Q: What does A-Level Physics: 15) Currents Guide cover? A: From drift velocity to diode rectifiers, this post unpacks Sub-topic 15 Currents of the 2026 H2 Physics syllabus for IP students and parents.
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=tQ.
1.1 Mini-drill
A charge of
12mC
passes a point in
4.0s
.
I=3.0mA.
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 velocityv when an external field is applied. Equating the charge that crosses a cross-section per second gives
I=nAvq
where
Symbol
Meaning
\( n \)
number density of charge carriers
\( A \)
conductor cross-section area
\( q \)
charge on one carrier (\( 1.60 \times 10^{-19} \pu{C} \) for an electron)
Tip for WA practice: treat v as μE when mobility (μ) and field (E) are given.
3 Potential difference (V)
Potential difference is the electrical work done per unit charge:
V=QW.
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,P=I2R,P=RV2.
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,
Irms=2I0,Vrms=2V0.
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=x0sinωt,
where ω=2πf.
7 Mean power in a resistive load
For R purely resistive and current I=I0sinωt:
Pmean=21I02R=Irms2R,
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-2 slips.
For rectifier graphs, label axes with units first, then plot.
Comprehensive revision pack
9478 Section V, Topic 15 Syllabus outcomes at a glance
Outcome (a) - describe current, potential difference, and resistance microscopically.
Outcome (b) - derive and use the drift velocity relation I=nAvq.
Outcome (c) - distinguish e.m.f. and p.d., including internal resistance effects.
Outcome (d) - analyse alternating currents, r.m.s. values, and power.
Outcome (e) - explain rectification and smoothing circuits.
Concept map (in words)
Charge carriers move with drift velocity when electric fields act. Potential difference measures energy per charge; power relations follow directly. e.m.f. supplies energy; p.d. dissipates it. AC descriptions require r.m.s. quantities, and rectifiers convert AC to pulsating DC.
Drift velocity formula: equate charge passing per second through cross-section to nAvq.
Power relationships: combine Ohm's law V=IR with P=VI.
r.m.s. derivation: integrate I02sin2ωt over one period to show the 21 factor.
Rectifier output: sketch the current waveform with diode removal of the negative half-cycle and compute average/mean power.
Worked example 1 - drift velocity
A copper wire (area 1.5mm2) carries 4.0A. Given free-electron density 8.5×1028m−3, find the drift velocity. Comment on why it is much smaller than electron random speeds.
Outline: convert area to m2, apply v=nAqI. Typical answer ≈2mm⋅s−1.
Worked example 2 - internal resistance & power
A 12V cell with internal resistance 0.40ohm supplies a 4.0ohm resistor. Find terminal p.d., current, and power dissipated in cell vs load. Suggest how results change if a second identical resistor is added in parallel.
Method: current I=R+rE, compute terminal voltage E−Ir, compare powers I2R and I2r.
Practical & data tasks
Measure I-V characteristics of a diode using datalogger; observe forward conduction and reverse blocking.
Use oscilloscope to compare peak and r.m.s. values of mains waveform; annotate timebase.
Investigate internal resistance by plotting terminal voltage vs current for a cell and finding gradient/intercept.
Common misconceptions & exam traps
Confusing conventional current direction with electron flow.
Forgetting to convert mm2 to m2 in drift velocity calculations.
Using peak rather than r.m.s. values when working with power.
Ignoring internal resistance when analysing supply voltage drops.
Quick self-check quiz
Define current in terms of charge flow. - I=dtdQ.
What physical quantity does potential difference represent? - Work done per unit charge.
For I=0.5sin100t A, find Irms. - 20.5≈0.354A.
How does adding a smoothing capacitor to a half-wave rectifier affect output? - Reduces ripple, increasing average voltage.
If a cell's internal resistance doubles, what happens to terminal voltage for same load? - It decreases further due to a larger Ir drop.
Revision workflow
Re-derive drift velocity and r.m.s. relations weekly to keep intuition fresh.
Practise I-V graph interpretation for resistors, bulbs, diodes, and identify regions of operation.
Work through past exam problems involving internal resistance and power distribution.
Summarise AC vs DC terminology (peak, r.m.s., average) on a cheat sheet and review before quizzes.
Practice Quiz
Test yourself on the key concepts from this guide.
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.
