A-Level Physics — 15) Currents (IP-Friendly Guide)
Download printable cheat-sheet (CC-BY 4.0)14 Jul 2025, 00:00 Z
TL;DR
Treat current electricity as the “traffic system” of Paper 2. Mastering charge flow, r.m.s. AC values and rectifiers turns once-scary graph questions into free marks — and locks in concepts needed for Magnetism, Quantum and Practical.
1 Electric current \((I)\)
Definition
Electric current is the rate of flow of charge through a surface:
\[ I = \frac{Q}{t}. \]
1.1 Mini-drill
A charge of \(12 \space \pu{mC}\) passes a point in \(4.0 \space \pu{s}\).
\(I = 3.0 \space \pu{mA}\).
Exam cue: always convert milli-, micro- and nano-coulombs to coulombs before substituting.
2 Microscopic view — drift velocity
In a metal, free electrons move randomly but acquire a drift velocity \(v\) when an external field is applied. Equating the charge that crosses a cross-section per second gives
\[ I = n A v q \]
where
| Symbol | Meaning |
| \(n\) | number density of charge carriers |
| \(A\) | conductor cross-section area |
| \(q\) | charge on one carrier (\(1.60 x 10^{-19} \space \pu{C}\) for an electron) |
Tip for WA practice: treat \(v\) as \(\mu E\) when mobility \((\mu)\) and field \((E)\) are given.
3 Potential difference \((V)\)
Potential difference is the electrical work done per unit charge:
\[ V = \frac{W}{Q}. \]
Parents: remind your child to track units — joule per coulomb is the volt.
4 Electrical power
Combine charge-flow and Ohm 's law ideas to get the “power trio”
\[ P = VI, \qquad P = I^2 R, \qquad P = \frac{V^2}{R}. \]
Timing hack: write \(P = VI\) at the top of data-handling questions; deriving the other two takes <15 s.
5 e.m.f. vs p.d.
| e.m.f. | p.d. | |
| Energy picture | energy supplied per coulomb by a source | energy converted to other forms per coulomb in a component |
| Circuit location | inside cells, generators | across resistors, lamps, etc. |
| Sign convention | raises potential | drops potential |
Remember: a cell 's internal resistance turns some of its e.m.f. into heat inside the cell — that lost voltage never reaches the external circuit.
6 Alternating current essentials
6.1 Period & frequency
- Period \((T)\): time for one full cycle.
- Frequency \((f)\): \(f = 1/T\).
6.2 Peak & r.m.s. values
For a sinusoid,
\[ I_{\text{rms}} = \frac{I_0}{\sqrt{2}}, \qquad V_{\text{rms}} = \frac{V_0}{\sqrt{2}}. \]
6.3 Why r.m.s.?
R.m.s. current produces the same heating effect in a resistor as a d.c. current of the same magnitude.
6.4 Equation of a sine wave
\[
x = x_0 \sin \omega t,
\]
where \(\omega = 2\pi f\).
7 Mean power in a resistive load
For \(R\) purely resistive and current \(I = I_0 \sin \omega t\):
\[ P_{\text{mean}} = \frac12 I_0^2 R = I_{\text{rms}}^2 R, \]
hence mean power is half the peak power.
8 Half-wave rectification
A single diode placed in series with a load blocks one half-cycle of the a.c. supply, allowing only positive (or negative) halves to pass. The output is a “pulsating d.c.” that still requires smoothing if a steady voltage is needed.
9 Three WA timing rules (Currents edition)
- 1 mark ≈ 1.5 min.
- Sketch peak and r.m.s. values before calculating — prevents factor-of-\(\sqrt{2}\) slips.
- For rectifier graphs, label axes with units first, then plot.
10 Further reading
11 Call-to-action
Parents: book a focused Currents clinic 1 week before WA 2 — it shores up both Electricity and upcoming EM induction.
Students: condense Sections 6-8 onto an A5 “AC cheat sheet” and quiz yourself on every bus ride.
Last updated 14 Jul 2025. Next review when SEAB releases the 2027 draft syllabus.
