October/November 2025 Paper 33 Worked Answers (IGCSE Physics 0625 Core)

10 questions · 10 marks · 75 minutes

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1. Question 1

   1 marksDistance from a speed-time graph
   Step 1: On a speed-time graph, the distance travelled is the area below the line. For uniform acceleration from rest, this is a triangle. Step 2: Area $= \tfrac{1}{2} \times 3.4 \times 33 \approx 56\ \text{m}$.
   Method:

   Apply the triangle area formula to the speed-time graph.
   Examiner tips

   • Use the triangle formula for uniform acceleration from rest.

2. Question 2

   1 marksPower from work and time
   Step 1: Power = work done / time, so $P = W/t$. Step 2: $P = 5800 / 7.4 \approx 784\ \text{W} \approx 780\ \text{W}$.
   Method:

   Divide energy by time.
   Examiner tips

   • Watt (W) = joule per second; keep units consistent.

3. Question 3

   1 marksFloating prediction by density
   Step 1: An object floats only if its density is less than the liquid's density. Step 2: Plastic ($0.905$) is less dense than sea water ($1.030$), so it floats in sea water. Step 3: Plastic ($0.905$) is denser than baby oil ($0.815$), so it sinks in baby oil.
   Method:

   Compare the plastic's density to each liquid's density.
   Examiner tips

   • An object floats if its density is less than the surrounding liquid's density.

4. Question 4

   1 marksBoiling point from a heating curve
   Step 1: When a pure substance changes state, its temperature stays constant. Step 2: The first plateau (at $60\ ^\circ\text{C}$) is the melting point. Step 3: The second plateau (at $140\ ^\circ\text{C}$), which appears after the liquid stage, is the boiling point.
   Method:

   Identify the second flat region as the boiling point.
   Examiner tips

   • Two plateaus on a heating curve are the melting and boiling points (in that order).

5. Question 5

   1 marksWavelength of visible light
   Step 1: Wave equation $v = f\lambda$, so $\lambda = v/f$. Step 2: $\lambda = \frac{3.0 \times 10^8}{6.0 \times 10^{14}} = 0.5 \times 10^{-6} = 5.0 \times 10^{-7}\ \text{m}$.
   Method:

   Divide speed by frequency with careful power-of-ten arithmetic.
   Examiner tips

   • Always check the sign of the exponent on the final answer.

6. Question 6

   1 marksPrincipal axis of a lens
   Step 1: The line through the centre of the lens, perpendicular to the lens, is the principal axis. Step 2: Focal length is a distance along this axis, not a line. Normal applies to a surface (such as a flat mirror), not the lens as a whole. Wavefronts are surfaces of constant phase, drawn perpendicular to the rays.
   Method:

   Match the geometric description to the standard term.
   Examiner tips

   • Memorise the standard labels on a lens diagram.

7. Question 7

   1 marksResistance from V and I
   Step 1: Ohm's law: $R = V / I$. Step 2: $R = 6.0 / 0.25 = 24\ \Omega$.
   Method:

   Divide voltage by current.
   Examiner tips

   • Use the right operation: $R = V/I$, never $V \times I$.

8. Question 8

   1 marksAdvantage of high-voltage transmission
   Step 1: For a given power $P = VI$, raising $V$ lowers $I$ in the cables. Step 2: Cable power loss is $I^2 R$, which falls sharply with smaller current. Step 3: Less power is lost as heat in the cables.
   Method:

   Link high voltage to lower current and to lower $I^2R$ loss.
   Examiner tips

   • $P_\text{loss} = I^2 R$: minimise current to minimise loss.

9. Question 9

   1 marksIdentifying radiations by penetration
   Step 1: Alpha is stopped by a sheet of paper, so Q is alpha. Step 2: Beta passes paper but is stopped by a few mm of aluminium, so P is beta. Step 3: Gamma passes both paper and aluminium, so R is gamma.
   Method:

   Match each behaviour to the standard penetration rule.
   Examiner tips

   • Paper stops alpha; few mm of aluminium stop beta; gamma needs thick lead/concrete.

10. Question 10

    1 marksDay, year and month
    Step 1: A day is one spin of the Earth on its axis. Step 2: A year is one full orbit of the Earth around the Sun. Step 3: A month is one full orbit of the Moon around the Earth.
    Method:

    Pair each event with its natural unit of time.
    Examiner tips

    • Match each motion to the natural unit it defines.