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# Ohm's Law

## When electricity passes through a resistor it loses voltage and releases power.

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Practice Ohm's Law
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Guess The Efficiency

### Guess The Efficiency

Credit: James Bowe
Source: http://www.flickr.com/photos/jamesrbowe/3338776771/

Thomas Edison created the first practical incandescent light bulb in 1878. Since then the light bulb has permanently changed the way the human world lives. Even though the light bulb has gone through several variations of material and shape, it is still highly inefficient.

#### News You Can Use

• Regular incandescent light bulbs convert electrical energy into heat and light. The light is then radiated through the glass while the heat raises the temperature of the bulb. By performing a simple experiment that measures the rate at which the thermal energy is given off Pheat and the total heat energy of the light bulb Ptotal, one is able to determine the efficiency using the following equation,

ε=PtotalPheatPtotal\begin{align*}\varepsilon=\frac{P_{total}-P_{heat}}{P_{total}}\end{align*}

• Surprisingly the efficiency of most light bulbs is 10% +/- 2%. When compared to other light bulbs, incandescent light bulbs produce the most heat. While incandescent light bulbs are extremely inefficient compared to other types that are currently available, they are still widely used due to their low purchase price.
• Recent laws have been passed to slowly phase out some of the most common incandescent light bulbs by 2014. These bulbs will be replaced with longer lasting, more energy efficient light bulbs. While these newer bulbs are more expensive to purchase, their longer life and lower power drain will cost individuals less in the long run.
• Watch how a light bulb is made:

#### Show What You Learned?

Using the information provided above, answer the following questions.

1. What is the benefit of coating the filament in the light bulb?
2. How does filling an incandescent light bulb with an inert gas extend the life of the light bulb?
3. If electrical current heats the filament to temperatures in the range of 2,000 – 3,000 Kelvin, where would you expect the melting point of tungsten to be?