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

Common mistakes and how to avoid them

1. Half-life

   MistakeYou halve a time read from the graph instead of finding when the count rate halves, or divide the starting amount by the number of half-lives.

   Fix

   Halve once for each half-life: 80 g becomes 40, 20, then 10 g after three.

   On a decay curve, find how long the count rate takes to halve.

2. Magnetic field direction

   MistakeYou know the shape of the magnetic field around a bar magnet or a straight wire, but choose arrows pointing the wrong way.

   Fix

   Outside a magnet, field lines go from N to S; a compass's N end points along them.

   For a wire: right thumb along the current, curled fingers show the field.

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

   Stone's volume = rise in water level: 60 cm³ − 40 cm³ = 20 cm³.

   Liquid's mass = full container − empty.

   Use a balance for mass, and a ruler only for regular blocks.

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, or staying at rest, means the forces are balanced and the resultant force is zero.

   A resultant force changes speed or direction.

5. Period and frequency

   MistakeYou divide the total time by the number of oscillations and call the result the frequency. That gives the period, the time for one oscillation.

   Fix

   Period = total time ÷ number of oscillations, in seconds.

   Frequency = number of oscillations ÷ time, in hertz.

   Check which one the question wants before choosing.

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 the amount of matter, in kg.

   Weight is the gravitational force on it, in N.

   Link them with W = mg, using the value of g given.

7. Speed-time graphs

   MistakeWhen the speed is changing, you multiply one speed by the total time to get the distance, and pick the option that matches.

   Fix

   First check the axes show speed against time.

   Distance is the area under the line: split it into rectangles and triangles, using only the time interval asked about.

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 the units the answer needs first: for m/s, use metres and seconds.

   Milli means a thousandth, kilo a thousand, mega a million.

   Wrong options often use unconverted numbers.

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 0.20 kg ball hits a wall at 5.0 m/s, rebounds at 3.0 m/s.

   Δv = 3.0 − (−5.0) = 8.0 m/s, not 2.0; momentum change 1.6 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, then halve.

   Reading 420 counts/min, background 20: the source gives 400, so after one half-life the detector shows 200 + 20 = 220, not 210.

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.

   Double the diameter: area × 4, so R ÷ 4.

   Double both length and diameter: R × 2 ÷ 4, which halves it.

4. Terminal velocity

   MistakeYou think an object falling at terminal velocity is still speeding up, because its weight is still acting on it.

   Fix

   Air resistance grows with speed until it equals the weight.

   Then the resultant force is zero, so there is no acceleration and the speed stays constant.

5. Impulse and momentum

   MistakeYou divide the change in momentum by the time and call it impulse, or assume kinetic energy is conserved in a collision just because momentum is.

   Fix

   Impulse = force × time = change in momentum, in N s.

   Change in momentum ÷ time is the resultant force.

   With no external force, momentum is conserved; kinetic energy usually is not.

6. Electromagnetic induction

   MistakeYou choose an option where the induced current helps the movement, such as a coil attracting a magnet that is being pushed into it.

   Fix

   In a complete circuit, the induced current opposes the change causing it.

   A magnet pushed in meets a like pole and is repelled; a magnet pulled out is attracted back.

7. Refraction angles

   MistakeYou use the angle between the ray and the surface, or divide one angle by the other, when finding a refractive index.

   Fix

   Measure both angles from the normal.

   n = sin(angle in air) ÷ sin(angle in the material), even when light leaves the block.

   For speeds: n = speed in air ÷ speed in the material.

8. Moments

   MistakeYou compare the forces on each side of a pivot without their distances, or use a distance that is not perpendicular to the force.

   Fix

   Moment = force × perpendicular distance from the pivot to the force's line of action.

   In equilibrium, total clockwise moment equals total anticlockwise moment, about any point.

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

Common mistakes and how to avoid them

1. Resistance and power

   MistakeYou multiply voltage by current to find resistance, divide one by the other to find power, or use V = IR when the question asks for power.

   Fix

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

   A lamp on 6.0 V taking 0.50 A has R = 12 Ω and P = 3.0 W.

2. Moments and work done

   MistakeYou divide force by distance to find a moment or the work done, use a distance not measured from the pivot, or give a moment's unit as N/m.

   Fix

   Both multiply.

   Moment = force × perpendicular distance from the pivot: 20 N at 0.5 m gives 10 N m.

   Work done = force × distance moved in the force's direction, in joules.

3. Nucleon and proton numbers

   MistakeIn nuclide notation you mix up the two numbers: you give the top number as the neutron count, swap top and bottom, or add them together.

   Fix

   Top number = nucleons (protons + neutrons).

   Bottom number = protons.

   Neutrons = top − bottom: uranium with 238 on top and 92 below has 92 protons and 146 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 your values, then give the answer with its unit.

   Show one method only.

   A correct method can still earn marks after an arithmetic slip.

5. Describing motion

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

   Fix

   Name the quantity: 'speed increases', 'constant speed' or 'at rest'.

   On a speed-time graph, a flat line means constant speed, or at rest if it lies on the time axis.

6. Thermal radiation

   MistakeWhen a question asks about emitting infrared, you answer about absorbing it, or you cannot say why a shiny surface reduces energy loss.

