Real World Connections Atomic Concepts Hannah

Directions: Use the passage below and your knowledge of Chemistry to answer questions 1-5.

Incandescent vs. Fluorescent vs. LED Lights

How do they work?

A "normal light bulb" is also known as an incandescent light bulb. These bulbs have a very thin tungsten filament that is housed inside a glass sphere. The basic idea behind these bulbs is simple. Electricity runs through the filament. Because the filament is so thin, it offers a good bit of resistance to the electricity, and this resistance turns electrical energy into heat. The heat is enough to make the filament white hot, and the "white" part is light. The filament glows because of the heat -- it incandesces. A fluorescent bulb uses a completely different method to produce light. There are electrodes at both ends of a fluorescent tube, and a gas containing argon and mercury vapor is inside the tube. A stream of electrons flows through the gas from one electrode to the other. These electrons bump into the mercury atoms and excite them. As the mercury atoms move from the excited state back to the ground state, they give off ultraviolet photons. These photons hit the phosphor coating the inside of the fluorescent tube, and this phosphor creates visible light. It sounds complicated, so lets go through it again in slow motion:  There is a stream of electrons flowing between the electrodes at both ends of the fluorescent bulb.  The electrons interact with mercury vapor atoms floating inside the bulb.  The mercury atoms become excited, and when they return to the ground state they release photons of light in the ultraviolet region of the spectrum.  These ultraviolet photons collide with the phosphor coating the inside of the bulb, and the phosphor creates visible light. The phosphor fluoresces to produce light. An LED bulb produces light when electrons move around within its semiconductor structure. LED bulbs are made of two silicon layers. One layer is positively charged and the other layer is negatively charged. The positive layer has "holes" -- openings for electrons; the negative layer has free electrons floating around in it. When an electric charge strikes the semiconductor, it activates the flow of electrons from the negative to the positive layer. Those excited electrons emit light as they flow into the positively charged holes.

Efficiency

The problem with incandescent light bulbs is that the heat wastes a lot of electricity. Heat is not light, and the purpose of the light bulb is light, so all of the energy spent creating heat is a waste. 90% of the energy consumed by an incandescent bulb is wasted heat. This makes the bulb extremely hot to touch and very unsafe. An incandescent light bulb can be up to 350°F. Conventional incandescent light bulbs also emit a good bit of invisible ultraviolet light, but they do not convert any of it to visible light. Consequently, a lot of the energy used to power an incandescent lamp is wasted. One advantage of a fluorescent lamp is its efficiency. First, fluorescent light bulbs waste much less energy as heat, operating at a much cooler 90°F. Second, the invisible ultraviolet light is not wasted, but put to work as the phosphor converts it to visible light. LED lamps are even more efficient than fluorescent bulbs. The increase in efficiency is not huge, approximately 5%, but it is the most efficient. To produce the same amount of light as a 60 Watt incandescent light bulb, the fluorescent bulb only uses 13 Watts, and the LED bulb uses a miserly 8.5 Watts. Cost

In December, 2015, the cost of a 60 W incandescent bulb on Amazon was about $2.50. The equivalent fluorescent bulb on Amazon was about $4.65, and the LED bulb was about $4.95. The typical annual electricity cost is $10.95 for the incandescent bulb, $2.56 for the fluorescent twist bulb, and $1.10 for the LED bulb.

Durability

The typical incandescent bulb will last 1,200 hours. Fluorescent, 8,000 hours, and LED bulbs 50,000 hours.

Other Factors in Choosing

So, why choose incandescent lighting? People generally use incandescent lights in the home because they emit a "warmer" light -- a light with more red and less blue. Fluorescent bulbs, as was discussed earlier in the article, use mercury. Mercury is a serious environmental problem due to its toxicity. The used bulbs should not be thrown into the trash, but brought to a place that will recycle the mercury. Unfortunately, as of this writing, I am not aware of any place that recycles fluorescent bulbs. Not exactly convenient.