GENERAL PHYSICS - Third Quarter

Page 1: Electric Charge and Electrostatics

• Electric Charge

  • Electricity at rest involves electric charges, forces between them, and their behavior in materials.

  • Charging by Friction:

    • The simplest method to charge an object by rubbing two different materials together.

    • Transfer of electrons causes one object to become positively charged (electron loser) and the other negatively charged (electron gainer).

    • The net charge remains zero, showing conservation of charge.

    • Similar charges repel; opposite charges attract.

• Charging by Conduction:

  • Involves direct physical contact between two objects.

  • Diagram Explanation:

    • (A) Negatively charged metal and uncharged metal.

    • (B) Charge separation occurs.

    • (C) The objects are touched.

    • (D) Negative object is removed; both have negative charge.

  • A positive object touching an uncharged one will cause the neutral object to donate electrons to the positively charged object.

• Charging by Induction:

  • Charging a conductor without direct contact.

  • Involves grounding wires which allow electrons to escape or enter.

  • Diagram Explanation:

    • (A) Neutral object grounded.

    • (B) Electrons escape to the ground.

    • (C) Wire is snipped, leaving a net positive charge.

• Electrostatic Series

  • Helps identify materials' tendency to gain or lose electrons when rubbed together.

  • Electroscope:

    • A scientific instrument to detect and measure electric charge.

    • Functions by responding to electrostatic forces acting on conductors.

Page 2: Electric Force and Fields

• Electric Force

  • Defined through quantity of charge (q).

  • Units of Charge:

    • Coulomb (C): 1 C = 6.25 x 10^18 electrons; 1 electron = -1.6 x 10^-19 C.

    • Commonly used metric prefixes: 1 μC = 1 x 10^-6 C, 1 nC = 1 x 10^-9 C, 1 pC = 1 x 10^-12 C.

• Coulomb’s Law:

  • The force between two point charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them.

• Superposition Principle:

  • Total electric force on a charge is the vector sum of forces from other charges.

• Van de Graaff Generator:

  • An electrostatic machine for generating high voltages, allowing charged particles to experience electric forces.

• Electric Field:

  • Electric field E is established by a charge and is defined at any point by the force experienced by a +q test charge placed in that field.

  • E direction is away from positive charges and toward negative charges.

• Electric Field Lines:

  • Imaginary lines illustrate the direction of the electric field.

  • Lines are tangent to the electric field vector at any point.

Page 3: Electric Flux and Potential

• Electric Flux (φ):

  • Describes the flow of electric field through a surface.

  • Depends on strength of electric field (E), surface area (A), and orientation of the field.

  • Flux conditions: Positive when pointing outward, negative inward.

• Gauss’ Law:

  • Total electric flux through a closed surface is proportional to net charge inside.

• Electric Potential (V):

  • Work done in moving a charge against an electric field.

  • Positive work, raising potential energy.

  • Potential due to a positive charge is positive; due to a negative charge is negative.

  • Unit: Joules per Coulomb (J/C).

  • Potential difference (VAB) is found by subtracting potentials at two points.

Page 4: Capacitance and Dielectric

• Electric Field vs. Electric Potential:

  • Electric field exists with a potential difference.

  • Potential lines indicate areas of equal electric potential.

• Capacitance (C):

  • Device that stores electric charges, consists of two plates separated by an insulator (dielectric).

  • Capacitance Definition: C = Q/V (Coulombs per Volt).

  • Factors affecting capacitance: area of plates, distance between them, and insulating material between them.

• Energy in Capacitors:

  • Stored energy in a capacitor: U = ½ QV.

  • Circuits can include series and parallel capacitor configurations.

Page 5: Current and Resistance

• Electric Current (I):

  • Rate of flow of charge through a section.

  • 1 Ampere = 1 Coulomb/second.

• Resistance (R):

  • Determined by material properties: length (L), area (A), and temperature (T).

  • Resistance is linearly related to the applied voltage in ohmic materials.

• Ohm's Law:

  • I = V/R; describes relationship for ohmic conductors vs. non-ohmic.

Page 6: Electromotive Force and Power

• Resistivity (ρ):

  • Property influencing electrical resistance, measured in ohm-meters (Ω·m).

• Electromotive Force (EMF):

  • Energy per unit charge; source for current flow in circuits.

  • Examples: batteries, power lines.

• Electric Power:

  • Calculated via P = I * V.

Page 7: Electric Symbols**

• Common symbols used in electrical circuits:

  • Ammeter, battery, fuse, light bulb, resistor, variator, voltmeter, switch, and electric wire.

Page 8: Conclusion

• This section contains general notes for reference throughout the semester.