Unit 8

Insulators and Conductors

• Insulators: Materials that do not allow electrons to flow easily.

  • Examples: Plastic, rubber, glass.

• Conductors: Materials that allow the flow of electrons.

  • Examples: Metals, such as copper and aluminum.

Charging Methods

• Charging by Friction: When two different materials are rubbed together, electrons transfer from one material to another.

• Charging by Contact: When a charged conductor touches an uncharged conductor, electrons move between them until they reach equilibrium.

Coulomb's Law and Charge

• Coulomb's Constant (K): K = 8.99 × 10^9 N·m²/C²

• Elementary Charge (e):

  • Charge of a proton, Q = Ne

  • 1e = 1.6 × 10^-19 C

• Charge Units:

  • 1 mC = 10^-3 C

  • 1 µC = 10^-6 C

  • 1 nC = 10^-9 C

  • 1 pC = 10^-12 C

• Coulomb's Law: Expresses the force between two point charges.

Charging a Metal Ball by Induction

1. Step 1 of Induction - Uncharged Metal Ball: Start with an uncharged metal ball on an insulating stand.

2. Step 2 of Induction - Negative Charged Rod: Bring a negatively charged rod near the ball; electrons in the ball are repelled.

3. Step 3 of Induction - Electron Buildup: Creates an induced positive charge on the side nearer to the rod and a negative charge on the far side.

4. Step 4 of Induction - Grounding the Ball: A wire connecting the ball to the ground allows excess electrons to exit, leaving the ball positively charged.

5. Step 5 of Induction - Removing the Rod: Once the rod is removed, electrons rearrange, leaving the ball with a net positive charge.

Key Concepts in Electrostatics

• Newton’s Third Law in Electricity - the force between two point charges follows the principle of action and reaction

• The net force on a charge can be calculated by vector addition of forces due to each charge acting on it.

• Equilibrium - When the total force acting on a charge is zero, it is in equilibrium.

Important Constants

• Gravitational Constant (G): G = 6.67 × 10^-11 N·m²/kg²

Contact Forces and Field Forces

• Contact Forces - forces that act with contact, such as the force of friction

• Field Forces - forces that act at a distance, such as the force of gravity, electricity, and magnetism