Secondary 4 Combined Science Physics Composition Situational Writing Quiz

Secondary 4 Combined Science Physics Quiz - Composition Situational Writing

Answer Key

Question 1 [3]

During the first flat section, the substance is melting (changing from solid to liquid). The energy supplied is used to break the bonds between particles, not to increase the temperature. The particles gain enough energy to overcome the forces of attraction holding them in fixed positions. The arrangement changes from a fixed, regular pattern (solid) to a random, closely packed arrangement (liquid). The temperature remains constant during this change of state.

Marking notes:

• 1 mark for identifying the process as melting / change of state.
• 1 mark for stating that energy is used to break bonds / overcome intermolecular forces.
• 1 mark for describing the change in particle arrangement (fixed → random / regular → irregular).
• Common error: saying particles move faster during the flat section (temperature is constant, so average KE does not change).

Question 2

(a) [1] Refraction

(b) [2] The light ray slows down as it enters the glass (glass is optically denser than air). Because the light slows down, it bends towards the normal. This is because one side of the wavefront enters the glass first and slows down while the other side is still travelling faster in air, causing the direction to change towards the normal.

Marking notes:

• 1 mark for stating the light slows down in glass.
• 1 mark for linking the decrease in speed to bending towards the normal.
• Common error: saying light speeds up in glass.

Question 3 [3]

As the ball leaves the thrower's hand, it has maximum kinetic energy. As it rises, it slows down due to the downward gravitational force. Kinetic energy is converted to gravitational potential energy. At the maximum height, the ball's speed is momentarily zero, so all the kinetic energy has been converted to gravitational potential energy (assuming no energy is lost to air resistance).

Marking notes:

• 1 mark for stating the ball has kinetic energy at the start.
• 1 mark for stating kinetic energy is converted to gravitational potential energy.
• 1 mark for stating that at maximum height, speed is zero / KE is zero / GPE is maximum.
• Common error: saying energy is "lost" rather than "converted."

Question 4

(a) [1] The current is the same at all points in a series circuit.

(b) [2] When the resistors are connected in parallel, the total resistance decreases. This is because connecting resistors in parallel provides more paths for the current to flow, making it easier for charge to pass through the circuit. The total resistance of parallel resistors is less than the resistance of either individual resistor.

Marking notes:

• 1 mark for stating total resistance decreases.
• 1 mark for explaining that more paths are available for current / equivalent resistance is lower.
• Common error: saying total resistance increases because there are more resistors.

Question 5

(a) [1] South (S)

(b) [1] The compass needle (which is itself a small magnet) points towards the opposite pole. Since the compass needle points towards the bar magnet, the nearest end of the bar magnet must be a south pole (unlike poles attract).

Marking notes:

• Accept "S" or "South."
• 1 mark for correct explanation referencing attraction of unlike poles.
• Common error: confusing the direction the compass needle points with the polarity of the bar magnet.

Question 6

(a) [1] Between t = 4 s and t = 6 s (speed is constant at 12 m/s).

(b) [2] Between t = 6 s and t = 10 s, the car is decelerating (slowing down). The speed decreases from 12 m/s to 0 m/s over this 4-second interval. The car comes to a stop at t = 10 s.

Marking notes:

• 1 mark for identifying deceleration / slowing down.
• 1 mark for referencing data from the table (speed decreases from 12 to 0 m/s).
• Common error: saying the car is stationary between 6–10 s.

Question 7 [2]

The graph shows that resistance increases as the length of the wire increases. Since the graph is a straight line passing through the origin, resistance is directly proportional to the length of the wire. This means that if the length doubles, the resistance also doubles.

Marking notes:

• 1 mark for stating resistance increases with length.
• 1 mark for using the term "directly proportional" correctly.
• Common error: saying "inversely proportional."

Question 8

(a) [2]

Acceleration = change in velocity / change in time
= (8 − 0) / (4 − 0)
= 8 / 4
= 2 m/s²

Marking notes:

• 1 mark for correct formula or method.
• 1 mark for correct answer with unit (m/s²).

(b) [1] Between t = 8 s and t = 12 s, the cyclist is decelerating (the velocity decreases from 8 m/s to 0 m/s).

