Top 10 IGCSE Physics MCQ Mistakes (2022-2025)
The most common Multiple Choice mistakes from Cambridge Examiner Reports. These errors appear year after year - learn to avoid them.
Why MCQ mistakes matter: Multiple choice papers are worth 30% of your grade. Physics MCQs test understanding of concepts, not just recall. The same errors appear every year - especially in forces, electricity, and waves.
The Top 10 MCQ Mistakes
Constant Speed = Zero Resultant Force
Appears: 2022, 2023What the Syllabus Expects:
Understand that if an object moves at constant velocity, the resultant force on it is zero.
What Goes Wrong:
- • Thinking constant speed means there's a net forward force
- • Not recognizing that "terminal velocity" means forces are balanced
- • Forgetting that drag force increases with speed
Example Question:
Q: A skydiver falls at constant velocity. What is the resultant force?
WRONG
Downward force (weight > drag)
CORRECT
Zero (weight = drag)
Why: Constant velocity means zero acceleration. F = ma, so if a = 0, then F = 0. The forces are balanced.
Remember:
- ✓ Constant velocity (including rest) = zero resultant force
- ✓ Acceleration = unbalanced forces
- ✓ Terminal velocity = drag equals weight
Moments & Perpendicular Distance
Appears: 2022, 2023, 2024, 2025What Goes Wrong:
- • Using distance along the ruler instead of perpendicular distance
- • Not considering ALL forces when calculating moments
- • Forgetting that equilibrium means moments balance about ANY point
Example Question:
Q: A force acts at an angle. How do you calculate the moment?
WRONG
Force × distance along object
CORRECT
Force × perpendicular distance from pivot
Why: Moment = Force × perpendicular distance. The perpendicular distance is from the pivot to the LINE OF ACTION of the force.
Remember:
- ✓ Moment = Force × PERPENDICULAR distance
- ✓ For equilibrium: clockwise moments = anticlockwise moments
- ✓ Draw a free-body diagram to identify all forces
Density Calculations
Appears: 2022, 2023What Goes Wrong:
- • Using TOTAL volume instead of the INCREASE in volume
- • Thinking "per" means multiply (it means divide!)
- • Confusing mass and volume in the formula
Example Question:
Q: A stone is added to water. Volume rises from 50cm³ to 70cm³. Stone mass is 40g. Find density.
WRONG
40 ÷ 70 = 0.57 g/cm³ (used total volume)
CORRECT
40 ÷ 20 = 2 g/cm³ (used increase: 70-50)
Why: The stone's volume is the INCREASE in water level (20cm³), not the total volume (70cm³).
Remember:
- ✓ Density = mass ÷ volume (mass PER unit volume)
- ✓ "Per" always means DIVIDE
- ✓ Use the CHANGE in volume for displacement method
Resistance is NOT the Gradient
Appears: 2023, 2024What Goes Wrong:
- • Thinking resistance is the gradient of an I-V graph
- • Believing resistance decreases when current increases
- • Not understanding how diameter affects resistance (it's the square!)
Example Question:
Q: How do you find resistance from a V-I graph?
WRONG
R = gradient of the line
CORRECT
R = V ÷ I (at any point)
Why: R = V/I, not the gradient. For an I-V graph (I on y-axis), resistance = 1/gradient. Always use R = V/I directly.
Remember:
- ✓ R = V ÷ I (always use this formula)
- ✓ Doubling length doubles resistance
- ✓ Doubling diameter QUARTERS resistance (area ∝ d²)
Electromagnetic Induction (Lenz's Law)
Appears: 2022, 2023What Goes Wrong:
- • Not understanding that induced current OPPOSES the change
- • Thinking only about conductor/field motion, ignoring flux linkage
- • Confusing the right-hand grip rule with the motor rule
Example Question:
Q: A N-pole moves toward a coil. What pole is induced at that end?
WRONG
S-pole (attracts the magnet)
CORRECT
N-pole (repels the magnet - opposes the change)
Why: Lenz's Law: the induced current creates a magnetic field that OPPOSES the change. Approaching N-pole induces N-pole to repel it.
Remember:
- ✓ Lenz's Law: induced current OPPOSES the change
- ✓ Approaching magnet → induced pole REPELS
- ✓ Retreating magnet → induced pole ATTRACTS
Thermal Radiation (Black vs Shiny)
Appears: 2022What Goes Wrong:
- • Thinking shiny surfaces are better radiators (they're worse!)
