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.
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.
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.
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.
Worked example 2 - RC timing
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.
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.
