Saturday, 13 August 2011

Physics: Dailylife applications of energy I (elective) -- lighting

To access the document version of this chapter of notes click the "Notes Corner" above.

End-use energy efficiency = useful energy output/energy output * 100%. For example in light bulb, useful energy means light energy.
Overall energy efficiency refers the percentage of conversion from generating electricity to end-use efficiency. Since generating electricity or transmission of it consumes energy, this efficiency is usually lower than the end-use energy efficiency.
-          Principle: when p.d. is applied across a bulb, the elements are excited through collision, and the electrons are in excited state. The transmission of electron from excited to ground state by emitting photon obeying ΔE = hf, where h is the Planck constant = 6.63*10-34 Js. In Bohr's model the emission spectrum is discrete but for liquid or solid, molecules has complex energy level and hence the emitted light can be regarded as continuous spectrum.
-          Incandescent lamp: Bulb emitting light by heating the tungsten (to 2500K) wire in the bulb with argon gas inside. Tungsten vapour may accumulate on bulb's wall and reduce light emitted, so tungsten halogen lamp is used to eliminate bulb wall blackening so it has a longer lifetime.
Advantage: controllable, cheap, small and light; disadvantage: low efficiency (2%)
-          Fluorescent lamp: When a p.d. is applied across the two electrodes, electron collides with mercury vapour inside and mercury emits UV light, which shines on the phosphor coating and gives out visible light. Since this is a gas discharge lamp, a very high p.d. is needed so it's connected to a starter. The resistance of the tube falls sharply as the gas ionizes, so ballast is used to maintain constant current. Note that efficiency is related to surface area/volume, so smaller FL has higher efficiency.
Advantage: comparatively high efficiency (10~15%), disadvantages: too long, high initial cost, heavy, uncontrollable, toxic mercury vapour
-          Compact fluorescent lamp: a smaller version of FL for home use. It's also called energy-saving bulbs.
-          High intensity discharge lamp: It relies on gas discharging principle and uses mercury vapour as well, but it's put in a arc tube such that higher pressure is given, and hence more intense light given out. It's expensive but very useful in outdoor. The three types of HID includes mercury vapour (for street lamp), metal halide (in stadium) and high pressure sodium lamps (for highways since it has longer life).
-          Light emitting diode(LED): it's connected with a pair of p and n type semiconductor. When p.d. is applied across the two semiconductors (p-type +ve), the electrons and positive holes jump across the junction and release energy in terms of visible light. It has very long life and high efficiency.
Light emitted per unit time by an object is measured in luminous flux Φ with unit lumen (lm). Note that 1 lm is equal to 1/683 W of monochromatic light of wavelength 555nm.
Efficacy = luminous flux in lm / input electrical power is used to measure the efficiency.
Illumination on a surface is measure by illuminance E = Φ/A with unit lux.
Considering the fact that light spreads spherically, E = Φ/A = Φ/4πr2 if the surface is perpendicular to the light source.
Consider a point on the plane, E = Φ/4πr2 cosθ = Φ/4πd2 cos3θ where d is the perpendicular distance between light source and the plane, θ is the angle between light and the plane. Note that when we consider the illuminance on a planet against a star, the distance remains almost the same, so illuminance is given by E = Φ/4πd2 cosθ only.

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