# Electrical Fields Learning Objectives

Content

• Force between point charges in a vacuum:
• F = 1/4πƐ0  Q1Q2/r2
• Permittivity of free space, Ɛ0
• Appreciation that air can be treated as a vacuum when calculating force between charges.
• For a charged sphere, charge may be considered to be at the centre.
• Comparison of magnitude of gravitational and electrostatic forces between subatomic particles.
• Representation of electric fields by electric field lines.
• Electric field strength.
• E as force per unit charge defined by E = F/Q
• Magnitude of E in a uniform field given by E = V/d
• Derivation from work done moving charge between plates: Fd = QΔV
• Trajectory of moving charged particle entering a uniform electric field initially at right angles.
• Magnitude of E in a radial field given by E = 1/4πƐ0  Q/r2
• Understanding of definition of absolute electric potential, including zero value at infinity, and of electric potential difference.
• Work done in moving charge Q given by ∆ W = Q ∆ V
• Equipotential surfaces.
• No work done moving charge along an equipotential surface.
• Magnitude of V in a radial field given by V = 1/4πƐ0  Q/r
• Graphical representations of variations of E and V with r.
• V related to E by E = ∆ V/∆ r
• ∆V from the area under graph of E against r.
• Similarities and differences between gravitational and electrostatic forces:
• Similarities: Both have inverse-square force laws that have many characteristics in common, e.g. use of field lines, use of potential concept, equipotential surfaces etc.
• Differences: masses always attract, but charges may attract or repel