Cambridge IGCSE Physics (0625) — Past Papers, Revision Notes & Exam Prep

Common mistakes and how to avoid them

1. Half-life

   MistakeYou take half the longest time on the graph's time axis instead of finding when the count rate halves, or divide the starting amount by the number of half-lives.

   Fix

   On a curve, half-life = time for 740 counts/min to fall to 370, not half the time axis.

   Three half-lives: 312 g → 156 → 78 → 39 g, not 312 ÷ 3 = 104 g.

2. Magnetic field direction

   MistakeYou choose field arrows pointing the wrong way around a bar magnet or around a straight wire carrying a current.

   Fix

   Outside a magnet, field lines run N to S.

   Beside a magnet's middle, a compass points to the S pole.

   Current into the page: right thumb in, fingers curl clockwise.

3. Density measurements

   MistakeYou use one reading where a difference is needed, such as the final water level as the stone's volume, or choose the wrong measuring instrument.

   Fix

   Volume by displacement = rise in level: 45 to 59 cm³ is 14 cm³, not 59 cm³.

   Mass needs a balance; a ruler works only for a regular block's volume.

4. Balanced forces

   MistakeYou assume something moving at a steady speed needs a resultant force to keep it going, so you choose an option with unbalanced forces.

   Fix

   Steady speed in a straight line: forces balanced, resultant zero.

   A scooter at steady speed on level ground: driving force 60 N, friction plus drag 60 N, not less.

5. Period and frequency

   MistakeYou give the time for many oscillations as the period, forgetting to divide by the number of oscillations, or mix up period and frequency.

   Fix

   Period = time for one oscillation.

   Frequency = oscillations per second.

   30 oscillations take 24 s: period = 24 ÷ 30 = 0.80 s, not 24 s.

   Frequency = 30 ÷ 24 = 1.25 Hz, not 0.80 Hz.

6. Mass and weight

   MistakeYou treat mass and weight as the same quantity, so you accept an option giving weight in kilograms or mass in newtons.

   Fix

   Mass is in kg; weight is a force, in N, and W = mg.

   A 4.0 kg bag has weight 4.0 × 9.8 = 39.2 N, not 39.2 kg or 4.0 N.

7. Speed-time graphs

   MistakeWhen the speed is changing, you multiply one speed by the total time, or leave out the ½ for a triangle, and pick the option that matches.

   Fix

   Distance = area under a speed-time graph.

   Speed rising steadily from 0 to 24 m/s over 7.0 s: ½ × 7.0 × 24 = 84 m, not 24 × 7.0 = 168 m.

8. Units and prefixes

   MistakeYou put numbers into an equation without converting them, such as minutes instead of seconds or mm instead of m, and pick the option matching that answer.

   Fix

   Convert to metres and seconds first.

   5.4 km in 6.0 minutes: speed = 5400 m ÷ 360 s = 15 m/s, not 5.4 ÷ 6.0 = 0.90.

   Kilo means × 1000; milli means ÷ 1000.

More multiple-choice traps: Top 10 IGCSE Physics MCQ Mistakes

Common mistakes and how to avoid them

1. Rebounds and momentum

   MistakeWhen something bounces back, you subtract one speed from the other to get the change in velocity or momentum, forgetting that the direction has reversed.

   Fix

   Rebound direction positive: a 0.60 kg trolley hits a buffer at 0.70 m/s, rebounds at 0.50 m/s.

   Δv = 0.50 − (−0.70) = 1.2 m/s, not 0.20.

   Δp = 0.60 × 1.2 = 0.72 kg m/s.

2. Half-life and background

   MistakeYou halve the whole detector reading without taking off background first, or forget to add background back when asked what the detector will show.

   Fix

   Subtract background, halve, then add it back for the detector reading.

   Reading 616 counts/min, background 16: source 600.

   Detector after one half-life: 300 + 16 = 316, not 308 or 300.

3. Wire thickness and resistance

   MistakeYou treat resistance as inversely proportional to diameter rather than to area, or lose track when length and thickness change together, so you pick the wrong factor.

   Fix

   R ∝ length ÷ area, and area ∝ diameter².

   Diameter × 3: area × 9, so R ÷ 9, not ÷ 3.

   Length × 3 and diameter × 2: R × 3 ÷ 4 = 0.75 R, not 1.5 R.

