Energy Transferred by Working
Electricity and Magnetism

Changing one bulb won't make a difference

Teaching Guidance for 14-16 Supporting Physics Teaching

Quantification is essential if you want to be convincing

Save it campaigns that are based on switching off electric lights or changing from incandescent (filament) to compact fluorescent bulbs are often met by the public with a shrug of the shoulders and It'll make no difference.

Experience has shown that students can become very motivated by carrying out energy surveys for their home or school. The approach is the same in both cases and involves the following steps:

  1. Track down all of the incandescent bulbs in current use in the house/school. (For the school survey, split the class into teams that take a different building each.)
  2. Make the assumption that each incandescent bulb has a power output of 75 watt. This is an interesting and helpful assumption to make – talk it through with the class. This simplifying assumption avoids having to inspect each and every bulb to find its power rating. The figure of 75 watt seems reasonable, lying somewhere between high (100–150 watt) and low (30–50 watt) power bulbs.
  3. For each bulb there is a saving of 60 % of the rate at which energy is shifted from the supply. That is for each 75 watt bulb there's an energy saving at the rate of 45 watt or 45 joule every second.
  4. Multiply the total number of incandescent bulbs used by 45 watt to find the rate at which energy is being dissipated through bulbs warming the surroundings, for the school or home. In a large public building, such as a school, this rate of wastage can become significant and, if this is the case, the class may wish to prepare a report for the school governors or local authority. But take care in interpreting your results – what you save on warming by light bulbs you may need to spend on extra warming by other means during the winter. In the summer you might counter the excess temperatures caused by the inefficient light bulbs by opening the windows or doors.
Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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