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Q: What does A-Level Physics: 16) Circuits Guide cover? A: From resistor networks to exponential RC transients, this post unpacks Topic 16 of the 2026 H2 Physics syllabus for IP students and parents.
TL;DR Circuits is not “just Ohm's Law” - it is the control panel behind practical Paper 4 and every data-logger question. This guide turns the SEAB bullet-points into classroom-tested check-lists, sensor hacks and WA timing tricks.
Keep the full circuits + electromagnetism toolkit handy via the H2 Physics notes hub; it links this post with the preceding Currents/Electric Fields chapters and the upcoming electromagnetism topics.
1 Circuit symbols & diagrams
Memorise SEAB's full symbol set - cell, switch, fixed/variable resistor, LDR, NTC thermistor, diode, capacitor, ammeter and voltmeter. Symbols must be drawn with a ruler in Paper 2 for the mark.
Parent tip: have your teen print the symbol sheet and stick it on the inside cover of their graph-book.
1.1 Mini-drill
Sketch a potential-divider with a fixed resistor and an LDR controlling Vout. Mark the sensing node. Time limit = 30 s.
2 Resistance, resistivity and internal resistance
2.1 Ohm's law refresher
The definition is R=IV
(Ω)
. Use it
only
when the graph through the origin is linear.
2.2 Microscopic link
For a uniform wire R=ρAl,
where ρ is resistivity, l (m) is length and A (m2) is cross-sectional area. Double l → double R; halve A → double R.
2.3 Temperature stories
Metals: hotter lattice ⇒ more collisions ⇒ higher resistivity.
NTC thermistors: heat frees charge carriers in the semiconductor ⇒ lower resistivity.
2.4 Internal resistance (r)
Real cells obey V=ε−Ir,
so increasing load current drops terminal p.d.
Graph cue: gradient = −r, intercept = ε. Quote ε in V not eV.
3 Resistors in series & parallel
Arrangement
Combined resistance
Series
Rtot=R1+R2+…
Parallel
Rtot1=R11+R21+…
Be ready to spot hidden series chains in messy WA diagrams.
3.1 Timing hack
Write the total resistance before inserting numbers; algebra first reduces keypad slips.
4 Potential divider circuits
4.1 Core formula
Vout=VinR1+R2R2.
4.2 Sensor combos
Brightness probe: replace R2 with LDR → Vout↑ in the dark.
Fire alarm: replace R2 with NTC → Vout↑ when hot.
Include a buffer op-amp in higher-ability tutorials to prevent loading.
5 I-V characteristics
Component
Key graph feature
Exam explanation
Ohmic resistor
Straight line through origin
Constant R
Filament lamp
Curve flattens as I↑
T↑⇒R↑ in tungsten
Diode
Conducts after ≈0.6V
Forward bias overcomes p−n barrier
NTC thermistor
Steep at low V
T↑⇒R↓ due to more carriers
Plot with current on the y-axis - SEAB marks for axis labels.
6 Capacitors in series & parallel
Arrangement
Combined capacitance
Series
Ctot1=C11+C21+…
Parallel
Ctot=C1+C2+…
Mnemonic: series for resistors adds R; series for capacitors adds 1/C.
7 RC circuits with d.c. source
7.1 Time constant
τ=RC.
At t=τ a discharging capacitor's Q,V or I falls to e1≈37% of its initial value.
7.2 Exponential laws
Charging: Q=Q0[1−e−t/τ],V=V0[1−e−t/τ].
Discharging: Q=Q0e−t/τ,V=V0e−t/τ,I=I0e−t/τ.
Plot lnV vs t to obtain a straight line of gradient −1/RC - a favourite Paper 4 practical.
8 Three WA timing rules (Circuits edition)
Label units first for every numerical answer - avoids unit-free slips.
Sketch a quick circuit even if not asked; you see hidden series legs faster.
Log-log check: if your answer for R or C is < 0.1 or > 10 Ω/F, re-read prefixes.
