How to Ace IGCSE Physics Paper 3: Theory (Core)

The Paper Most Students Underestimate

Here's something that might surprise you: the students who struggle most on IGCSE Physics Paper 3 aren't the ones who don't understand the physics. They're the ones who understand it perfectly well but can't translate that understanding into marks on paper.

Sound familiar? You can explain refraction to your friends over lunch. You know exactly why a parachutist reaches terminal velocity. But when you sit down in the exam hall and read "Explain why the ball eventually reaches a constant speed," something goes wrong between your brain and the answer booklet.

That gap between knowing physics and writing physics answers is exactly what Paper 3 tests. And the good news? It's a gap you can close with the right technique.

What Paper 3 Actually Looks Like

Paper 3 is the Theory (Core) paper for Cambridge IGCSE Physics (0625). Here are the numbers you need to know:

• Duration: 1 hour 15 minutes (75 minutes total)
• Total marks: 80
• Question type: Structured questions with short and longer written answers
• Calculator: Yes, you can use one
• Weighting: 50% of your final grade if you're a core candidate

That 80 marks in 75 minutes works out to roughly one mark per minute, with a few minutes of reading time built in. Keep that ratio in your head. It will save you later.

The Command Word Cheat Sheet

Every question on Paper 3 starts with a command word, and each one tells you exactly how much detail the examiner expects. Miss this signal and you'll either write too much (wasting time) or too little (dropping marks).

The biggest pitfall? Treating "describe" and "explain" as the same thing. They're not. "Describe what happens to the temperature" wants observations: "The temperature rises quickly at first, then levels off." "Explain why the temperature levels off" wants physics: "The rate of heat loss to the surroundings equals the rate of heat input, so the thermal energy of the water stops increasing."

Mixing these up is one of the fastest ways to lose marks on Paper 3.

Time Allocation: Your 75-Minute Game Plan

Most students don't run out of knowledge on Paper 3. They run out of time. Here's a practical breakdown that keeps you moving:

1. First 3 minutes: Read through the entire paper. Yes, the whole thing. Circle any questions you immediately feel confident about. Star any that look tricky. This small investment pays for itself by preventing the panic of discovering a 6-mark question on the last page with two minutes left.
2. Minutes 3 to 68: Work through the questions roughly in order. Spend no more than one minute per mark. A 2-mark question gets two minutes. A 6-mark question gets six. If you're stuck after the allocated time, leave a gap and move on.
3. Minutes 68 to 75: Return to any questions you skipped. Check your calculations for arithmetic errors. Make sure every "calculate" answer has units.

Here's the counterintuitive bit: spending 8 minutes perfecting a 3-mark answer and then rushing a 6-mark answer is worse than giving both answers a solid attempt. The marks don't care how polished your prose is. They care whether you hit the marking points.

How to Write Answers That Actually Score

The mark scheme for Paper 3 is built on "marking points." Each mark corresponds to a specific piece of physics the examiner is looking for. Your job isn't to write a beautiful paragraph. Your job is to hit those points, clearly and efficiently.

For "State" and "Define" questions (1-2 marks)

Keep it tight. One sentence maximum.

For "Describe" questions (2-4 marks)

Think chronologically. What happens first? Then what? Use words like "initially," "then," "as... increases," "eventually."

Don't explain WHY unless the question asks you to. If it says "Describe the motion of the ball after it is released," just tell the story of what happens to the ball. Speed increases, it bounces, speed decreases between bounces. That's it.

For "Explain" questions (2-4 marks)

This is where most marks are lost on the whole paper. An explanation needs a CAUSE and an EFFECT linked by physics. Think of it as a chain:

• Physical cause (what's happening at a fundamental level)
• Physics principle or law that connects cause to effect
• Observable consequence

See the difference? The weak answer states a fact. The strong answer builds a chain of reasoning. Examiners call this a "developed explanation," and it's the single most valuable skill you can bring into Paper 3.

For "Calculate" questions (2-4 marks)

There's a formula for calculations, and it's not a physics one. It's the routine that protects you from losing marks:

1. Write down the equation you're using
2. Substitute the values (with correct units)
3. Show the arithmetic
4. Write the final answer with the correct unit

Even if you get the final number wrong, you can still pick up method marks for steps 1-3. Students who jump straight to the answer and make an arithmetic error get zero. Students who show their working might lose only the final accuracy mark. That's the difference between 0/3 and 2/3 on a single question.

And units matter more than you think. "450" is not an answer. "450 J" is.

The Five Topics That Keep Coming Back

Paper 3 draws from the entire core syllabus, but certain areas appear with real consistency. If you're short on revision time, these are your priorities:

• Energy transfers and efficiency: Sankey diagrams, calculating efficiency, identifying useful versus wasted energy. You can almost guarantee at least one question here.
• Forces and motion: Speed-time and distance-time graphs, calculating acceleration, applying Newton's laws to everyday situations like falling objects, cars braking, and terminal velocity.
• Thermal physics: Specific heat capacity calculations, changes of state, conduction versus convection versus radiation comparisons.
• Waves: The wave equation (v = f x wavelength), properties of light including reflection and refraction, and sound wave characteristics.
• Electricity basics: Ohm's law calculations, series and parallel circuit rules, choosing the right fuse rating.

This doesn't mean you should skip other topics entirely. It means that if you're revising the night before (we've all been there), these five areas give you the most coverage per hour of study.

