Lab: Electrical equivalent of heat


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    We use electricity to heat water many times in our lives: cooking, cleaning, bathing and so on. How efficient are these heating methods? That's what you are going to determine, using two methods: a hot plate and a microwave oven.

    The Hot Plate part:
    Measure a certain amount of water ( 200 mL) in a beaker and put on the hotplate.
    Put a temp sensor in the water and measure temp for a few minutes to stabilize the temperature.
    Once you are confident it has equilibrated, plug the hot plate into a Kill-a-watt unit, which is plugged into the 120Volt outlet.
    Turn on the hotplate, and start recording the temperature. A good interval might be every few seconds for at least 2 minutes.
    Calculate the slope of the graph, including the units.

    Stuff you need to know:
    The amount of energy needed to change the temperature is:
    Q = mc∆T
    Q = energy in Joules
    m = mass of water in grams (1 ml = 1 cc = 1 cm2 = 1 gram for water)
    c = specific heat of water = 4.18 J/g ˚C
    ∆T = temperature change in °C

    From the Kill-a-watt unit
    • What was the power reading on the hot plate while you were heating the water? This is usually measured in Watts, or joules/sec

    From the data:
    • What was the measured slope of the graph, in what units?
    • At what rate was the electrical energy transformed into heat?
    • Calculate the efficiency of the hot plate.
    The Microwave part:
    Measure about 200 grams of water in a glass beaker.
    Using a temp sensor, record the temperature of the water, as you did above.
    Plug the microwave into a kill-a-watt unit
    Set the microwave oven to 50% power.
    Heat the water for two minutes.
    Measure the change in temperature, record.

    • What was the change in temperature for the water?
    • How much energy was absorbed by the water?
    • How many seconds did the microwave heat the water?
    • How many Watts were used by the microwave oven?
    • Multiply the Watts x seconds, this should be the number of joules
    • How efficient is the hot plate?

    1. List at least three ways energy is lost in the microwave heating
    2. what are three reasons not to use a microwave to cook with
    3. How would you make the hot plate more efficient?
    4. What makes the hot plate so inefficient?
    5. About how much energy is wasted in cooking in our country?