The goal of this experiment is to understand, in a concrete way, what specific heat capacity is and how it can be measured.
You will need a microwave oven, a styrofoam coffee cup, and a clock or watch.
In the range of temperature where water is a liquid (0 C to 100 C), it is approximately true that it takes 4.2 J of energy (1
calorie) to raise the temperature of 1 gram of water through 1 degree Kelvin. To measure this specific heat capacity of
water, we need some way to raise a known mass of water from a known initial temperature to a final temperature that can
also be measured, while we keep track of the energy supplied to the water. One way to do this, as discussed in the text, is
to put water in a wellinsulated container within which a heater, whose power output is known, warms up the water. In this
experiment, instead of a wellinsulated box with a heater, we will use microwave power, which preferentially warms up
water by exciting rotational modes of the water molecules, as opposed to burners or heaters that warm up water in a pan by
first warming up the pan.
Use a styrofoam coffee cup of known volume in which water can be warmed up. The density of water is 1 gram/cm3.
It is a good idea not to fill the cup completely full, because this makes it more likely to spill.
One method of recording the initial temperature of the water is to get water from the faucet and wait for it to equilibrate
with room temperature (which can either be read off a thermostat or estimated based on past experience). After waiting
about a half hour for this to happen, place the cup in the microwave oven and turn on the oven at maximum power. The
cup needs to be watched as it warms up, so that when the water starts to boil, the elapsed time can be noted accurately.
BE CAREFUL! A styrofoam cup full of hot liquid can buckle if you hold it near the rim. Hold the cup near
the bottom. If the cup is full, do not attempt to move the cup while the water is hot. A spill can cause a
painful burn.
On the back of the microwave oven (or inside the front door), there is usually a sticker with specifications that says “Output
Power = …Watts” which can be used to calculate the energy supplied. If there is no indication, use a typical value of 600
watts for a standard microwave oven. Using all the quantities measured above and knowing the temperature interval over
which you have warmed up the water, you can calculate the specific heat capacity of water.
(a) Show and explain all your data and calculations, and compare with the accepted value for water (4.2 J/K per gram).
(b) Discuss why your result might be expected to differ from the accepted value. For each effect that you consider, state
whether this effect would lead to a result that is larger or smaller than the accepted value.