagrams considering the nature of the charges involved.

Units and Charge

  • Coulomb (C): Standard unit of charge.

    • One coulomb equals approximately 6.24 \times 10^{18} electrons or protons.

  • Elementary Charge:

    • Charge of an electron: -1.602 \times 10^{-19} C

    • Charge of a proton: +1.602 \times 10^{-19} C

    • The magnitudes of charges are equal but opposite.

Example Problem

Sphere A, which has a charge of +6.0 µC, is positioned 4.0 cm to the left of Sphere B, which carries a charge of -3.0 µC.

Sketching and Analyzing the Problem

  • Create a diagram to represent the positions of the spheres.

  • Free-body diagram showing forces acting on B.

  • Known variables:

    • q_A = +6.0 \mu C ,

    • r_{AB} = 4.0 cm

    • q_B = -3.0 \mu C

    • r_{BC} = 5.0 cm

Solve Known Forces

  • Apply Coulomb’s law for each pair:

    • Attraction between A and B:
      F{AB} = K \cdot \frac{qA \cdot qB}{r{AB}^2}

    • Attraction between C and B:
      F{CB} = K \cdot \frac{qC \cdot qB}{r{BC}^2}

    • Calculate magnitudes of these forces substituting in respective values.

Use Vector Addition for Net Force

  • Determine the direction of forces:

    • FA on B to the left (attractive).

    • FC on B downward (attractive).

  • Combine both forces using vector addition:
    F{net} = \sqrt{F{AB}^2 + F_{CB}^2}

Applications of Electrostatic Forces

  • Automotive Painting: Charged paint droplets ensure uniform coverage.

  • Photocopy Machines: Use static electricity to selectively apply toner.

  • Pollution Control: Electrostatic forces help to collect emissions in smokestacks, reducing air pollution.