Illumination and Electric Heating
What is tested
Illumination quantities and laws, lamp types and selection, and the principles of resistance, induction, dielectric and arc heating, including welding.
Illumination basics
Luminous flux is in lumens, illuminance in lux (lm/m²), luminous intensity in candela and luminance in cd/m². The inverse-square law gives E = I/d² directly below a point source; the cosine law gives E = (I cos θ)/d² on a tilted surface. Utilization and maintenance factors reduce the installed flux: number of lamps N = (E × A)/(Φ × UF × MF).
Worked example
A 1000 lm lamp is mounted 3 m above a bench. Treating it as radiating uniformly, I = Φ/4π = 1000/12.57 = 79.6 cd. Directly below, E = 79.6/3² = 8.8 lux - poor for reading, so a reflector or a higher-output lamp is required.
Lamps
Incandescent: 10–12 lm/W, warm, being phased out. Fluorescent tube: 60–80 lm/W, needs a choke and starter. LED: 90–130 lm/W with a driver. Mercury vapour: bluish, poor colour rendering. Sodium vapour: 100–150 lm/W, yellow, used for street lighting.
Electric heating
Resistance heating uses nichrome (80/20 Ni-Cr) elements up to about 1100 °C for ovens, irons and geysers. Induction heating melts metals; dielectric heating glues PVC and wood; arc heating serves arc furnaces and arc welding. Example: a 2 kW geyser heating 50 litres of water through 40 K needs 50 × 4186 × 40 / 2000 ≈ 4186 s, about 70 minutes, ignoring losses.
Traps
Using the cosine law without converting degrees, mixing lux and lumens, and comparing lamps by wattage rather than by lumens per watt.