Key Equations

Charge & Current - OCR A-Level Physics

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.
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