Describe vs Explain: A Side-by-Side Walkthrough

This distinction trips up so many IGCSE students that it deserves its own worked example. Consider this scenario:

A student drops a ball from a height. The ball bounces several times, each bounce lower than the last, until it eventually stops.

"Describe the motion of the ball." (3 marks)

The ball accelerates downwards after being released. It hits the ground and bounces back upwards, but reaches a lower height than before. Each subsequent bounce reaches a lower height until the ball comes to rest.

Notice: no physics concepts, no equations, no "because." Just what happens.

"Explain why each bounce is lower than the previous one." (3 marks)

At each bounce, some kinetic energy is transferred to thermal energy and sound energy in the ball and the ground. The total energy of the ball decreases after every impact. With less kinetic energy at each bounce, the ball cannot convert as much energy to gravitational potential energy, so it reaches a lower height.

Every sentence in the explanation contains a physics reason. Cause, mechanism, consequence. "Describe" is a camera recording what happens. "Explain" is the physicist telling you why.

Common Traps and How to Dodge Them

The Precision Language Problem

Paper 3 is a theory paper, which means your ability to use correct physics vocabulary directly affects your marks. The examiner is checking for specific terms, and casual language won't cut it.

Some swaps you should make automatic:

• "Heat rises" becomes "less dense warm air rises by convection"
• "The object slows down" becomes "the resultant force acts opposite to the direction of motion, causing deceleration"
• "The light bends" becomes "the light refracts as it passes from one medium to another"
• "Electricity flows" becomes "current flows" or "charge flows through the conductor"
• "The waves bounce off" becomes "the waves are reflected"

Each of these swaps could be worth a mark. Over an 80-mark paper, precise language can easily add 5 to 8 marks to your total. That's the difference between a C and a B for some students.

A Full Worked Question from Start to Finish

Let's walk through a typical IGCSE Paper 3 structured question to show these techniques working together.

Question: A car of mass 1200 kg is travelling at 20 m/s. The driver applies the brakes and the car comes to rest in 8 seconds.

(a) Calculate the deceleration of the car. [2 marks]

(b) Calculate the braking force. [2 marks]

(c) Explain why the braking distance would be longer on a wet road. [2 marks]

Notice how part (c) uses the cause-mechanism-consequence chain. Two marks, two linked physics ideas. Clean and efficient.

Graphs: The Secret Mark Goldmine

Paper 3 frequently asks you to draw, read, or interpret graphs. These questions are often worth 3-4 marks, and students either ace them or bomb them. There's rarely a middle ground.

For drawing graphs:

• Label both axes with quantity AND unit (e.g., "Time / s" not just "Time")
• Use a sharp pencil and ruler for straight lines
• Plot points with small, neat crosses, not big blobs
• Draw a smooth best-fit line or curve. Don't connect the dots like a zigzag

For reading graphs:

• Use a ruler to draw construction lines from the axis to the curve and back
• Read the scale carefully. If each small square represents 2 units, not 1, you'll get the wrong answer
• The gradient of a distance-time graph gives speed. The gradient of a speed-time graph gives acceleration. The area under a speed-time graph gives distance. These three facts unlock a huge number of marks

Quick self-test: if a speed-time graph shows a horizontal line at 15 m/s for 20 seconds, what distance has the object travelled? If you said 300 m (area = 15 x 20), you're ready for these questions.

The Equation Sheet: Friend or Foe?

You'll get a list of equations at the front of the paper. That's helpful, obviously. But here's what students don't always realise: knowing the equations isn't the same as knowing when to use them.

The exam won't say "use E = mc delta-theta to find the answer." It'll say something like "Calculate the thermal energy needed to raise the temperature of 2 kg of water from 20 degrees C to 80 degrees C." You need to recognise that this is a specific heat capacity problem, find the right equation, identify the values (m = 2 kg, c = 4200 J/kg degrees C for water, delta-theta = 60 degrees C), substitute, and solve.

Practise this translation skill by doing past papers without looking at the equation sheet first. Try to recall the equation from the context of the question. Then check. You'll build the pattern recognition that turns a 4-mark calculation from a puzzle into a routine.

Your Pre-Exam Checklist

The week before your IGCSE Physics Paper 3, run through this list:

• Can you write all the core equations from memory? (v = s/t, F = ma, W = mg, P = F/A, E = mc delta-theta, and so on)
• Can you draw and interpret speed-time graphs, distance-time graphs, and current-voltage graphs?
• Can you explain the difference between mass and weight, heat and temperature, speed and velocity?
• Have you practised at least 3 full past papers under timed conditions?
• Can you both describe AND explain each core phenomenon? Not just one or the other.
• Do you know which equations need unit conversions? Minutes to seconds, km/h to m/s, grams to kg?

If you answered "no" to any of those, you know exactly where to spend your remaining revision time.

What to Do in the Last 5 Minutes

Those final minutes are more valuable than most students realise. Here's your checklist:

1. Every calculation has units on the final answer
2. You haven't left any questions completely blank. Write something. Anything relevant can score.
3. Your "explain" answers contain physics reasons, not just descriptions of what happens
4. Numbers make physical sense. A car doesn't weigh 0.5 kg. A temperature change isn't 50,000 degrees.

Paper 3 rewards students who think like examiners. Every mark has a corresponding marking point. Your job is straightforward: identify those points and hit them, one by one, for 75 minutes. The physics knowledge you already have is probably enough. The technique of expressing it clearly on paper is what separates the grades.