4. Terminal velocity

   MistakeYou think something falling at terminal velocity still has a resultant downward force on it, so it keeps speeding up.

   Fix

   Weight still acts, but air resistance has grown to equal it.

   A hailstone weighing 0.040 N meets 0.040 N of air resistance: resultant 0, so constant speed.

5. Impulse and momentum

   MistakeYou divide the change in momentum by the time and call the result the impulse, or define impulse as force per unit time.

   Fix

   Impulse = force × time = momentum change.

   Momentum changes by 18 kg m/s in 0.40 s: impulse = 18 N s.

   18 ÷ 0.40 = 45 N is the resultant force, not impulse.

6. Electromagnetic induction

   MistakeYou get induced poles or forces the wrong way round because you do not apply the rule that an induced current opposes the change that causes it.

   Fix

   Induced current opposes whatever change produces it.

   A magnet pushed into a coil is repelled, not attracted.

   S pole pulled out: the coil's near end becomes N, attracting it back.

7. Refraction angles

   MistakeYou use the angle between the ray and the surface instead of the normal, or put the angles themselves into the formula instead of their sines.

   Fix

   Measure from the normal and use sines.

   Ray at 24° to the surface: angle of incidence 66°.

   Refraction angle 39°: n = sin 66° ÷ sin 39° = 1.45, not 66 ÷ 39 = 1.69.

8. Moments

   MistakeYou multiply a force by a distance that is not perpendicular to its line of action, such as a length measured along a sloping arm.

   Fix

   Moment = force × perpendicular distance.

   A 70 N load hangs 0.50 m along a sloping arm, 0.30 m horizontally from the pivot.

   70 × 0.30 = 21 N m, not 70 × 0.50.

More multiple-choice traps: Top 10 IGCSE Physics MCQ Mistakes

Common mistakes and how to avoid them

1. Resistance and power

   MistakeYou find resistance by multiplying voltage by current or dividing current by voltage, or find power by dividing voltage by current or using V = IR.

   Fix

   Resistance divides, power multiplies: R = V ÷ I and P = I × V.

   A 30 V heating pad takes 0.12 A.

   R = 30 ÷ 0.12 = 250 Ω.

   P = 0.12 × 30 = 3.6 W.

2. Moments and work done

   MistakeYou divide force by distance to find a moment or the work done, multiply a force by the other force's distance, or give a moment's unit as N/m.

   Fix

   Both multiply, never divide.

   Moment = 40 N × 0.30 m (its perpendicular distance from the pivot) = 12 N m, not N/m.

   Work done = 18 N × 4.0 m (distance moved) = 72 J.

3. Nucleon and proton numbers

   MistakeIn nuclide notation you mix up the numbers: you confuse the top number with the neutron count, or swap the top and bottom numbers.

   Fix

   Top number = nucleons (protons + neutrons).

   Bottom number = protons.

   Neutrons = top − bottom.

   Nickel with 63 on top and 28 below: 28 protons, 63 − 28 = 35 neutrons.

4. Showing working

   MistakeYou write only a final number. If it is wrong, nothing on the page shows that you knew the method.

   Fix

   Write the equation, substitute, then give the answer and unit.

   Speed = distance ÷ time = 98 m ÷ 7.0 s = 14 m/s.

   A slip to 1.4 m/s can still earn marks for working.

5. Describing motion

   MistakeYou say the motion 'increases' or 'is constant' without naming what is increasing or constant.

   Fix

   Name the quantity: speed, not 'the motion'.

   Speed-time line rising: 'speed increases'.

   Line flat at 5.5 m/s: 'constant speed of 5.5 m/s'.

   Line along the time axis: 'at rest'.

6. Thermal radiation

   MistakeYou mix up which surfaces are good emitters of infrared, or cannot explain how a shiny surface reduces energy loss.

   Fix

   Dull black surfaces are good emitters and absorbers of infrared.

   Shiny surfaces are poor at both.

   A shiny teapot emits less infrared, so the tea stays hot longer.

7. Energy resources

   MistakeYou call an energy resource 'clean', 'green' or 'eco-friendly' without saying exactly what makes it better.

   Fix

   Give a specific reason, not 'clean' or 'green'.

   Solar cells: 'release no carbon dioxide while generating electricity'.

