Executive Summary
A consistent weakness across all years is the confusion between E.M.F. and terminal potential difference, particularly when internal resistance is involved. Candidates frequently struggle with the principle of potentiometers, often failing to recognize that a null reading (zero current) implies balanced potentials, not zero voltage across the source. Additionally, the application of Kirchhoff's Second Law is often flawed by incorrect loop choices or sign errors, especially in circuits with multiple sources or parallel branches.
The "Hall of Shame"
These specific errors appear in almost every exam session. Memorize them to avoid losing easy marks.
The "Lost Volts" Trap
EMF ≠ Terminal PD
Candidates assume the voltmeter reading across a battery equals its EMF. If current flows, V = E - Ir. The reading is less than EMF.
Null Point Logic
Zero Current Meaning
Thinking a null reading means zero voltage everywhere. It means the PD across the wire segment exactly balances the test cell's EMF.
Loop Analysis
Sum of PDs Definition
Omitting "around a closed loop" when defining the law. Also, failing to sum individual PDs before equating to EMF.
Circuit Rules
PD in Parallel
Thinking current splits equally or PD is different across parallel branches. PD is ALWAYS the same across parallel components.
March 2025
What Went Wrong
- Terminal PD vs EMF: Many candidates confused terminal PD with EMF. They failed to recognize that with an open circuit, the voltmeter reading equals the EMF, but in a closed circuit, it reads the terminal PD.
- Internal Resistance: Candidates frequently neglected the effect of internal resistance on voltmeter readings in closed circuits.
Success Point
Candidates who recognized the open circuit condition correctly identified V as the EMF.
March 2024
What Went Wrong
- Definitions: Candidates knew PD and EMF were different but often swapped their definitions.
- Internal vs Total Resistance: Some candidates confused the total circuit resistance (e.g., 7.2Ω) with the internal resistance r of the source.
Success Point
Stronger candidates correctly distinguished between internal resistance and load resistance.
November 2023
What Went Wrong
- Parallel Formula: Using R = 1/R1 + 1/R2 instead of 1/R = 1/R1 + 1/R2.
- LDR Behaviour: Stronger candidates incorrectly assumed current in an LDR was equal to the current in a parallel fixed resistor.
- Potentiometer Slider: Difficulty showing the correct slider position for null deflection, indicating a lack of understanding of the balancing principle.
- Series PD Ratio: Assuming PD splits equally across resistors when calculating ratios, leading to reciprocal answers.
Success Point
Most successfully recalled that LDR resistance decreases as light intensity increases.
June 2023
What Went Wrong
- Battery Polarity: Failing to notice when one cell's polarity is opposite to others in a loop (subtracting instead of adding EMFs).
- Energy Calculation: Calculating energy transferred but failing to subtract it from initial stored energy to find remaining energy.
- Internal Resistance Effect: Not realizing that adding a parallel resistor decreases total resistance, increases total current, and thus increases lost volts (Ir), decreasing terminal PD.
- Potentiometer Principle: Struggling to apply the principle of comparing PDs to find unknown EMFs.
Success Point
Stronger candidates correctly deduced that the voltmeter reading would drop below 4.8V due to increased current.
March 2023
What Went Wrong
- Variable Resistor PD: Assuming PD across a variable resistor was the full supply voltage (e.g., 12V or 6V) rather than the actual calculated value (e.g., 4.5V).
- Kirchhoff's 1st Law: Many candidates failed to reference all three currents at a junction when explaining circuit behavior.
- Lost Volts: Not understanding that as current increases, terminal PD (E - Ir) decreases.
Success Point
Most candidates correctly identified that increasing light intensity increases LDR power dissipation.
November 2022
What Went Wrong
- Power & Resistance: Misconception that P = V2/R implies power is inversely proportional to resistance, forgetting that V might change or I might be the constant factor.
- Current Calculation: To find potentiometer resistance, one must first find the current using Kirchhoff's laws. Many skipped this step.
- Lamp Brightness: Incorrectly predicting lamp brightness changes when a slider moves. This often requires analysing parallel branch currents.
- Internal Resistance: Calculating resistance of a conductor assuming it's in parallel with cells, when it was in series.
Success Point
Stronger candidates realized the need to calculate current in the internal resistance first.
June 2022
What Went Wrong
- Potentiometer Circuit: Choosing a diagram where cell polarities were incorrect. Driver cells must drive current in the same direction as the test cell for a null point.
- Circuit Symbols: Poor drawing of symbols for heaters and LDRs.
- Series Derivation: Omitting the first step: Vtotal = V1 + V2.
