Household Appliances

This week's laboratory focuses on common household appliances. There will be five stations to visit. At each station you will measure the current through and the voltage across the appliances while in operation. At several of the stations, you will also make qualitative observations of the behavior of the current.

The general setup for each station is shown. The circuits consist of the appliance, a junction box, a power cord, a voltmeter, an ammeter and several electrical leads with banana plug fittings. The ammeter has very little resistance and is connected in series with the appliance. The voltmeter has a very high resistance and is in parallel with the appliance. Neither instrument has any significant effect on the circuit.

WARNING: Although the circuits have been set up with safety in mind, you must use caution. Many of the connections utilize banana plugs, which can be easily removed. Keep the area clear to minimize the chances of accidently removing a "live" plug from its receptor. Make sure all connections are securely in place before plugging the power cord into the electrical outlet.

  1. Incandescent and Fluorescent Lights

    1. Place the 25 W incandescent light bulb in the lamp holder, check all connections and plug the power cord into an electrical AC outlet. Turn on the lamp and record the voltage and current.
    2. Turn off the lamp and replace the 25 W bulb with the 100 W bulb. Again record voltage and current.
    3. Repeat using the fluorescent light . For all three bulbs, calculate the power being consumed by the bulbs. Are the power consumption values consistent with your expectations?

  2. Resistance and Temperature

    1. Place the 100 W tungsten filament bulb in the holder. Using the variable transformer (Variac), record the current and voltage at settings of approximately 20 VAC, 60 VAC and 120 VAC. Record from the voltmeter, not the Variac. Calculate the power consumption and resistance at each of these settings.
    2. Replace the 100 W bulb with the special carbon filament bulb. Calculate the power consumption and resistance at the same settings as above. What qualitative statement can you make about the temperature dependence of the resistance of the two types of filaments?

  3. Fan and Air Supply Unit

    1. The air supply unit is used for the air tracks in the lab. It consists of a motor which turns an internal fanblade. Its operation is quite similar to any common vacuum cleaner. Turn the air supply on while carefully watching the ammeter. Write down your observations, estimating the intial and final currents. Explain the behavior in terms of the back emf created by the motor.
    2. While the unit is running, place your hand over the air outlet and observe the ammeter and voltmeter. What conclusion would make concerning the power consumption of the unit with and without a "load".
    3. Repeat procedure "a" with the fan. Unlike the air supply, you can more easily read the initial current by using a metal spatula to hold the fan blade still. Quickly read the ammeter and then release the blade and note the final value.

  4. Microwave and Electric Heating Efficiencies

    This section consists of two stations, one with a microwave oven and the other with an electric "hot plate" heater. You will use both to heat a small sample of water. You will determine the efficiencies of each, defined by the ratio of heat energy produced in the water divided by electrical energy consumed. The electric energy consumed is simply the electrical power times the time. That is,

    Electrical Energy = Power x time = IV x t.

    The heat energy produced is directly related to the increase in temperature of the water. The heat energy required to raise the temperature one gram of water by 1 oC is 4.186 Joules.

    Microwave Oven

    1. Place 200 gm of water in a glass beaker and measure its temperature in oC with the thermometer. Place the beaker in the micowave oven and set the oven for 30 seconds at full power. Record the current and voltage to the oven. Calculate the electrical energy consumption. You may assume the oven timer is accurate.
    2. Remove the beaker of water and insert the thermometer. Gently stir the water and allow the thermometer a minute or so to come to equilibrium.
    3. Multiply the mass times the change in temperature times 4.186 J/gmoC to find the heat energy produced. Compute the heating efficiency of the microwave oven.

    Hot Plate

    1. Turn the hot plate heater to 'Hi" and let it warm up for at least 5 minutes.
    2. Place 200 gm of water in an aluminum beaker and measure its temperature in oC with the thermometer.
    3. Place it on the hot plate and start the stop watch. Monitor the current and voltage to the hot plate. Heat the water for 2 minutes. Remove the aluminum beaker using an oven mitt, place it in the holder and measure the temperature as before. (The holder will minimize heat loss while making measurements.)
    4. Calculate the electrical energy consumption, the heat energy produced and the efficiency of the hot plate.

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