   Fix

   Dull black surfaces are good absorbers and good emitters of infrared.

   Shiny surfaces are poor at both, so a shiny surface cuts energy lost or gained by radiation.

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, such as 'releases no carbon dioxide while generating electricity' or 'renewable, so it will not run out'.

   Be just as specific about drawbacks.

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 container walls.

   Heating at fixed volume: they move faster, hitting 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, add the alpha particle's numbers instead of subtracting them, or give the product of beta decay the same proton number.

   Fix

   Top = protons + neutrons; bottom = protons (−1 for an electron); both rows balance.

   Alpha: top −4, bottom −2.

   Beta: a neutron becomes a proton plus an electron: bottom +1, top unchanged.

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: 2 kΩ and 4 kΩ across 12 V take 4 V and 8 V.

   Brighter light: lower LDR resistance, smaller share.

3. Gas pressure explanations

   MistakeYou write 'more collisions' instead of more collisions per second, leave out that each hit exerts more force in a hotter gas, or never mention force per unit area.

   Fix

   Hotter gas, fixed volume: faster particles hit the walls more often and with more force, so force per unit area rises.

   Smaller volume, same temperature: same speed, more frequent hits.

4. Final answers

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

   Fix

   Keep every digit until the end, then give two or three significant figures to match the data.

   Always add the unit, such as kg m/s for momentum.

5. Key definitions

   MistakeYour definitions are loose, such as a kilowatt-hour as 'kilowatts per hour', or a spring rule stated with 'length' instead of 'extension'.

   Fix

   Learn definitions word for word.

   A kilowatt-hour is a unit of energy: 1 kW for 1 hour.

   Extension, not length, is proportional to load, up to the limit of proportionality.

6. Energy stores

   MistakeYou write 'potential energy' or 'electrical energy' when asked to name an energy store.

   Fix

   Name the store in full: kinetic, chemical, gravitational potential, elastic (strain), electrostatic, nuclear or internal (thermal).

   An electric current transfers energy between stores; it is not a 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.

   In metals, free electrons also move through the metal carrying energy, which is why metals conduct so well.

8. Fission and fusion

   MistakeYou mix up fission and fusion, or write that the whole atom splits.

   Fix

   Fission: a large nucleus absorbs a neutron, splits into two smaller nuclei and releases neutrons.

   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: °C ÷ s gives °C/s, s × s gives s², Ω ÷ cm gives Ω/cm.

   A length divided by a length has no unit.

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

   Draw one thin straight line or smooth curve with a sharp pencil, with points balanced on both sides along its length.

   Say which point you ignore, and why.

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

   Make the statement fit your results, then quote values naming both quantities, such as 'resistance rose from 2.1 Ω at 20.0 cm to 8.3 Ω at 80.0 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 each reading to the smallest scale division, or the nearest half division if it falls between marks.

   Keep decimal places equal down a column; units go in headings.

5. Sources of uncertainty

   MistakeYou write 'human error', or 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 this experiment, such as judging the exact moment something happens or a reading that keeps changing, and say which measurement it affects.

6. Cooling rates

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

   Fix

   Compare the temperature fall over the same time interval, from the same starting temperature.

   A hotter object cools faster, so unequal starting temperatures make the test unfair.

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 measuring instrument.

   State what you change, what you measure and what you keep the same.

   Draw a table with a column, and unit, for each quantity.

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

   Make each large (2 cm) square worth 1, 2 or 5 units (or 10, 20, 50), with points spread over half the grid both ways.

   Label axes as quantity / unit.

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 otherwise, axes need not start at zero.

   Make each large square 1, 2 or 5 units (or 10, 20, 50), with points over half the grid both ways.

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 exactly what repeats: 'repeat for at least five lengths, such as 20, 40, 60, 80 and 100 cm, taking each reading twice and finding the average'.

3. Significant figures

   MistakeYour calculated values have different significant figures down one column, often from giving them all the same decimal places, or you chop off digits instead of rounding.

   Fix

   Give calculated values the same significant figures, usually 2 or 3 to match the readings.

   Keep full values until the end, then round: 0.0467 to 2 s.f. is 0.047.

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.

   Where you can, measure several identical objects together, or time many oscillations, then divide by how many there are.

5. Sources of uncertainty

   MistakeYou list careless mistakes as errors, or give a general difficulty that does not fit the object or apparatus in the question.

   Fix

   Describe a difficulty built into the method, tied to the object you are measuring, such as seeing exactly where a rounded edge meets the ruler.

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

   Space the pins at least 5 cm apart.

   Then draw one straight line through both pin marks with a sharp pencil and a ruler.

7. Describing technique

   MistakeYou write 'make sure it is level' or 'find the balance point' without saying how you would actually check it.

   Fix

   Give the check: measure a metre rule's height above the bench at each end and make them equal, or nudge the pivot until the rule stays level unaided.

8. Circuit symbols

   MistakeYou draw component symbols with parts missing, such as the arrow on a variable resistor, or leave gaps where wires should join.

   Fix

   Learn each standard symbol exactly: a variable resistor is a resistor rectangle with a diagonal arrow through it.

   Draw wires with a ruler and close every connection.