IP Physics Notes (Upper Secondary, Year 3-4): 10) Static Electricity

Quick recap -- Static electricity tracks how charges accumulate, move, and interact via electric fields. Watch electron transfer, sketch field lines correctly, and relate the principles to photocopiers, paint sprayers, and safety controls.

Charge Basics

• Charge comes in positive or negative units; SI unit coulomb (( \pu{C} )).
• Like charges repel; unlike charges attract.
• Conservation of charge: total charge in an isolated system stays constant.
• Only electrons move in everyday charging processes; protons remain fixed in nuclei.
• Charging methods:
  • Friction: electrons transferred between insulators (e.g., polythene rod and wool).
  • Conduction: direct contact lets electrons flow between conductors.
  • Induction: charged object polarises a neutral conductor; grounding can lock in opposite charge.

Conductors vs Insulators

• Earthing connects the object to ground, allowing excess electrons to flow away.

Electric Fields

• Electric field: region where a charge experiences a force.
• Field direction defined by the force on a positive test charge.
• Field line rules:
  • Start on positive charges, end on negative charges.
  • Density of lines reflects field strength.
  • Lines never cross and meet conductors at right angles.
• Patterns to memorise: isolated positive/negative (radial), dipoles, parallel plates (uniform field).

Worked Example: Force Between Suspended Spheres

Two identical spheres of mass ( m ) carry equal charges ( q ) and hang from threads of length ( L ), forming angle ( \theta ) with the vertical. At equilibrium, resolve forces: ( T \cos \theta = m g ) and ( T \sin \theta = F_\text{electric} = k q^2 / r^2 ), hence ( \tan \theta = k q^2 / (m g r^2) ).

Applications of Static Electricity

• Photocopiers/laser printers: charged drum attracts toner to image regions; heat fuses toner onto paper.
• Electrostatic paint spraying: charged droplets repel each other and attract to earthed car bodies for even coating.
• Electrostatic precipitators: dust particles charged and collected on oppositely charged plates in chimneys.
• Inkjet printing: fine charged droplets deflected by electric fields to form images.
• Van de Graaff generator: accumulates high voltages for demonstrations.

Hazards & Safety

• Sparks near fuel vapour can ignite explosions. Mitigation: earth fuel tankers, use antistatic hoses, keep humidity moderate.
• Sensitive electronics damaged by small discharges. Use antistatic wrist straps, mats, and storage bags.
• Lightning: large-scale static discharge. Lightning rods provide a safe conducting path to ground.

Key Takeaways

• Track electron movement to explain charging and discharging.
• Electric-field sketches communicate direction and relative strength; follow the rules strictly.
• Static applications leverage attraction/repulsion for deposition or removal; hazards revolve around unintended sparks.
• Always connect theory to mitigation strategies (earthing, humidity control, shielding).