   Be as specific about drawbacks: 'output falls on cloudy days'.

8. Gas pressure

   MistakeYou say a gas's pressure changes but leave out the particles, or forget that they collide with the container walls.

   Fix

   Pressure comes from particles hitting the walls.

   Heated, same volume: particles move faster, hitting the walls harder and more often.

   Smaller volume, same temperature: more frequent hits.

   Both raise pressure.

Common mistakes and how to avoid them

1. Decay equations

   MistakeYou put the neutron number on top or swap the two numbers, add the alpha particle's numbers instead of subtracting, or keep the same proton number after beta decay.

   Fix

   Top = nucleons, bottom = protons.

   Both sides must balance.

   Alpha from gadolinium-152 (64 protons): 152 − 4 = 148, 64 − 2 = 62.

   Beta from potassium-40 (19 protons): 40 stays, 19 + 1 = 20, calcium-40.

2. Potential dividers

   MistakeYou put the whole supply p.d. across one resistor, or get a sensor backwards, saying the p.d. across an LDR rises in brighter light.

   Fix

   In series, p.d. is shared in proportion to resistance.

   1.5 kΩ and 3.5 kΩ across 10 V take 3.0 V and 7.0 V.

   Brighter light: lower LDR resistance, smaller share.

3. Gas pressure explanations

   MistakeYou write 'more collisions' instead of more collisions per second, or leave out that each hit exerts more force in a hotter gas.

   Fix

   Pressure is force per unit area from particles hitting the walls.

   Sun-warmed football: faster particles make more collisions per second, each with more force.

   So pressure rises.

4. Final answers

   MistakeYou round in the middle of a calculation, give the answer to one significant figure, or leave off the unit.

   Fix

   Round only at the end: 2 or 3 significant figures, plus the unit.

   KE = ½ × 3.0 kg × (2.333… m/s)² = 8.2 J.

   Rounding the speed to 2.3 m/s first gives 7.9 J.

5. Key definitions

   MistakeYour definitions are loose, such as a kilowatt-hour with no mention of energy, or you use a spring's length where its extension is needed.

   Fix

   Learn definitions word for word.

   Kilowatt-hour (3.6 MJ): the energy transferred at 1 kW for 1 hour.

   Up to the limit of proportionality, extension (not length) is proportional to load.

6. Energy stores

   MistakeYou name an energy store vaguely or wrongly, such as 'potential energy' or 'electrical energy'.

   Fix

   Name the store in full.

   A stretched catapult: elastic (strain) store, not 'potential energy'.

   There is no electrical store.

   A toaster: energy is transferred electrically to its internal (thermal) store.

7. Conduction in solids

   MistakeTo explain conduction in a metal, you mention only vibrating particles, or bring in convection, which cannot happen in a solid.

   Fix

   In all solids, vibrating particles pass energy to their neighbours.

   Steel poker in a fire: free electrons also carry energy through the metal to the handle.

8. Fission and fusion

   MistakeYou mix up fission and fusion, or write that atoms split or fuse when it is their nuclei.

   Fix

   Fission: a heavy nucleus absorbs a neutron and splits.

   Pu-239 + n → Te-135 + Mo-102 + 3n.

   Fusion: light nuclei join.

   Both lose a little mass, releasing energy.

   Say 'nucleus', not 'atom'.

Common mistakes and how to avoid them

1. Units of calculated values

   MistakeYou leave the unit off a value you worked out, or guess it, such as s for T² instead of s², or cm for a magnification.

   Fix

   Build the unit from the working.

   T = 1.40 s gives T² = 1.96 s².

   32 °C ÷ 400 s = 0.080 °C/s.

   Length ÷ length has no unit: 9.6 cm ÷ 3.2 cm = 3.0.

2. Drawing best-fit lines

   MistakeYou join the points dot to dot, draw a thick line, or rule a straight line through points that clearly bend into a curve.

   Fix

   One thin pencil line, never dot to dot.

   If points clearly bend, draw a smooth curve.

   With 6 points, leave about 3 above and 3 below, spread along the line.

3. Backing up conclusions

   MistakeYou state a conclusion with no numbers from your own table, or write what theory predicts when your results actually show something different.

   Fix

   Write what your results show, not what theory says.

   Quote values from your table.

   'Wider parachutes took longer to fall: 1.2 s at 20 cm, 2.0 s at 40 cm'.

