AQA Physics A-level: Section 5 - Electricity

3.5.1 Current Electricity

3.5.1.1 Basics of Electricity

• Electric Current (I): Flow of charge per unit time.
  • Formula: ( I = \frac{Q}{t} )
• Potential Difference (V): Energy transferred per unit charge.
  • Formula: ( V = \frac{W}{Q} )
• Resistance (R): Measure of how difficult it is for charge carriers to pass through a component.
  • Formula: ( R = \frac{V}{I} )

3.5.1.2 Current-Voltage Characteristics

• Ohm's Law: For ohmic conductors, current is directly proportional to potential difference if physical conditions are constant.
  • Ohmic conductor: Straight line graph through the origin.
  • Semiconductor diode: High resistance in reverse bias, allows current in forward bias beyond threshold voltage.
  • Filament lamp: Resistance increases with current due to heating.

3.5.1.3 Resistivity

• Resistivity (( \rho )): Measure of how easily a material conducts electricity.
  • Formula: ( \rho = \frac{RA}{L} )
• Temperature effects:
  • Metal conductor: Resistance increases with temperature.
  • Thermistor: Resistance decreases with temperature.
• Superconductors: Materials with zero resistivity below critical temperature.
  • Applications: Power cables, magnetic fields (maglev trains, medical applications).

3.5.1.4 Circuits

• Resistors in Series: ( R_T = R_1 + R_2 + R_3 + \ldots + R_n )
• Resistors in Parallel: ( \frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \ldots )
• Power (P): Rate of energy transfer.
  • Formula: ( P = \frac{E}{t} )
  • Variations: ( P = VI = I^2R = \frac{V^2}{R} )
• Series Circuits:
  • Current same everywhere; voltage summed across components equals supply voltage.
• Parallel Circuits:
  • Total current: Sum of current through branches.
  • Voltage same across each branch.

3.5.1.5 Potential Divider

• Circuit with resistors in series across a voltage source.
  • Produces a specific fraction of source voltage.
  • Can use variable resistors for variable p.d.
  • Application: Thermistors and LDRs for temperature/light sensors.

3.5.1.6 Electromotive Force and Internal Resistance

• Internal Resistance (r): Resistance within the battery causing energy loss.
• Electromotive Force (emf, ( \mathcal{E} )): Energy transferred per coulomb.
  • Formula: ( \mathcal{E} = \frac{E}{Q} )
• Equations:
  • Total resistance: ( R_T = R + r )
  • ( \mathcal{E} = I(R + r) = IR + Ir )
  • Terminal p.d and lost volts: ( \mathcal{E} = V + v )
• Example Calculations:
  • Current, lost volts, and internal resistance in circuits.