Electric charge

  • Charge is a physical property of matter
  • Both positive and negative charge exists
  • When two bodies with a net electrical charge come together, they experience a force
  • The SI unit for charge is the Coulomb (C)

Coulomb’s law

Not in the course, but

coloumbs law

FIf F is positive, the force is repulsion; if it is negative, it is attraction.

Note the similarity with Newton’s Law of Gravitation.

Electric fields

  • Electric charge produces electric fields
  • The electric field is a vector – has both magnitude and direction
  • Represented by lines of force with arrows
  • These represent the direction that a positive charge would move in if it were placed in that location

Electrical field strength

Measured in N/C or V/m

E = F / q

Note that E is electrical field strength not energy.

Electrical work

  • To move charges, we need to do work
  • E.g. when two positive charges are put together
  • This work is stored as electrical potential energy


W = Vq = Fd

Where V = voltage, q = charge, d = distance moved, F = force applied

Note that 1 V gives a particle with 1 C of charge 1 J of electrical potential.

Electrical field between parallel plates

      • Between these plates is a potential difference
      • There is a uniform electric field across the plates

E = V / d

E = electrical field strength, V = voltage across the plates, d = distance between them


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