- • Confusing radiation with conduction
- • Thinking black is a better CONDUCTOR (it's a better RADIATOR)
Example Question:
Q: Which surface is the best emitter of thermal radiation?
WRONG
Shiny silver surface
CORRECT
Dull black surface
Why: Dull, dark surfaces are good absorbers AND good emitters of radiation. Shiny surfaces are poor at both - they reflect instead.
Remember:
- ✓ Dark, matte surfaces = good absorbers AND emitters
- ✓ Shiny, light surfaces = poor absorbers AND emitters
- ✓ Good absorber = good emitter (same surface)
Electromagnets & Polarity
Appears: 2022What Goes Wrong:
- • Thinking both ends of a coil attract ALL magnets
- • Using the LEFT-hand rule for coils (that's for motors!)
- • Not determining the polarity of each end
Example Question:
Q: Current flows through a coil. What happens when a N-pole approaches each end?
WRONG
Both ends attract the N-pole
CORRECT
One end attracts (S-pole), one end repels (N-pole)
Why: Use the RIGHT-hand grip rule: thumb points to N-pole of coil when fingers curl in current direction. One end is N, other is S.
Remember:
- ✓ RIGHT-hand grip rule for coils/solenoids
- ✓ LEFT-hand rule for motors only
- ✓ A coil has TWO poles: one N, one S
Pressure: Weight not Mass
Appears: 2022What Goes Wrong:
- • Using mass instead of weight in pressure calculations
- • Forgetting to convert units (cm to m)
- • Multiplying weight by g when weight is already given
Example Question:
Q: Calculate the pressure exerted by a 600N person standing on 0.02m² area.
WRONG
600 × 10 ÷ 0.02 = 300,000 Pa (multiplied by g twice)
CORRECT
600 ÷ 0.02 = 30,000 Pa
Why: P = F/A. If weight (in N) is given, use it directly. Only multiply by g if MASS (in kg) is given.
Remember:
- ✓ P = Force ÷ Area = Weight ÷ Area
- ✓ Weight = mass × g (only if mass given)
- ✓ Always check units: convert cm² to m²
Radioactive Decay Changes
Appears: 2023What Goes Wrong:
- • Not realizing beta decay changes the ELEMENT
- • Confusing how alpha and beta affect atomic/mass numbers
- • Not understanding decay graphs show total atoms constant
Key Facts:
| Decay Type | Mass Number | Atomic Number |
|---|---|---|
| Alpha (α) | −4 | −2 |
| Beta (β) | 0 | +1 (new element!) |
| Gamma (γ) | 0 | 0 |
Remember:
- ✓ Alpha: loses 2 protons + 2 neutrons (helium nucleus)
- ✓ Beta: neutron → proton (atomic number +1)
- ✓ Beta decay = new element formed!
Gas Pressure & Volume
Appears: 2022, 2023What Goes Wrong:
- • Thinking high pressure = high volume (they're INVERSELY related)
- • Believing molecules get closer together when gas cools at fixed volume
- • Not understanding that cooling at fixed volume reduces pressure
Example Question:
Q: A gas is compressed. What happens to pressure?
WRONG
Pressure decreases (less space = less pressure)
CORRECT
Pressure increases (same particles, less space)
Why: P × V = constant (at constant temperature). Smaller volume = more frequent collisions with walls = higher pressure.
Remember:
- ✓ Boyle's Law: P₁V₁ = P₂V₂ (at constant T)
- ✓ Smaller volume → higher pressure
- ✓ Lower temperature → slower molecules → lower pressure
Quick Reference: The 10 Physics MCQ Killers
- 1. Constant speed: Zero resultant force
- 2. Moments: Use PERPENDICULAR distance
- 3. Density: Use INCREASE in volume
- 4. Resistance: R = V/I (not gradient)
- 5. Induction: Induced field OPPOSES change
- 6. Radiation: Dark/matte = good emitter
- 7. Electromagnets: Right-hand grip rule
- 8. Pressure: P = Weight/Area (not mass)
- 9. Beta decay: Changes the element
- 10. Gas laws: P × V = constant
Source: Cambridge IGCSE Physics 0625 Examiner Reports (2022-2024)
Analysis of 137 MCQ mistakes across 9 exam sessions.