Question 9 [3]

Metal is a good thermal conductor. When the metal spoon is placed in hot water, free electrons in the metal gain kinetic energy and transfer this energy rapidly through the spoon by collisions with other particles. This means thermal energy is quickly conducted from the water to the handle of the spoon, making it feel hot. Plastic is a thermal insulator — it does not have free electrons, so thermal energy is transferred much more slowly by vibrations between particles, and the plastic spoon does not feel as hot.

Marking notes:

• 1 mark for identifying metal as a good conductor / plastic as an insulator.
• 1 mark for mentioning free electrons in metal transferring energy.
• 1 mark for explaining why plastic does not conduct heat well (no free electrons / slower transfer).
• Common error: saying "heat is trapped" in plastic without explaining the mechanism.

Question 10 [4]

Property 1: Real
Explanation: The image is formed where the refracted rays actually converge (meet) on the other side of the lens. This can be determined from the diagram because the rays cross at the image position.

Property 2: Inverted
Explanation: The image is upside down compared to the object. This can be seen from the diagram as the image is drawn below the principal axis while the object is above it.

Marking notes:

• 1 mark each for correctly identifying a property (real, inverted, diminished) — accept any two.
• 1 mark each for a valid explanation linked to the diagram.
• Accept "diminished" with explanation that the image is smaller than the object.

Question 11

(a) [2]

a = (v − u) / t
= (24 − 0) / 8
= 3 m/s²

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit.

(b) [2]

F = ma
= 1200 × 3
= 3600 N

Marking notes:

• 1 mark for correct formula.
• 1 mark for correct answer with unit (N).
• Allow error carried forward from (a).

(c) [2] The deceleration is greater than the acceleration in (a). The car slows from 24 m/s to 0 m/s in only 4 seconds, compared to speeding up from 0 to 24 m/s in 8 seconds. Since the change in velocity is the same (24 m/s) but the time is shorter, the rate of change of velocity (deceleration) is greater.

Marking notes:

• 1 mark for stating "greater than."
• 1 mark for correct reasoning (same change in velocity, shorter time → greater deceleration).

Question 12 [4]

The phenomenon is refraction. Light from the person's legs travels from water to air. When light passes from water (optically denser medium) to air (optically less dense medium), it speeds up and bends away from the normal. To the observer, the light rays appear to come from a higher point than the actual position of the legs. This makes the legs appear shorter (or the person appears to have shorter legs) when viewed from above the water.

Marking notes:

• 1 mark for naming refraction.
• 1 mark for stating light travels from water to air.
• 1 mark for stating light speeds up and bends away from the normal.
• 1 mark for explaining that the apparent position is higher, making legs look shorter.
• Common error: saying light bends towards the normal when going from water to air.

Question 13

(a) [1] Total resistance = R₁ + R₂ = 2 + 4 = 6 Ω

(b) [2]

I = V / R
= 6 / 6
= 1 A

Marking notes:

• 1 mark for correct formula.
• 1 mark for correct answer with unit (A).

(c) [2]

V₂ = I × R₂
= 1 × 4
= 4 V

Marking notes:

• 1 mark for correct formula or method.
• 1 mark for correct answer with unit (V).
• Allow error carried forward from (b).

Question 14

(a) [2]

v = u + gt
= 0 + 10 × 3
= 30 m/s

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit (m/s).

(b) [1] Air resistance is negligible / the ball falls freely / g is constant at 10 m/s².

(c) [3] Terminal velocity is the constant maximum speed reached by a falling object when the downward gravitational force (weight) is balanced by the upward air resistance (drag force). At this point, the resultant force is zero, so the object stops accelerating and continues to fall at a constant speed. Air resistance increases with speed until it equals the weight of the object.

Marking notes:

• 1 mark for defining terminal velocity as constant speed.
• 1 mark for stating weight = air resistance (forces are balanced).
• 1 mark for explaining that resultant force is zero so acceleration is zero.
• Common error: saying the ball stops moving at terminal velocity.

Question 15

(a) [1] The type of surface (or the surface material).

(b) [1] The distance travelled by the toy car after leaving the ramp.

(c) [2] The height of the ramp must be kept constant. This is because if the height changes, the toy car will have a different speed at the bottom of the ramp (different gravitational potential energy converted to kinetic energy), which would affect the distance travelled and make the results unreliable / not a fair test.

Marking notes:

• 1 mark for identifying a valid controlled variable (ramp height, same car, same starting point, etc.).
• 1 mark for explaining why it must be kept constant (to ensure a fair test / so that only the independent variable affects the result).

