Apparatus for Teaching Physics

Wattage: Household Appliances

John J. Lynch, Physics Department, Wheeling Jesuit University, Wheeling, WV 26003; 

	Our students become acquainted with electricity early in life. By the time we meet them in a physics class they are familiar with most electrical appliances on the market–-TV’s, computers, lamps, clock radios, fans, and space heaters to name a few. Yet, when we set out to teach them the basic concepts of electricity we use unfamiliar devices such as DC power supplies, color-coded resistors, rheostats, and Helmholtz coils. The reason for doing so is obvious: low voltage DC circuits are safer to experiment with. Using familiar AC appliances, however, would be more meaningful. The following is a brief description of a lab that addresses this concern. 
	At the heart of this lab is an ordinary 8-foot extension cord that has been modified to be used safely with an ammeter (see Fig. 1). To make such a cord take it and snip the neutral wire about a foot from the receptacle end. The neutral wire is the one that runs between the larger slot on the receptacle end and the larger blade on the plug. Strip both of the cut ends and install the banana plugs. Set up the multimeter to measure AC current and plug the banana plugs into the appropriate jacks (see Fig. 2). It would be unwise to install the banana plugs on the live wire because students might unplug them with the cord still plugged in to the wall. 
	In this lab students measure the current drawn by a wide range of familiar appliances using an ammeter with the modified extension cord. From the current drawn they calculate the rate of energy consumption--wattage. They estimate how often each appliance might be used in a typical household each year and for how long. From this they estimate the annual energy consumption in kwatt@hours (kWh). Using the rate from a utilities bill they estimate the annual cost (in dollars) of operating each appliance. This number is meaningful to them.
	The students (1) learn how to relate the joule to the kwatt@hour and the kwatt@hour to dollar and cents, (2) learn that some appliances such as night lights and clock radios cost mere pennies per year to operate, (3) learn that appliances that produce heat (e.g. hair dryers, space heaters) or reject it (e.g. refrigerators, air conditioners) cost the most to operate by an order of magnitude, (4) get a sense of what an ampere is.

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Fig. 1: Modified extension cord. 

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Fig. 2: Desk fan running in series with ammeter.