Electric Charge, Electric Force, Electric Fields, and Electric Potential

Symbols and Constants

This section introduces key symbols and constants used in electrostatics.

• Prefixes for Powers of 10:

  • m = (milli)

  • μ = (micro)

  • n = (nano)

• Fundamental Constants:

  • Coulomb's constant (k):

  • Elementary charge (electron):

  • Elementary charge (proton):

  • Proton mass:

Coulomb's Law

Coulomb's Law describes the electric force between two point charges. The force is attractive for opposite charges and repulsive for like charges.

• Formula:

• Direction: The force acts along the line joining the two charges.

• Newton's Third Law: The force on due to is equal in magnitude and opposite in direction to the force on $q_2$ due to $q_1$.

• Example: Two charges of and separated by experience a force calculated using the above formula.

Electric Field

The electric field is a vector field that represents the force per unit charge at a point in space due to other charges.

• Electric Field due to a Point Charge:

  • Points away from

  • Points toward

• Electric Field between Parallel Plates:

  • is the potential difference between the plates.

  • is the distance between the plates.

• Example: The electric field between two plates with and is .

Electric Potential and Potential Energy

Electric potential (V) is the electric potential energy per unit charge at a point in space. The unit of potential is the volt (V), where .

• Potential due to a Point Charge:

  • is negative when is negative.

• Change in Electric Potential Energy:

  • Describes the work done by the electric field when a charge moves from point A to point B.

• Example: Moving a charge across a potential difference changes its potential energy by .

Equipotential Lines

Equipotential lines are lines or surfaces where the electric potential is constant. No work is required to move a charge along an equipotential line.

• Electric field lines are always perpendicular to equipotential lines.

• Closer equipotential lines indicate a stronger electric field.

Conceptual Topics

• Creating a Charged Object: Objects become charged by gaining or losing electrons (e.g., by friction, conduction, or induction).

• Charged Object's Effect: Charged objects exert forces on other charged and neutral objects (via polarization).

• Electric Force Vectors: The direction and magnitude of electric forces can be represented as vectors, following the rules of vector addition.

• Electric Field Vectors: The direction of the electric field at a point is the direction of the force on a positive test charge placed at that point.

• Finding Potential from Multiple Charges: The total potential at a point due to several point charges is the algebraic sum of the potentials from each charge:

Summary Table: Key Electrostatics Quantities