Question 16

(a) [2] This is a step-up transformer because the secondary coil has more turns (200) than the primary coil (100). A transformer with more turns on the secondary side produces a higher output voltage than the input voltage.

Marking notes:

• 1 mark for stating "step-up."
• 1 mark for correct explanation (more turns on secondary → higher voltage).

(b) [2]

V_s / V_p = N_s / N_p
V_s / 12 = 200 / 100
V_s = 12 × 2
= 24 V

Marking notes:

• 1 mark for correct formula or ratio.
• 1 mark for correct answer with unit (V).

(c) [2] One energy loss is heat loss in the copper coils (due to resistance of the wire). This can be reduced by using thick copper wire with low resistance. (Also accept: eddy currents in the core — reduced by using a laminated core; or magnetic flux leakage — reduced by using a soft iron core.)

Marking notes:

• 1 mark for identifying a valid energy loss.
• 1 mark for stating a valid method to reduce it.

Question 17 [4]

When the jar is in the refrigerator, both the metal lid and the glass jar contract (shrink) due to the lower temperature. However, metal has a higher coefficient of thermal expansion than glass, so the metal lid contracts more than the glass jar opening, making it very tight and difficult to open. When hot water is poured over the lid, the metal lid expands more quickly and to a greater extent than the glass jar because metal is a better thermal conductor and has a higher coefficient of expansion. The lid becomes looser relative to the jar opening, making it easier to open.

Marking notes:

• 1 mark for stating that metal expands more than glass (higher coefficient of thermal expansion).
• 1 mark for explaining that heating causes the lid to expand.
• 1 mark for stating that metal conducts heat better / responds faster to temperature change.
• 1 mark for linking the greater expansion of the lid to it becoming easier to open.
• Common error: saying the glass also expands by the same amount.

Question 18

(a) [2]

GPE = mgh
= 60 × 10 × 4.0
= 2400 J

Marking notes:

• 1 mark for correct substitution.
• 1 mark for correct answer with unit (J).

(b) [2]

Power = Work done / time
= 2400 / 5.0
= 480 W

Marking notes:

• 1 mark for correct formula.
• 1 mark for correct answer with unit (W).
• Allow error carried forward from (a).

(c) [1] The student also does work to gain kinetic energy (moving horizontally / accelerating), not just gravitational potential energy. Therefore the total energy transferred is greater than just the GPE, so the actual power is greater.

Marking notes:

• Accept any valid reason: work done against friction, energy used for horizontal movement, kinetic energy gained, energy lost as heat.

Question 19

(a) [1] The angle of reflection is 30° (angle of incidence = angle of reflection).

(b) [3] The diagram should show:

• The normal (a dashed line perpendicular to the mirror surface at the point of incidence).
• The incident ray approaching the mirror at 30° to the normal.
• The reflected ray leaving the mirror at 30° to the normal, on the opposite side of the normal.
• The angle of incidence labelled as 30° (between incident ray and normal).
• The angle of reflection labelled as 30° (between reflected ray and normal).

Marking notes:

• 1 mark for showing the normal correctly.
• 1 mark for showing the reflected ray on the correct side at the correct angle.
• 1 mark for correctly labelling both angles as 30°.
• Common error: measuring the angle from the mirror surface instead of the normal.

Question 20

(a) [2] The hypothesis is incorrect. Resistance decreases when the thickness (cross-sectional area) of the wire increases. A thicker wire has a larger cross-sectional area, which means there is more space for electrons to flow, resulting in lower resistance. The student's reasoning is wrong because having more material does not increase resistance — a wider path makes it easier for current to flow.

Marking notes:

• 1 mark for stating the hypothesis is incorrect.
• 1 mark for correct explanation (larger cross-sectional area → lower resistance).

(b) [1] "If the thickness (cross-sectional area) of the wire increases, then the resistance will decrease because a thicker wire provides a larger area for electrons to flow through."

(c) [1] The student could repeat the experiment and calculate the average (mean) resistance for each wire thickness to improve reliability. (Also accept: use a more precise measuring instrument; take readings at different points along the wire; ensure the wire is at a constant temperature.)

Marking notes:

• Accept any valid method to improve reliability (repeating and averaging is the most common and expected answer).