Q: What does IP Physics Notes (Upper Secondary, Year 3-4): 12) Direct Current Circuits cover? A: Combine series/parallel rules, potential dividers, and sensor behaviour to solve IP DC circuit problems.
Quick recap -- DC circuit analysis hinges on current and voltage sharing rules. Once you master how resistances combine and how potential dividers behave, thermistor/LDR sensor circuits become straightforward.
The core idea is simple: Series shares current; parallel shares voltage.
Use it as a working check: Simplify resistance first, then find total current, voltage drops, and branch currents. Potential dividers turn resistance changes into voltage changes.
Then go one layer deeper: Work through the mixed network and sensor notes to practise drawing the simplified circuit, applying the correct sharing rule, and checking current and voltage totals.
Keep your practice loop tight via our Sec 3 IP Physics tuition hub. It links each topic here to quizzes, diagnostics, and WA-style problem sets.
For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.
Eclat core: circuit diagrams, series and parallel rules, effective resistance, whole-circuit calculations, potential dividers, thermistors, LDRs, and sensor problems follow Marcus Pang's current Chapter 12 route.
Eclat application depth: mixed networks, bulb-power comparisons, divider thresholds, and switching interpretation prepare integrated questions but may be sequenced differently by school.
2027 national comparison: K323 Topic 15 overlaps with the full Eclat core, including NTC thermistors and LDR input transducers.
Check your school: use the current circuit-symbol sheet and the exact sensor orientation shown in the task before applying a memorised output rule.
Before analysing a circuit, draw it with the standard symbols supplied by your school or examination. Be ready to represent cells, batteries, d.c. and a.c. supplies, switches, lamps, fixed and variable resistors, potentiometers, fuses, ammeters, voltmeters, bells, LDRs, thermistors, LEDs, and ordinary diodes. A junction is shown by a filled connection point; crossing wires without a junction are not electrically connected.
Series
Current is identical everywhere in the loop.
Potential differences add: V=V1+V2+⋯.
Resistances add: Req=R1+R2+⋯.
Parallel
Potential difference across each branch is the same.
Currents add: I=I1+I2+⋯.
Inverse resistances add: 1/Req=1/R1+1/R2+⋯
Power expressions: P=VI=I2R=RV2
Ammeter -> series (assume R≈0); voltmeter -> parallel (assume R→∞).
Power formula choice checkpoint
All three power formulae describe the same component, but each one is convenient only after you know the matching current, voltage, or resistance.
What you know for one component
Fastest expression
Why this is safer
Common trap
Current through the component and its resistance
P=I2R
Series components share current, so this often avoids finding each voltage first.
Using the supply voltage for every series resistor.
Voltage across the component and its resistance
P=RV2
Parallel branches share voltage, so this often avoids finding branch current first.
Using total current in every parallel branch.
Component voltage and component current
P=VI
This is the direct definition once both quantities are already known.
Multiplying supply voltage by a branch current unless the branch is across the full supply.
Worked check: two resistors, 3Ω and 6Ω, are in series with a 9.0V supply. The total resistance is 9Ω, so the current is 1.0A. The 6Ω resistor has power P=I2R=1.02×6=6.0W, not 9.02/6, because it does not receive the full supply voltage.
Misconception check: choose the formula after deciding whether the component is in a series path or across a parallel branch. The formula does not decide the circuit sharing rule for you.
Meter-placement checkpoint
Before inserting a meter, decide whether you are measuring flow through a component or potential difference across it. The connection rule follows that decision.
Measurement wanted
Where the meter goes
Ideal resistance reason
Common trap
Current through one component
Ammeter in series with that component
Very low resistance so it does not significantly reduce the current
Connecting the ammeter across the component and creating a near short circuit.
Potential difference across one component
Voltmeter in parallel across that component
Very high resistance so it takes negligible current
Putting the voltmeter in series and almost opening the circuit.
Total current from the supply
Ammeter in the main loop before any branch split or after branches recombine
The same total current passes through that main section
Measuring only one branch current and calling it total current.
Voltage across a parallel block
Voltmeter across the two junctions of the whole block
Every branch shares the same potential difference
Placing the voltmeter across just one resistor outside the block.
Worked check: to measure the current through a 6Ω branch in a parallel circuit, place the ammeter inside that branch. To measure the voltage across the branch, connect the voltmeter across the two branch terminals.
Misconception check: a meter should not be placed where it changes the quantity you are trying to measure. Ammeter readings are meaningful only when the ammeter is part of the current path; voltmeter readings are meaningful only when the voltmeter compares two points.
Mixed-circuit simplification checkpoint
For a mixed circuit, simplify one clear group at a time before calculating currents and voltages.
Circuit clue
First move
Rule to apply
Common trap
Components share one path with no junction between them
Treat them as a series group.
Add their resistances directly.
Splitting the same current into different values inside a series path.
Components connect across the same two junctions
Treat them as a parallel group.
Add inverse resistances, then invert the result.
Adding parallel resistances directly.
One resistor is outside a parallel block
Replace the parallel block first, then combine with the outside resistor.
Parallel first, then series.
Using the supply voltage across each resistor before simplifying.
Asked for a branch current
Find the voltage across the whole parallel block first.
Use I=V/R for that branch.
Using the total circuit current as every branch current.
Misconception check: the supply voltage belongs to the whole circuit. A resistor or branch gets the supply voltage only if it is connected directly across the supply terminals.