4. Recording readings

   MistakeYou record readings less precisely than the instrument allows, such as to the nearest cm when the ruler reads to the nearest mm.

   Fix

   Record to the smallest step the instrument shows.

   Ruler marked in mm: 42.0 cm, not 42 cm.

   Meter shows 0.70 A: write 0.70 A, not 0.7 A.

5. Sources of uncertainty

   MistakeYou name a slip you could avoid by working carefully, such as misreading a scale, instead of a difficulty that stays however careful you are.

   Fix

   Name a difficulty built into the experiment, not a careless slip.

   Say which reading the difficulty affects.

   'Hard to judge the instant the parachute lands, so fall time is uncertain'.

6. Cooling rates

   MistakeYou compare how fast things cool by looking only at their final temperatures.

   Fix

   Compare temperature falls, not final readings.

   'In 200 s, A's temperature fell 17 °C, B's 11 °C: B cooled more slowly'.

   Start both equally hot: hotter water cools faster.

7. Planning an investigation

   MistakeYour plan leaves out obvious apparatus, lets a variable change that should stay fixed, or has a table that cannot hold every reading.

   Fix

   List every instrument, fixed variable and table column.

   A parachute test needs a stopwatch and metre rule.

   Keep drop height and load the same.

   Columns: width / cm, fall time / s.

8. Graph scales

   MistakeYou pick an awkward scale, such as 3 per square, or one so small that your points cover less than half the grid across or up.

   Fix

   Per large square: 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50.

   Largest reading 2.6 N, 6-square axis: 0.5 N per square.

   1 N per square fills under half.

Common mistakes and how to avoid them

1. Choosing graph scales

   MistakeYou start both axes at zero when your readings are far from zero, so points cover less than half the grid, or count in awkward steps like 3 or 7.

   Fix

   Unless told, start axes near your data.

   2 cm = 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50.

   Readings 46 to 78, 10 cm axis: from 40, 2 cm = 10.

2. Repeating in plans

   MistakeYou end a plan with just 'repeat the experiment', without saying what changes each time or how many readings you take.

   Fix

   Say what changes and how many values you use.

   'Repeat for at least five masses: 50, 100, 150, 200 and 250 g'.

   'Take each reading twice and use the average'.

3. Significant figures

   MistakeYou write calculated values in one column with neither the same significant figures nor the same decimal places, or you round wrongly, for example by chopping off digits.

   Fix

   Give calculated values in a column the same significant figures.

   3 s.f.: 0.857, 1.46, 11.6, not 0.9, 1.4583, 11.63.

   Round, don't chop: 1.4583 becomes 1.46, not 1.45.

4. Small measurements

   MistakeYou measure a small length only to the nearest cm, so the reading error is a large fraction of the length.

   Fix

   Read to the nearest mm: 3.7 cm, not 4 cm.

   Better, measure many identical objects together and divide.

   10 beads in a row: 7.4 cm, so one is 0.74 cm.

5. Sources of uncertainty

   MistakeYou give a careless slip, such as reading a scale wrongly, as a source of uncertainty, rather than a difficulty that stays even with careful work.

   Fix

   Name a difficulty that stays even with careful work.

   Not 'misreading the ruler'.

   Instead: 'the sponge squashes as the ruler touches it, so its thickness is hard to judge'.

6. Ray tracing

   MistakeYou place the two pins marking a ray close together, so a small error in either pin swings the direction of your line a lot.

   Fix

   Place the pins well over 5 cm apart.

   A pin 1 mm off turns the line by under 1° with pins 8 cm apart, but nearly 6° at 1 cm.

7. Describing technique

   MistakeYou say how you would check something is level, such as 'measure its height above the bench', but not which readings would show it is level.

   Fix

   Say the readings that prove it.

   Measure the plank's height above the bench at each end.

   Equal heights, such as 12.5 cm and 12.5 cm, mean it is level.

8. Circuit symbols

   MistakeAsked for a variable resistor, you draw a thermistor or a mix of the two, or you leave gaps where wires should join.

   Fix

   Draw symbols exactly.

   Variable resistor: rectangle crossed by a diagonal arrow.

   Thermistor: rectangle crossed by a diagonal line with a flat foot, no arrow.

   Leave no gaps where wires meet.