Top 10 IGCSE Physics MCQ Mistakes
Ten multiple-choice mistakes that cost IGCSE Physics students easy marks, with the fix for each.
Why MCQ mistakes matter: Your multiple-choice paper (Paper 1 Core or Paper 2 Extended: 40 questions in 45 minutes) is worth 30% of your grade. Physics MCQs test understanding of concepts, not just recall. The traps below are in forces, electricity, magnetism, matter and radioactivity.
The Top 10 MCQ Mistakes
Steady Speed in a Straight Line = Zero Resultant Force
What the Syllabus Expects:
Understand that if an object moves at constant velocity, the resultant force on it is zero.
What Goes Wrong:
- • Thinking a car at steady speed needs a resultant forward force (its driving force only balances friction and air resistance), or that steady speed round a bend means balanced forces
- • 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: Steady speed in a straight line means no acceleration, so there is no resultant force: air resistance up equals weight down. (Extended: F = ma with a = 0 gives F = 0.)
Remember:
- ✓ Constant velocity (including rest) = zero resultant force
- ✓ Acceleration = unbalanced forces
- ✓ Terminal velocity = drag equals weight
Moments & Perpendicular Distance
What 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 20 N force pulls a door handle 0.80 m from the hinge, at an angle. The perpendicular distance from the hinge to the force's line of action is 0.60 m. What is the moment about the hinge?
WRONG
20 × 0.80 = 16 N m (distance along the door)
CORRECT
20 × 0.60 = 12 N m (perpendicular distance)
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
What 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 from a Graph: Use V ÷ I
What Goes Wrong:
- • Using the slope of a curved graph (e.g. a filament lamp) as the resistance
- • Thinking more current means less resistance: a fixed resistor's R stays the same, and a filament lamp's R goes UP as it gets hotter
- • Extended: forgetting that area depends on diameter squared
Example Question:
Q: A filament lamp's current-voltage graph is a curve. At 3.0 V the current is 0.25 A. What is its resistance at 3.0 V?
WRONG
The slope of the curve at 3.0 V
CORRECT
R = V ÷ I = 3.0 ÷ 0.25 = 12 Ω
Why: R = V ÷ I uses the two readings at that point. The gradient gives R (V-I graph) or 1/R (I-V graph) only for a straight line through the origin, such as a resistor at constant temperature.
Remember:
- ✓ R = V ÷ I (always use this formula)
- ✓ Longer wire = more resistance; thicker wire = less (Extended: doubling length doubles R)
- ✓ Extended: doubling diameter QUARTERS resistance (area ∝ d²)
Electromagnetic Induction: Direction (Extended)
What Goes Wrong:
- • Not understanding that induced current OPPOSES the change
- • Forgetting that a changing magnetic field through a coil induces an e.m.f. even when nothing moves
- • Using the left-hand (motor) rule for an induced current; that needs the right-hand (generator) rule
Example Question:
Q: A N-pole is pushed towards one end of a coil connected to a meter. What pole is induced at that end of the coil?
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)
What 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: A metal plate on a satellite must radiate energy into space as fast as possible. Which finish is best?
WRONG
Shiny silver (metals are good at transferring heat)
CORRECT
Dull (matt) black
Why: In space there is no air, so only radiation can remove energy (it needs no medium). Dull black is the best emitter of infrared; shiny surfaces are poor emitters and poor absorbers because they reflect. Being a good conductor does not help here.
Remember:
- ✓ Dark, matte surfaces = good absorbers AND emitters
- ✓ Shiny, light surfaces = poor absorbers AND emitters
- ✓ Thermal radiation is infrared; it is the only transfer that crosses a vacuum
Electromagnets & Polarity
What 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
The end that is a S-pole attracts it; the end that is a N-pole repels it
Why: Use the RIGHT-hand grip rule: thumb points to N-pole of coil when fingers curl in current direction. One end is N, the other is S. Any unmagnetised magnetic material (iron, steel, nickel or cobalt) is attracted to both ends.
Remember:
- ✓ RIGHT-hand grip rule for coils/solenoids
- ✓ LEFT-hand rule: force on a current in a magnetic field (motor effect)
- ✓ A coil has TWO poles: one N, one S
Pressure: Weight not Mass
What Goes Wrong:
- • Using mass instead of weight in pressure calculations
- • Converting cm² to m² by dividing by 100 instead of 10 000
- • Multiplying weight by g when weight is already given
Example Question:
Q: A crate of mass 40 kg stands on a floor. Its base has area 800 cm². What pressure does it exert? (g = 9.8 N/kg)
WRONG
40 ÷ 0.080 = 500 N/m² (used mass, not weight)
CORRECT
W = 40 × 9.8 = 392 N; 800 cm² = 0.080 m²; 392 ÷ 0.080 = 4900 N/m²
Why: P = F/A, and here F is the crate's weight in newtons. 1 m² = 10 000 cm², so divide cm² by 10 000. If the weight in N is given, use it directly: do not multiply by g again.
Remember:
- ✓ P = Force ÷ Area = Weight ÷ Area
- ✓ Weight = mass × g with g = 9.8 N/kg (only if mass given)
- ✓ cm² → m²: divide by 10 000
Radioactive Decay Changes
What Goes Wrong:
- • Not realising that alpha AND beta decay both change the element (gamma does not)
- • Confusing how alpha and beta affect atomic/mass numbers
- • Thinking decayed nuclei disappear: each becomes a nucleus of a new element, so as undecayed nuclei fall, daughter nuclei rise
Key Facts (Extended: decay equations):
| Decay Type | Nucleon number (A) | Proton number (Z) |
|---|---|---|
| Alpha (α) | −4 | −2 (new element!) |
| Beta (β) | 0 | +1 (new element!) |
| Gamma (γ) | 0 | 0 |
Remember:
- ✓ Alpha: loses 2 protons + 2 neutrons (helium nucleus)
- ✓ Beta (Extended): neutron → proton + electron; the electron leaves as the β-particle
- ✓ Alpha or beta = new element; gamma = same element, same numbers
Gas Pressure & Volume
What Goes Wrong:
- • Thinking a smaller volume gives a lower pressure (for a fixed mass at constant temperature, it is the opposite)
- • Believing molecules get closer together when gas cools at fixed volume
- • Not understanding that cooling at fixed volume reduces pressure
Example Question:
Q: The outlet of a bicycle pump is blocked and the handle is pushed in slowly until the trapped air has half its volume. The temperature stays the same. What happens to the pressure?
WRONG
It halves (less space = less pressure)
CORRECT
It doubles
Why: The same particles at the same speed now hit each square metre of wall twice as often, so pressure rises. Extended: pV = constant for a fixed mass at constant temperature, so p × ½V needs 2p.
Remember:
- ✓ Extended: p₁V₁ = p₂V₂ (fixed mass, constant temperature)
- ✓ Smaller volume → higher pressure
- ✓ At constant volume: lower temperature → slower particles → fewer, gentler hits → lower pressure
Quick Reference: The 10 Physics MCQ Killers
- 1. Constant velocity: Zero resultant force
- 2. Moments: Use PERPENDICULAR distance
- 3. Density: Use INCREASE in volume
- 4. Resistance: R = V/I at the point (not the slope of a curve)
- 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. Alpha and beta decay: Both change the element
- 10. Gas laws: Smaller volume = higher pressure (Extended: pV = constant)
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