- Parallel PD: Failing to realize that PD across two parallel resistors of different values is the same.
Success Point
Most candidates attempted the parallel resistance calculation correctly, though some forgot to invert 1/R.
March 2022
What Went Wrong
- Cell Polarity: Weaker candidates ignored reversed cell polarities in calculations.
- Parallel PD: Most candidates failed to use Kirchhoff's 2nd law to show PD across a parallel wire is constant (equal to the source if no internal resistance). Misconception that PD would increase.
Success Point
None explicitly stated.
November 2021
What Went Wrong
- Load Resistor Change: Increasing load resistance decreases current, decreasing internal power loss, and increasing terminal PD. Many got this chain of logic wrong.
- Efficiency Calculation: Premature rounding of individual power values led to incorrect efficiency percentages.
- Total Resistance: Calculating conductor resistance assuming parallel connection with cells instead of series.
- Kirchhoff's 2nd Law Steps: Omitting intermediate steps in calculations. Examiners require seeing the sum of PDs equated to sum of EMFs.
- Power Decrease: Correctly stating power decreases but failing to explain it's due to increased total resistance and decreased current.
Success Point
Stronger candidates found applying Kirchhoff's second law straightforward.
June 2021
What Went Wrong
- Meaning of 9.0V: Confusion over a battery marked "9.0V". This is EMF. Terminal PD is less due to lost volts.
- Internal Resistance from Graph: Failing to identify the gradient of a V-I graph as the internal resistance (magnitude).
- Derivation Steps: Omitting the statement "Sum of PDs = Total PD" in series derivations.
- Potentiometer Balance: Not realizing that at the balance point, internal resistance of the test cell is irrelevant because current is zero.
Success Point
Most realized that sliding the contact changes effective resistance and thus current.
March 2021
What Went Wrong
- Internal Resistance Calc: Simply dividing EMF by Current gives Total Resistance, not Internal Resistance. You must subtract Load Resistance.
- Kirchhoff's 1st Law: Stating the law for a "single current" instead of the "sum of currents" at a junction.
- Potential Divider Redraw: Weaker candidates couldn't visualize the circuit as a potential divider. Redrawing the circuit was a recommended strategy.
Success Point
Stronger candidates correctly identified the voltmeter reading as EMF when the switch was open.
November 2020
What Went Wrong
- Parallel Resistor Addition: Adding a resistor in parallel decreases total resistance. Many thought it increased resistance or didn't link it to increased "lost volts" due to higher total current.
- Lamp Resistance: Assuming lamp resistance is 1/Gradient of I-V graph. It is V/I.
- Zero Voltmeter Reading: Failing to deduce that if a voltmeter reads zero, the potentials at its terminals are equal.
- Solar Cell EMF: Confusing the EMF of the main battery with the small EMF of the solar cell being measured.
Success Point
Candidates who could recall the intensity-amplitude relationship usually applied it correctly.
March 2020
What Went Wrong
- Parallel Batteries: Not understanding that connecting identical batteries in parallel leaves PD across external resistors unchanged (so current remains same).
- Battery Current: In the above case, the total current splits, so current in each battery decreases.
- Energy Explanation: Explaining potential dividers in terms of "shared voltage" instead of "energy" as requested.
Success Point
Almost half correctly calculated combined EMF and internal resistance, avoiding polarity errors.
Master Tips for Full Marks
Check Cell Polarity
In loops with multiple cells, check arrows. If they push current in opposite directions, subtract their EMFs. If same direction, add them.
The "Lost Volts" Chain
Memorize this chain: Add parallel load → Rtotal decreases → Itotal increases → Lost Volts (Ir) increases → Terminal PD (E - Ir) decreases.
Null Point = Balance
If a galvanometer reads zero, the PD across the slide wire segment exactly equals the EMF of the test cell. No current flows in the test cell branch.
Examiner Voices (Direct Quotes)
Direct quotes from the reports that require no extra context:
"Candidates who recognised that V represents the terminal p.d. and that with an open circuit this would be equal to the e.m.f. selected the correct option."
"Candidates who used this method sometimes used the incorrect formula of R = 1 / R1 + 1 / R2 instead of the correct formula 1 / R = 1 / R1 + 1 / R2."
"Although some candidates were able to calculate the energy transferred from the battery, many did not realise that they then needed to subtract this... from the initial energy."
"Many candidates did not realise that, in order to calculate the resistance of the potentiometer, it was first necessary to find the current... by applying Kirchhoff's first law."
"Only a small percentage of the candidates explained that the potential difference would be the same across both [parallel] resistors."