Understanding Electricity in A-Level Physics

May 11, 2025

Topic 9: Electricity - CAIE Physics A-Level

9.1 Electric Current

  • Electric Current (I): Flow of charge (Q) per unit time (t), or rate of flow of charge.
  • Charge Carriers: Typically electrons, carrying elementary charge ( e = 1.60 \times 10^{-19} \text{C} ).
  • Charge Equation: ( Q = I \times t )
    • Example: 5 amps for 10 seconds = 50 coulombs.
  • Current Equation: ( I = nAqv )
    • Current ( I ) in a conductor with cross-sectional area ( A ), charge ( q ), number density ( n ), and mean drift velocity ( v ).

9.2 Potential Difference and Power

  • Potential Difference (V): Energy transferred per unit charge between two points in a circuit.
    • Formula: ( V = \frac{W}{Q} ); where ( W ) is energy/work done.
  • Power (P): Rate of energy transfer; ( P = \frac{E}{t} ).
    • Another formula: ( P = VI )
    • Variations: ( P = I^2R ) and ( P = \frac{V^2}{R} ).
  • Energy Transferred: ( E = Pt ) or ( E = VIt ).

9.3 Resistance and Resistivity

  • Resistance (R): Opposes the flow of charge; ( R = \frac{V}{I} ).
  • Ohm's Law: ( V = IR ) for ohmic conductors (constant physical conditions).
    • Ohmic Conductor: Follows Ohm's law, linear V-I graph.
    • Diode Graph: Forward bias allows current after threshold; reverse bias has high resistance.
    • Filament Lamp: Resistance increases with temperature and current.
  • Measurement Tools:
    • Ammeters have zero resistance.
    • Voltmeters have infinite resistance.
  • Resistivity (( \rho )): Measure of material's conductivity; ( \rho = \frac{RA}{L} ).
    • Resistance increases with temperature in metals; decreases in thermistors.
    • Thermistors: Resistance decreases with temperature increase.
    • Superconductors: Zero resistivity below critical temperature.
      • Applications: Power cables, magnetic fields (maglev trains, medical).
  • Light Dependent Resistor (LDR): Resistance decreases with light intensity.
    • Used for light-based circuit triggers.