AQA GCSE Physics: Topic 2 - Electricity

Circuit Symbols and Electrical Charge

• For Charge to Flow:
  • Circuit must be closed (no open switches).
  • There must be a source of potential difference (e.g., battery or cell).

Electrical Current

• Definition: Current is the flow of electrical charge.
• Characteristics:
  • Greater rate of flow of charge = greater current.
  • In a single closed loop, current has the same value at any point.
  • Relationship: ( Q = I \times t ) where:
    • ( Q ) = Charge flow (coulombs, C).
    • ( I ) = Current (amperes, A).
    • ( t ) = Time (seconds, s).
• Dependency:
  • Current through a component depends on resistance (R) and potential difference (V).
  • Greater resistance = smaller current for a given V.
  • Calculations: ( V = I \times R ).

Resistors

• Ohmic Conductor:
  • Constant resistance; current is directly proportional to potential difference.
  • Linear graph.
• Non-Ohmic Components:
  • Resistance changes with current; graph is nonlinear.
  • Examples: Filament lamps, diodes, thermistors, LDRs.

Resistance Changes

• With Current:
  • As current increases, electrons have more energy.
  • Electrons collide with atoms, increasing resistance.
• With Temperature:
  • Higher temperatures increase atom vibration and resistance.
  • Thermistors:
    • Lower resistance at higher temperatures.
• With Length:
  • Longer wires have higher resistance.
• With Light:
  • LDRs:
    • Lower resistance with more light.

Series and Parallel Circuits

• Series Circuits:
  • Single path for current.
  • Current is the same throughout.
  • Total Resistance = sum of individual resistances.
• Parallel Circuits:
  • Multiple paths for current.
  • Total current is the sum of currents in branches.
  • Total resistance is less than the smallest branch resistance.

Domestic Uses and Safety

• Mains Electricity:
  • AC supply, 50 Hz frequency, ~230 V in the UK.
  • Cabling:
    • Live (brown, 230V), neutral (blue, 0V), earth (green/yellow, 0V) wires.

Power

• Definition: Energy transferred per second; relates to current and voltage.
• Calculations:
  • Power ( P = V \times I ).
  • Energy Transferred ( E = P \times t ) or ( E = Q \times V ).
• National Grid:
  • Links power stations to consumers via cables and transformers.

Charge and Static Electricity

• Charge:
  • Positive and negative; like charges repel, opposites attract.
• Insulators vs. Conductors:
  • Insulators: Do not conduct as electrons are fixed.
  • Conductors: Electrons can flow.
• Static Electricity:
  • Occurs when two insulators rub together, transferring electrons.
  • Sparking occurs when charge builds up and objects are close.

Electric Fields

• Characteristics:
  • Analogous to magnetic fields but for charges.
  • Lines point in direction a positive charge would move.