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 ).

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 ).

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.