NECTA PHYSICS 1 2025 SOLUTION

NECTA Form 4 Physics 2025

Suggested Solutions & Revision Notes

SECTION A (16 Marks)

1. Multiple Choice Questions

(i) Answer: A (Electronic mails)
Reasoning: The question asks for a method to convey a message in the form of "writings". Electronic mail (email) is the only option that directly delivers written text.

(ii) Answer: D
Reasoning: The standard scientific method sequence is: Problem identification (4) → Asking questions (6) → Formulating hypothesis (1) → Performing experiment (3) → Data collection (8) → Interpretation (2) → Presentation (7) → Conclusion (5).

(iii) Answer: B
Reasoning: Wiping the anvil and spindle removes dirt/dust that would cause a zero error or false reading.

(iv) Answer: D
Reasoning: To push a car, you need an external force. Outside the car, friction acts on your feet, allowing you to push the ground backward and generate a forward reaction force. Inside, the forces cancel out.

(v) Answer: D
Reasoning: Density = Mass / Volume. Increasing the mass while keeping the volume fixed increases the density.

(vi) Answer: B
Reasoning: Water spreads because its adhesive force (attraction to the tile) is stronger than its cohesive force. Honey has stronger cohesive forces (viscosity) relative to adhesion.

(vii) Answer: A
Reasoning:
Mass, m = 5 g = 0.005 kg.
Force, F = mg = 0.005 × 9.8 = 0.049 N.
Radius, r = 0.1 mm = 1 × 10^-4 m.
Area, A = πr² = 3.14 × (10^-4)² = 3.14 × 10^-8 m².
Pressure, P = F / A = 0.049 / (3.14 × 10^-8) ≈ 1.56 × 10⁶ Pa.
Closest option is 1.59 × 10⁶ Pa.

(viii) Answer: A
Reasoning:
Object distance, u = 25 cm.
Radius = 24 cm, so Focal length f = 12 cm.
Mirror formula: 1/f = 1/u + 1/v
1/12 = 1/25 + 1/v
1/v = 1/12 - 1/25 = (25 - 12) / 300 = 13/300
v ≈ 23.1 cm.
Since v is positive, the image is Real. Real images are always Inverted. Since v is less than u (23.1 < 25), it is Diminished.

(ix) Answer: B
Reasoning:
Force F = mg = 200 × 9.8 = 1960 N.
Work = F × distance = 1960 × 60 = 117,600 J.
Power = Work / time = 117,600 / 20 = 5,880 W = 5.88 kW.

(x) Answer: C
Reasoning: Capacitance C is proportional to Area (A) and inversely proportional to separation (d). To decrease capacitance, you need to increase separation or decrease area.

2. Matching Items

• (i) Temperature of melting ice: F (Lower fixed point)
• (ii) SI Temperature scale: H (Kelvin scale)
• (iii) Average kinetic energy measure: G (Temperature)
• (iv) Temperature of boiling water: A (Upper fixed point)
• (v) Scale based on absolute zero: B (Thermodynamic scale)
• (vi) Molecular motion stops: E (Absolute zero)

SECTION B (54 Marks)

3. Light and Optics

(a) Lenses for Eye Defects:

• (i) Short-sightedness: Concave (Diverging) lens.
• (ii) Long-sightedness: Convex (Converging) lens.

(b) Concave Mirrors as Shaving Mirrors:
Concave mirrors are used because when the face is placed close to the mirror (between the pole and the focal point), it produces an upright and magnified virtual image.

4. Pressure and Moments

(a) Simple Mercury Barometer:

• (i) Torricellian Vacuum: The vacuum space created at the top of the tube.
• (ii) Atmospheric Pressure: Represented by the vertical height of the mercury column.

(b) Moments Calculation (Figure 2):
Case I (Finding A):
Clockwise Moment = Anticlockwise Moment
4N × 25cm = A × 20cm
100 = 20A → A = 5 N.

Case II (Finding B):
(1N × 20cm) + (2N × 40cm) = B × 25cm
20 + 80 = 25B
100 = 25B → B = 4 N.

Difference: A - B = 5N - 4N = 1 N.

5. Mechanics

(a) Impulse:

• (i) Bending knees increases the time of impact. Since Force = Change in Momentum / Time, increasing the time reduces the force, preventing injury.
• (ii) Law: Newton's Second Law of Motion.

(b) Inclined Plane:
Velocity Ratio (VR) = Distance of Effort / Distance of Load.
Distance of Effort = Length of plane (L).
Distance of Load = Height (h).
VR = L / h.
Using trigonometry: sin θ = h / L.
Therefore, L / h = 1 / sin θ.
So, VR = 1 / sin θ.

6. Thermal Physics

(a) Bimetallic Strip:

• (i) Made by joining two metals with different expansion coefficients (e.g., riveted together).
• (ii) When heated, the metal with higher expansion (brass) expands more, forcing the strip to bend with brass on the outside curve.

(b) Method of Mixtures:
Hot water: mass = 105g, Temp = 75°C.
Cold water: mass = 215g, Temp = 15°C.
Heat Lost = Heat Gained
105 × (75 - T) = 215 × (T - 15)
Divide both sides by 5: 21(75 - T) = 43(T - 15)
1575 - 21T = 43T - 645
2220 = 64T
T ≈ 34.7 °C.

7. Electricity

(a) Factors for Heat Generated (Joule's Law):

1. Current (I) squared.
2. Resistance (R) of the conductor.
3. Time (t) the current flows.

(b) Internal Resistance and EMF:
Equation: V = E - Ir
1.1 = E - 0.2r ... (eq 1)
1.3 = E - 0.1r ... (eq 2)
Subtract eq 1 from eq 2: 0.2 = 0.1r → r = 2 Ω.
Substitute r into eq 2: 1.3 = E - 0.1(2) → E = 1.5 V.

8. Geophysics

(a) Venus: Nicknamed Earth's twin due to similar size, mass, and density.

(b) Volcanic Activity:
Negative: Destruction of habitat, air pollution.
Positive: Formation of fertile soil, geothermal energy.

SECTION C (30 Marks)

9. Radioactivity

(a) Cloud Chamber: Detects radiation by condensation trails. Alpha tracks are thick and straight; Beta tracks are thin and wavy.

(b) Half-life:
Mass changes from 20g to 0.625g.
Sequence: 20 → 10 → 5 → 2.5 → 1.25 → 0.625.
This is 5 half-lives.
Total time = 150 days.
Half-life = 150 / 5 = 30 days.

10. Waves and Electronics

(b) Diffraction: Used in radio communication (bending around hills), X-ray crystallography, and spectroscopy.

(c) Vibrating Wire: Frequency is proportional to the square root of tension (mass).
Mass ratio: 12g / 3g = 4.
Frequency change = √4 = 2.
So, quadrupling the mass doubles the frequency.

11. Electromagnetism

(a) Transistors:
PNP: Arrow points IN.
NPN: Arrow points OUT.
Layers: Emitter, Base, Collector.

(b) Electric Bell: Works on the principle of electromagnetism. Closing the switch magnetizes the coil, attracting the armature and striking the gong. This breaks the circuit, demagnetizing the coil, and the armature returns, reconnecting the circuit to repeat the cycle.

(c) Induction:
(i) Fleming's Left Hand Rule: Thumb = Force, First Finger = Magnetic Field, Second Finger = Current.
(ii) Transformer Efficiency: Less than 100% due to heat losses (copper loss, eddy currents, hysteresis).
(iii) Factors for EMF: Strength of magnetic field, speed of conductor motion, number of turns in coil.

Disclaimer: These are suggested solutions for educational revision.