Need structured practice on Circuits? Our H2 Physics tuition programme covers this topic with weekly problem sets and Paper 4 practical drills.
Comprehensive revision pack
9478 Section V, Topic 16 Syllabus outcomes at a glance
Outcome (a) - interpret circuit diagrams and recognise standard symbols.
Outcome (b) - analyse series/parallel resistor and capacitor networks.
Outcome (c) - explain and design potential divider circuits with sensors.
Outcome (e) - describe and model RC charging/discharging transients.
Concept map (in words)
Start with circuit symbols to communicate clearly. Use Ohm's law and resistivity for basic components. Combine resistors/capacitors systematically. Potential dividers convert sensor resistance to voltage signals. RC circuits introduce exponential behaviour governed by time constant RC.
Key relations
Quantity / concept
Expression / highlight
Ohm's law
V=IR
Resistivity relation
R=ρAl
Series resistors
Rtot=∑Ri
Parallel resistors
Rtot1=∑Ri1
Potential divider
Vout=VinR1+R2R2
Series capacitors
Ctot1=∑Ci1
Parallel capacitors
Ctot=∑Ci
RC charging equation
V(t)=V0(1−e−t/(RC))
RC discharging equation
V(t)=V0e−t/RC,;I(t)=I0e−t/RC
Derivations & reasoning to master
Potential divider: derive ratio using loop current, or use voltage drop proportionality.
RC exponential: solve differential equation dtdQ=RV−Q/C to show exponential form.
Sensor behaviour: analyse how LDR/NTC resistance changes V_out and relate to control systems.
ln-linearisation: rearrange V=V0e−t/RC to lnV=lnV0−t/RC
Worked example 1 - potential divider sensor
Design a circuit that outputs 3.0V when an LDR (resistance 12kOhm in dark, 2.0kOhm in bright light) is exposed to daylight using a 9.0V supply. Determine the fixed resistor value and predict the output in darkness.
Method: solve Vout=VinRfixed+RLDRRLDR for the bright condition; check the dark condition to ensure the alarm threshold.
A 47kOhm resistor and 100uF capacitor form a delay circuit. (a) Find the time constant. (b) How long until the capacitor voltage reaches 90 percent of the supply? (c) If used with 5V logic, what is the voltage at 3τ?
Solution: The time constant is 4.7s(τ=RC).
V=V0(1−e−t/(RC))
Use this relation to solve for t and the specific voltages.
For 90\% charging: 0.90=1−e−t/τ⇒t=τln10=2.303τ≈10.8s.
At 3τ: V=V0(1−e−3)=0.950V0≈4.75V for V0=5V.
Practical & data tasks
Build potential divider with light sensor; logVout under different lux levels and fit calibration curve.
Record capacitor discharge using Logger Pro; plot lnV vs t to extract −1/RC.
Investigate loading effect by attaching low-resistance voltmeter to a divider; observe output change.
Common misconceptions & exam traps
Forgetting to convert mm2 to m2 when using resistivity equation.
Mixing up series/parallel rules for capacitors vs resistors.
Ignoring meter resistance when measuring delicate dividers (loading).
Failing to state exponential behaviour explicitly in written explanations.
Quick self-check quiz
In a potential divider, what happens to Vout if R2 decreases? - It decreases (assuming R1 is fixed).
How do you halve the time constant without changing capacitance? - Halve the resistance (since τ=RC).
What is the gradient of lnV vs t for a discharging capacitor? - −RC1.
Are thermistors ohmic? - No; resistance changes with temperature causing non-linear I−V behaviour.
Suggest one way to buffer a sensor output against loading. - Use an op-amp voltage follower.
Revision workflow
Redraw standard sensor-circuit templates and annotate expected Vout behaviour.
Parents: book a 60-min Circuit Masterclass four weeks before WA 2 - most careless marks hide in potential-divider algebra.
Students: screenshot the RC graphs above and recreate them without notes tomorrow.
Last updated 14 Jul 2025. Next review when SEAB issues the 2027 draft syllabus.
