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Sec 2 · Physics

Physical Systems

Write the relevant relationship first, then substitute values with units. Track energy transfers carefully — energy is never created or destroyed, only converted.

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MOE syllabus alignment

  • Lower Secondary theme: Interactions.
  • Core Ideas / Practices lens: Energy / System / Change.
  • Study focus: Energy transfers, electricity, forces and thermal effects as connected physical systems.

Alignment note: Independently written and mapped to public MOE/SEAB syllabus structures. Not affiliated with MOE, SEAB or Cambridge.

Key terms

Current (I)Voltage (V)Resistance (R)Ohm's LawPower (P)Series circuitParallel circuitForceMomentPressureEnergyEfficiencyTransverse waveLongitudinal waveAmplitudeWavelengthFrequencyWave speed

Electric circuits — key rules

Series circuits

  • Current is the same throughout all parts: I₁ = I₂ = I₃
  • Total voltage = sum of voltages across each component: V_total = V₁ + V₂ + V₃
  • Total resistance = sum of individual resistances: R_total = R₁ + R₂ + R₃
  • If one component fails (open circuit), the whole circuit stops.

Parallel circuits

  • Voltage is the same across each branch: V₁ = V₂ = V₃
  • Total current = sum of branch currents: I_total = I₁ + I₂ + I₃
  • Adding more branches decreases total resistance.
  • If one branch fails, other branches continue to work.
✓ Exam tip: Household appliances are wired in parallel — each appliance gets the full supply voltage and can be switched independently.
⚠ Common trap: "Current is used up" — FALSE. Current (charge flow) is not consumed; it is the same going into and coming out of a component. Energy is transferred, not current.

Ohm's Law and electrical calculations

V = I × R  |  I = V / R  |  R = V / I
Power: P = V × I  |  P = I² × R  |  P = V² / R
Energy: E = P × t  (where t is time in seconds, E in joules)

Example: A lamp with resistance 15 Ω carries a current of 0.4 A. Calculate the voltage across it and its power.

  • V = I × R = 0.4 × 15 = 6 V
  • P = V × I = 6 × 0.4 = 2.4 W
✓ Exam tip: Always state the formula first, substitute with units, then calculate. Show all working — method marks are available even if the final answer is wrong.

Forces and Newton's Laws (intro)

Types of forces

  • Gravity / weight — downward force on any mass; W = m × g (g ≈ 10 N/kg on Earth).
  • Normal reaction — perpendicular force from a surface.
  • Friction — opposes relative motion between surfaces.
  • Tension — force transmitted through a string or rope.

Balanced and unbalanced forces

  • If forces are balanced (net force = 0): object stays at rest or moves at constant velocity.
  • If forces are unbalanced: object accelerates in the direction of the net force.

Moments (turning effects)

Moment = Force × Perpendicular distance from pivot (unit: N m)

Principle of moments: For an object in equilibrium, sum of clockwise moments = sum of anticlockwise moments.

Example: A 30 N force acts 0.5 m from the pivot. Moment = 30 × 0.5 = 15 N m.

✓ Exam tip: The distance must be perpendicular to the line of action of the force. A force applied closer to the pivot has a smaller moment even if it is the same force.

Pressure

Pressure (Pa) = Force (N) ÷ Area (m²)
  • Same force over a smaller area → higher pressure (e.g. knife blade, stiletto heel, drawing pin tip).
  • Same force over a larger area → lower pressure (e.g. snowshoes, tractor tyres, camel's feet).
  • In fluids, pressure increases with depth: P = ρgh (density × g × height — for reference only).
  • Fluid pressure acts equally in all directions at a given depth.
⚠ Common trap: Pressure depends on the contact area, not the total weight. A heavier person in flat shoes may exert less pressure than a lighter person in stilettos.

Energy transfers and efficiency

Energy stores

Chemical, kinetic, gravitational potential, elastic potential, thermal, electrical, nuclear, light.

Energy transfer language

Always describe energy by: type stored → transferred by [mechanism] → type stored. Example: "Chemical energy stored in the battery is transferred electrically and then stored as thermal energy and light in the bulb."

Efficiency = Useful energy output ÷ Total energy input (× 100 for %)

Efficiency is always less than 100% in real systems — some energy is always wasted as thermal energy.

⚠ Common trap: Energy is not "used up" — it is transferred. The total energy is conserved (1st Law of Thermodynamics).

Waves — properties and types

Key wave quantities

  • Amplitude — maximum displacement from rest position; related to energy/loudness/brightness.
  • Wavelength (λ) — distance between two successive identical points (e.g. crest to crest).
  • Frequency (f) — number of complete waves passing a point per second; unit: Hz.
  • Wave speed (v) — distance travelled per second; unit: m/s.
Wave speed = Frequency × Wavelength  |  v = f × λ

Transverse vs longitudinal

  • Transverse — vibration is perpendicular to direction of travel. Examples: light, electromagnetic waves, water waves, waves on a string.
  • Longitudinal — vibration is parallel (along) the direction of travel. Example: sound waves (compressions and rarefactions).
⚠ Common trap: Sound cannot travel through a vacuum — it needs a medium (solid, liquid or gas). Light can travel through a vacuum.

Past-year style question set

  1. A 3 V lamp has a resistance of 6 Ω. Calculate: (a) the current through the lamp; (b) the power of the lamp.
  2. Two identical lamps are connected in parallel across a 6 V battery. Each lamp has resistance 12 Ω. Find: (a) voltage across each lamp; (b) current through each lamp; (c) total current.
  3. A force of 50 N acts over an area of 0.02 m². Calculate the pressure.
  4. A lever has its pivot 20 cm from one end. A 60 N weight is placed 10 cm to the right of the pivot. What force is needed 30 cm to the left of the pivot to balance it?
  5. A wave has a frequency of 200 Hz and a wavelength of 1.5 m. Calculate its speed.

Answer points

  1. (a) I = V/R = 3/6 = 0.5 A. (b) P = VI = 3 × 0.5 = 1.5 W.
  2. (a) 6 V (same as supply in parallel). (b) I = V/R = 6/12 = 0.5 A each. (c) Total = 0.5 + 0.5 = 1.0 A.
  3. P = F/A = 50/0.02 = 2500 Pa.
  4. Clockwise moment = 60 × 0.10 = 6 N m. Anticlockwise moment = F × 0.30 = 6. F = 6/0.30 = 20 N.
  5. v = f × λ = 200 × 1.5 = 300 m/s.

Must-know checklist

  • Can state current and voltage rules for series and parallel circuits.
  • Can apply Ohm's Law (V = IR) and power formula (P = VI).
  • Can calculate pressure (P = F/A) and solve moment problems.
  • Can describe balanced and unbalanced forces and their effects.
  • Can use the wave equation (v = fλ).
  • Can distinguish transverse and longitudinal waves with examples.
  • Can calculate efficiency and identify useful vs wasted energy.
See also in Sec 2 Reasoning & data → Matter & cycles → Living systems → Environment → All-topic study notes →