Charge & Current

The fundamental quantities that make circuits work - from elementary charge to drift velocity.

Spec Points Covered
  • I can define electric currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). and state its unit.
  • I can use $Q = It$ and $I = \Delta Q/\Delta t$ in calculations.
  • I can explain what chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). quantisation means and use $Q = ne$.
  • I can state the elementary chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).The magnitude of charge carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C. and relate it to electron/proton charge.
  • I can identify charge carriers in metals, electrolytes, and semiconductorsMaterials with resistivityA material property that quantifies how strongly it resists currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohm-metres (Ω m). between conductors and insulators. Their number densityMass per unit volume of a material. Measured in kg m⁻³. of charge carriers increases with temperature..
  • I can distinguish between conventional currentThe rate of flow of charge. Measured in amperes (A).The direction of current flow defined as from positive to negative. Opposite to the direction of electron flow. and electron flow.
  • I can describe electron motion with and without a p.d. applied.
  • I can define mean drift velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.The average velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of charge carriers through a conductor in the direction of current flow, typically very slow (~1 mm/s).The average velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. of charge carriers through a conductor in the direction of current flow. and use $I = Anev$.
  • I can explain how number densityMass per unit volume of a material. Measured in kg m⁻³.The number of charge carriers per unit volume in a material. Measured in m⁻³. relates to conductors, semiconductorsMaterials with resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m). between conductors and insulators. Their number densityMass per unit volume of a material. Measured in kg m⁻³. of charge carriers increases with temperature., and insulators.
  • I can state and apply KirchhoffKirchhoff's laws: (1) Conservation of charge at junctions. (2) Conservation of energyThe capacity to do work. Measured in joules (J). around closed loops. Sum of EMFs = sum of IR drops.'s first law using conservation of charge.
Notes
01 Electric charge Electric charge → 02 Electric current Electric current → 03 Charge carriers Charge carriers → 04 Conventional current flows from positive to negative terminal → 05 Mean drift velocity Mean drift velocity → 06 Number density (n) Number density (n) → 07 Rearranging I = Anev gives → 08 Kirchhoff's first law Kirchhoff's first law → 09 A copper wire has cross-sectional area A = 0 $v = \frac{I}{Ane} = \frac{2.90}{0.5 \times 10^{-6} \times 8.50 \times 10^{28} \times 1.6 \times 10^{-19}}$ → 10 A lightning strike delivers $I = 30 kA$ and transfers $Q = 15 C$ of charge $\Delta t = \frac{\Delta Q}{I} = \frac{15}{30 \times 10^{3}} = 5.0 \times 10^{-4} \text{ s}$ →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
Charge from current and time
$$Q = It$$
  • Where:
    • $Q$ = C
    • $I$ = A
    • $t$ = s
Rearrangement of the current definition. Use when you know current and time and need total charge. Always convert time to seconds.
Current as rate of flow of charge
$$I = \frac{\Delta Q}{\Delta t}$$
  • Where:
    • $I$ = A
    • $ΔQ$ = C
    • $Δt$ = s
The defining equation for current. 1 A = 1 C s^-1. This is the form given in the OCR data sheet.
Mean drift velocity equation
$$I = Anev$$
  • Where:
    • $I$ = A
    • $A$ = \(m^{2}\)
    • $n$ = m^-3
    • $e$ = C
    • $v$ = m s^-1
Key OCR equation (not in AQA). Links current to the physical properties of the conductor. Rearrange for v: v = I/(Ane). Cross-sectional area MUST be in \(m^{2}\).
Charge quantisation
$$Q = ne$$
  • Where:
    • $Q$ = C
    • $n$ = number of electrons (no units)
    • $e$ = C (1.6 x 10^-19)
Charge is quantised: it must be a whole-number multiple of e. Use to find the number of electrons transferred. Do not confuse this n (count) with number density n in I = Anev.