Lesson 16: Technology in Everyday Life

Every device around you is frozen science. Technology is the application of scientific knowledge for practical purposes — and the same physics from Lessons 6–8 explains your phone charger, your air-conditioner and the solar panels on your school roof. This lesson analyses everyday technology through efficiency, energy transfer and sensors, the way an examiner would.

Efficiency: the number that runs your bills

Recall efficiency = (useful energy output / total energy input) × 100%. An LED lamp converts about 90% of electrical energy to light; an old filament bulb manages only about 10%, wasting the rest as heat. Compare the annual cost of lighting a room for 5 hours every evening:

LED lamp, 9 W:  energy per year = 9 W × 5 h × 365 = 16 425 Wh ≈ 16.4 kWh
Filament bulb, 60 W: energy per year = 60 × 5 × 365 = 109 500 Wh ≈ 109.5 kWh
Saving = 109.5 − 16.4 ≈ 93 kWh per year ≈ $27.93 at $0.30/kWh

One bulb. Multiply across a block of flats and a nation, and lighting technology becomes energy policy — Singapore phased out incandescent bulbs for exactly this reason. Air-conditioners use a related idea, the coefficient of performance: a good unit moves 3–4 joules of heat out of a room per joule of electrical energy in, because it is moving heat, not converting electricity into cold.

Solar panels: light into electricity

A solar (photovoltaic) panel converts light energy directly into electrical energy. Output depends on three factors: the light intensity, the panel area, and the panel's efficiency. On a bright Singapore afternoon, sunlight delivers about 1000 W per square metre (more precisely, about 1000 W/m² at solar noon under clear skies; our annual average is lower because of clouds and the Sun's angle).

2 m² school-roof panel at 20% efficiency:
power = 1000 W/m² × 2 m² × 0.20 = 400 W

That runs about 40 LED lamps — or one small air-conditioner for a fraction of each hour. Cloud cover can halve the output within a minute, which is why grid-connected systems need the grid (or batteries) as backup: understanding this intermittency is essential to any exam discussion of renewable energy.

Sensors and automatic systems

A sensor converts a physical change (light, temperature, pressure) into an electrical signal that a system can act on:

• Light-dependent resistor (LDR) — resistance falls as light brightens. Street lamps and the auto-brightness on your phone use light sensing: when the LDR's signal crosses a threshold, the circuit switches.
• Thermistor — resistance changes with temperature; used in air-conditioner thermostats, water heaters and fever thermometers.
• Passive infrared (PIR) sensor — detects body heat movement; triggers automatic doors, corridor lights in HDB blocks and security systems, saving energy by lighting only occupied spaces.

The pattern to describe in answers: input change → sensor resistance/signal changes → control circuit compares with a set value → output device switches. Feedback completes the loop in a thermostat: temperature reached → heater off → temperature falls → heater on again.

Waves on the move: phones and fibre

Mobile phones communicate with microwaves — electromagnetic waves that pass through air and walls but carry little energy per photon, which is why phones do not damage tissue the way ultraviolet or X-rays can. Fibre-optic broadband (Singapore's Nationwide Broadband Network) sends data as pulses of light trapped inside glass fibres by total internal reflection: light striking the fibre wall above the critical angle reflects completely, zig-zagging along even around bends, with far less signal loss than copper wires.

Singapore focus: a smart, efficient city

HDB estates use pneumatic waste conveyance (air pressure moves refuse through underground pipes), solar on rooftops, and smart lighting with motion sensors. The Building and Construction Authority's Green Mark scheme rates buildings on energy efficiency — the same physics: insulation (Lesson 15 materials), efficient cooling, solar gain control. Technology questions in exams often hide inside such contexts; strip the context away and they are Lesson 7 and 8 problems in disguise.

Common mistakes

• Treating efficiency above 100% as possible for an energy converter. A lamp or motor cannot exceed 100%; air-conditioners can move more heat than the electrical energy they consume because they relocate heat — say "moves", not "produces".
• Assuming solar output is constant. Output varies with light intensity and angle; quote the conditions with any number.
• Describing a sensor as "deciding". Sensors only convert signals; the control circuit does the comparing and switching.
• Confusing microwaves with "radiation" damage. All electromagnetic waves are radiation; harm depends on photon energy — ionising radiation (UV, X-ray, gamma) damages cells, microwaves do not.