Bulb brightness checkpoint
For identical bulbs, brightness follows power. Decide how current and voltage change before comparing brightness.
Circuit change
First comparison
Brightness result
Common trap
Add another identical bulb in series
Total resistance increases, so total current decreases
Every series bulb becomes dimmer than a single bulb on the same supply
Saying the first bulb keeps the original current because it is nearest the battery.
Add another identical bulb in parallel
Each branch still gets the supply voltage
Each identical parallel bulb is about as bright as one bulb alone, while total current from the supply increases
Saying each bulb gets half the voltage just because there are two bulbs.
One bulb is removed from a series circuit
The circuit opens if there is no bypass path
All bulbs in that series path go off
Thinking the remaining bulbs get brighter automatically.
One branch is removed from a parallel circuit
Other branches still connect across the supply
Bulbs in the remaining branches stay lit with about the same brightness
Treating a parallel circuit as one broken loop.
Worked check: two identical bulbs in series share the supply voltage, so each has a smaller potential difference than one bulb connected alone. Two identical bulbs in parallel each connect across the supply, so each branch bulb receives the full supply voltage; the battery supplies more total current, not less voltage per branch.
Worked Example: Mixed Network
A 12V supply feeds a 4Ω resistor in series with a parallel pair of 6Ω and 3Ω. The combined parallel resistance Rparallel=(1/6+1/3)−1=2Ω, so total Req=4+2=6Ω.
Total current I=12/6=2.0A. Voltage across the parallel loop is V=I×2=4.0V, so branch currents are 4/6=0.67A and 4/3=1.33A, summing back to 2.0 as a consistency check.
Potential Divider Principle
Two series resistors R1 and R2 share supply Vin.
Output across R2: Vout=R1+R2R2Vin
Replace one resistor with a sensor (thermistor or LDR) to make the output voltage depend on temperature or light.
Divider direction map
Before substituting numbers, mark where Vout is measured. The output follows the resistor it is across:
Sensor position
Sensor resistance increases
Sensor resistance decreases
Output across the sensor
Vout increases because the sensor takes a larger share of the supply.
Vout decreases because the sensor takes a smaller share of the supply.
Output across the fixed resistor
Vout decreases because the fixed resistor takes a smaller share of the supply.
Vout increases because the fixed resistor takes a larger share of the supply.
Use this as a sense-check after calculation. A bigger share of total resistance gets a bigger share of the supply voltage.
Worked Example: Thermistor Divider Direction
A 9.0V supply is connected to a fixed 3.0kΩ resistor in series with an NTC thermistor. Vout is measured across the thermistor. At low temperature, the thermistor has resistance 6.0kΩ. At high temperature, it has resistance 1.5kΩ.
At low temperature:
Vout=3.0+6.06.0×9.0=6.0V.
At high temperature:
Vout=3.0+1.51.5×9.0=3.0V.
Misconception check: "hotter" does not automatically mean a larger output voltage. For an NTC thermistor, hotter means lower resistance. Whether Vout rises or falls depends on whether the output is taken across the thermistor or across the fixed resistor.
Sensor Behaviours
NTC thermistor: resistance decreases as temperature rises. In a divider, place it top or bottom depending on whether you want output to increase or decrease with temperature.
LDR: resistance decreases under intense light.
Potentiometer: a three-terminal variable resistor acting as an adjustable divider (volume knobs, contrast controls).
Tips for Divider Problems
Identify which resistor the output spans.
Express Vout symbolically using the divider formula.
Substitute the sensor's resistance at the specified condition.
For switching circuits, combine with transistor/relay thresholds or compare to reference voltages.
Threshold decision checkpoint
For a sensor divider question, calculate or reason the output voltage first, then compare it with the switching threshold. Do not decide from "hotter" or "brighter" alone.
Question clue
First decision
Then compare
Common trap
"Switch turns on when Vout>3.0V"
Does the condition raise or lower Vout?
Turn on only when the output is above the threshold.
Saying the switch turns on just because the sensor resistance changed.
"Output is taken across the fixed resistor"
If the sensor resistance falls, the fixed resistor takes a larger voltage share.
Compare the new fixed-resistor voltage with the threshold.
Using the sensor's voltage when the output terminals are not across it.
"Output is taken across the sensor"
If the sensor resistance falls, the sensor takes a smaller voltage share.
Compare the new sensor voltage with the threshold.
Assuming an LDR or thermistor always increases output when the stimulus increases.
"Choose a sensor position"
Decide whether the output should rise or fall when the stimulus rises.
Place the sensor across the output if you want its voltage share to control the signal directly.
Drawing a divider before deciding the required output direction.
Worked check: an LDR is in series with a fixed resistor and Vout is measured across the fixed resistor. In brighter light, the LDR resistance decreases, so the fixed resistor takes a larger share of the supply voltage. If the circuit switches on when Vout is high, this arrangement is suitable for a light-activated circuit.
Misconception check: a potential divider question has two separate steps. First decide how the sensor changes resistance; then decide whether the measured output is across the sensor or across the other resistor.
Practice Quiz
Rehearse series-parallel simplifications, potential divider logic, and sensor response questions with this DC circuits set.
Key Takeaways
Memorise the sharing rules and reciprocal resistance formula for rapid simplification.
Track total current first, then drop across series components to analyse parallel branches.
Potential dividers turn resistance changes into voltage signals -- crucial for sensor questions.
Always cross-check current splits and voltage sums to guard against arithmetic slips.
