General Physics 2 - Electric Charges and Potential Energy

Electric Charges

  • Positive and Negative Charges
  • Terms refer to electric charge states.

Conservation of Charge

  • Principle: Electric charge is conserved; electrons are not created or destroyed, only transferred between materials.

Charging Methods

  • Charging by Friction:
  • Charging occurs through the transfer of electrons when two materials are rubbed together.
  • Charging by Induction:
  • Occurs when a charged object is brought near a conductive material, inducing a charge in the conductive material without direct contact.

Coulomb’s Law

  • Definition: The force of attraction or repulsion (F) between two charged bodies is given by the formula:
  • F = k * (q1 * q2) / r²
  • Where k is Coulomb's constant, q1 and q2 are the magnitudes of the charges, and r is the distance between the charges.

Electric Field

  • Concept: An invisible force field around charged objects, representing the electric force per unit charge.
  • Direction: Pointing away from positive charges and toward negative charges.
Electric Field Examples
  • Two opposite charges with equal magnitude experience attraction.
  • Two like charges repel each other.

Electric Potential Energy (U)

  • Definition: The energy required to move a charge against an electric field.
  • Unit: Measured in Joules (J).
  • Relation: Positive work results in increased potential energy when moving a charge against the field direction.
Work as Potential Energy
  • If the force is conservative, work done is stored as potential energy.
  • Potential energy changes based on the charge's movement relative to the force direction:
  • Increases when moving against the field, decreases when moving with the field.

Electric Potential (V)

  • Definition: The electric potential energy per unit charge, expressed in volts.
  • Formula: V = U / q
Example Calculation of Electric Potential
  • If a charge of 6.5 x 10^-17 C is positioned 2 x 10^-8 m from another charge of -8 x 10^-19 C, calculate the potential energy.
  • U = -2.3 x 10^-17 Joules recorded for the configuration.

Voltage

  • Concept: The potential difference between two points in an electric field, calculated as Va - Vb.
  • Example: Potential at point A = 12 V, potential at B = 8 V, voltage difference = 4 V.

Electron Volts (eV)

  • Definition: Energy gained by an electron moving through a potential difference of 1 V.
  • Energy associated with potential differences can be calculated easily using the charge of electrons.

Equipotential Surface

  • A surface where the electric potential is constant; electric field lines are perpendicular to these surfaces.

Potential Gradient

  • Indicates how the electric field (E) is related to changes in electric potential (V).
  • Describes the relationship:
  • Electric field E = -dV/dx
  • Measured in volts per meter (V/m).

General Points

  • Potential Increases: When moving against the electric field.
  • Potential Decreases: When moving with the electric field.

Practice Problems (Seatwork)

  1. Calculate electric potential energy for a charge of 5 nC and another charge of 9.5 nC at 0.5 m.
  2. Determine potential difference when 600 J of energy is used to move a charge of 2 C.
  3. Find the distance required to achieve 0.600 J energy between two point charges.
  4. Identify distance for an electrical potential of 12.0 V for a charge of 8.00 x 10^-11 C.