Exam Experience

Question 1 (structured, data-based). A school tests two desk fans run for 8 hours. Fan A consumes 45 W and moves 2.2 m³ of air per second; Fan B consumes 60 W and moves 2.8 m³/s. Electricity costs $0.30 per kWh. (a) Calculate the energy used by Fan A in 8 hours, in kWh. [2] (b) Calculate the cost of running Fan B for 8 hours. [2] (c) A teacher says Fan B is "obviously better value". Evaluate this claim using the data. [3]

Question 2. In an automatic street lamp, an LDR is used so that the lamp switches on at dusk. Which statement is correct? [1] A. The LDR's resistance increases as light intensity falls. B. The LDR produces light when it gets dark. C. The LDR's resistance decreases as light intensity falls. D. The LDR stores charge during the day to power the lamp.

Question 3. Explain, in terms of energy transfers, why replacing a 60 W filament bulb with a 9 W LED of the same brightness reduces a household's electricity bill so much. [3]

Marking scheme. Q1(a) E = 0.045 kW × 8 h = 0.36 kWh [1 method, 1 answer with unit]. Q1(b) E = 0.060 × 8 = 0.48 kWh; cost = 0.48 × 0.30 = $0.144 (about $0.14) [1 method, 1 answer]. Q1(c) Fan B moves more air (2.8 vs 2.2 m³/s) but uses more power [1]; air moved per watt: A = 2.2/45 = 0.049 m³/s per W; B = 2.8/60 = 0.047 [1]; so B is slightly LESS efficient per watt — the claim is not supported unless the extra airflow itself is the priority [1]. Q2 A [1]. (LDR resistance is high in the dark; the control circuit uses that change to switch the lamp on.) Q3 The LED transfers a much larger fraction (about 90% vs about 10%) of electrical energy to light [1]; the filament bulb wastes most input energy as thermal energy [1]; so the LED needs far less electrical energy (9 W vs 60 W) per second for the same light output, reducing energy consumed and cost [1].

Going deeper: heating, cooling and the COP number

Air-conditioning dominates Singapore's electricity demand, and its efficiency measure rewards careful reading. An air-conditioner rated "3000 W of cooling" consuming 900 W of electrical power has a coefficient of performance (COP) of:

COP = heat moved / electrical energy used = 3000 / 900 ≈ 3.3

This is NOT an efficiency of 330% breaking physics: the machine MOVES 3000 W of heat from inside to outside; it does not convert 900 W of electricity INTO 3000 W of cooling. The distinction between moving and converting energy is exactly the kind of precision this course has practised since Lesson 7. Raising the set-point from 22 °C to 25 °C helps twice: less heat leaks in (smaller temperature difference to the outside) and the compressor cycles less.

Standby power — the vampire load. Devices on standby draw 1–10 W each, all day, all year: a 5 W standby load uses 5 × 24 × 365 = 43.8 kWh a year — about $13 spent on nothing. Smart plugs and switched sockets are technology deployed against arithmetic like this. A household audit of standby loads (TV, chargers, set-top boxes) is a genuine Lesson 16 investigation: measure, multiply, rank, act.

Fibre versus copper, quantified. Total internal reflection keeps light signals inside a fibre with very low loss over kilometres, and light's high frequency carries far more data per second than electrical pulses in copper. Singapore's fibre-to-the-home coverage is why video lessons stream smoothly to every flat — wave physics (Lesson 8's electromagnetic family) cashed out as infrastructure.

Exam-style extra

Question. A refrigerator moves 500 W of heat from its interior while consuming 160 W of electrical power. (a) Calculate its COP. [2] (b) Explain why a COP above 1 does not violate the conservation of energy. [2]

Marking points. (a) COP = 500/160 ≈ 3.1 [1 method, 1 answer]. (b) The electrical energy is used to MOVE existing heat, not to create cold or energy [1]; total energy is conserved — the heat dumped outside equals the heat removed plus the electrical energy input [1].

Workshop: meters, satellites and silent data

Singapore's smart electricity meters report every 30 minutes — 48 readings a day per household — letting families see the air-conditioning peak in their own data and letting the grid anticipate demand (the demand curve is Lesson 16's load graph drawn by a nation). Data of this kind is invisible infrastructure: no new power station is built until the curves prove one is needed.

Positioning is the other silent utility. A phone fixes its location by timing signals from at least four GPS satellites: each signal's travel time (at the speed of light, 3 × 10⁸ m/s) gives a distance; four distances intersect at one point. A timing error of just 1 microsecond would shift the calculated position by 300 m (3 × 10⁸ × 1 × 10⁻⁶ = 300 m) — which is why the satellites carry atomic clocks and why "GPS is accurate to metres" is an achievement of error management, tying Lesson 16 straight back to Lesson 17's precision theme. Contactless payment uses the same electromagnetic family at the opposite extreme: near-field communication works over mere centimetres, a deliberate range limit that doubles as a security feature.