Question

A 25-w fluorescent light emits 5.0 j of thermal energy each second. what is the effi- ciency of the fluorescent light

Answer 1

There are a couple of important questions to answer here. First is "how much heat do appliances produce and why?" Second is "Is it a good idea to use light bulbs as space heaters?" Heat is a measure of energy. Some people would say that heat is the "waste" energy of a process. Whenever energy is transferred, some heat is lost to the environment. When you say a light bulb uses 100 watts, you are talking about the power of the bulb; power is an energy rate. The metric unit for energy is the joule and a watt is equal to one Joule per second. Your 100W light bulb uses 100 joules of energy every second.

But how much of those 100 joules every second is converted to heat? The answer is a little shocking:

All of it.
Granted, the energy isn´t all transferred to the environment by heat all at once. Some of it (about 10% according to Wikipedia) is converted to light while the rest of the energy is dissipated as heat. But the light from the light bulb hits the walls and objects in your room and scatters. Every time the light bounces off something, some of it is absorbed as heat. (Which wavelengths of light an object absorbs determines its color). Eventually all of the energy from the light is absorbed by objects in your room and ends up as heat. This is why rooms get dark when you turn off a light switch.

This means that a 100W light bulb produces 100 joules of heat every second. Consequently, a 90W Nichrome heater produces 90 joules of heat every second.<./p>

Now on to the second question: Is it a good idea to use light bulbs as space heaters? The answer varies depending on your personal preferences, but in general is a resounding "Meh...not really." As you may have learned in school, hot air (and therefore heat) rises upward and cool air falls downward. Most people use light bulbs near their ceiling, but if you wanted to use them efficiently for heating you would need to install them near the floor. With your lights high up, the space near the ceiling would be nice and warm, but you would be stuck on the ground, cold but very well lit because the hot air cannot float down to the floor. Additionally, people don´t always want heat and light at the same time. If I´m reading a book on a warm summer night, I certainly don´t want to be bothered with extra heat from my lamp if I don´t have to be. So, will using incandescent light bulbs lower your heating bill? Maybe a little (when you actually want heat) but there are much more efficient ways to warm yourself up. Blankets, natural gas heaters, and cuddling are among my favorite ways to get warm, and they are all more effective than an incandescent bulb.