Utilization of Electrical Energy and Illumination
Utilization of Electrical Energy and Illumination tests electric heating, welding, traction, electrolysis, drives, and lighting design.
Core rules
- Heating: resistance ovens use I²R; induction heating uses eddy currents; dielectric heating suits insulators. Arc furnaces and welding rely on controlled arcs.
- Electric traction: DC series motors give high starting torque; AC traction uses induction motors with VVVF control. Regenerative braking returns energy.
- Illumination: luminous flux is in lumens; illuminance E in lux is lumens per square metre; luminous intensity in candela.
- Inverse-square law: E=I/d² for a point source normal to the surface; cosine law: E=(I/d²)cosθ.
- Efficacy is lm/W; LED and sodium lamps are more efficacious than incandescent. Fluorescent lamps need a ballast; sodium vapour lamps have a warm-up period.
Worked example
A point source of 800 cd is 4 m vertically above a work plane. Directly below, E=800/4²=800/16=50 lux. At a point where the ray makes 60° with the normal, E=50×cos60°=25 lux.
Traps
Do not confuse lumens with lux; lux depends on distance and angle. Do not add illuminance from several sources as scalar lux without vector geometry. A fluorescent lamp without a ballast can fail.