Take two Styrofoam cups, partially fill them with equal amounts of cold water, and drop
equal amounts of ice into each cup to obtain a mixture of ice and water.
a. Stir the water and ice mixture in one of the cups vigorously with a nonmetallic
stirrer until all of the ice has been melted. Note the time that it takes to melt all
the ice.
b. Set the second cup aside and observe it every 10 minutes or so until all the ice has
melted. Note the time that it takes for all the ice to melt.
c. How do the times compare? Where is the energy coming from to melt the ice in each cup.
Have you performed the experiment? What did you find out?
The answer to c. may not be what you think, or what the teacher thinks.
The energy of hand stirring is negligible compared to another source. Stirring affects convective heat transfer rates.
To compare the times it takes to melt the ice in each cup and understand where the energy is coming from, we need to consider the process of melting ice and the factors that influence it.
a. In the first cup, you vigorously stir the water and ice mixture. Stirring aids in the transfer of heat from the surroundings to the ice, speeding up the melting process. The mechanical energy you provide by stirring helps break down the ice crystals faster. Note the time it takes for all the ice to melt in this cup.
b. In the second cup, you set it aside without stirring and observe it every 10 minutes or so. Without any external agitation, the ice will gradually melt due to the energy transfer from the surroundings to the ice. This process is slower than stirring, as there is no additional mechanical energy input to aid the melting process. Note the time it takes for all the ice to melt in this cup as well.
c. Comparing the times, you will likely find that the ice melts faster in the stirred cup than in the cup that was left to melt on its own. This is because stirring increases the rate of heat transfer, allowing more energy to reach the ice and facilitate melting.
The energy to melt the ice in each cup comes from the surrounding environment. When the ice and water mixture is kept in contact with the surroundings, heat energy is transferred from the surroundings to the cup, and from the cup to the ice, causing it to melt. In the stirred cup, the stirring action enhances this heat transfer by increasing the contact between the water, ice, and the surrounding air, leading to a faster melting process. The non-stirred cup still receives energy from the surroundings, but the slower rate of heat transfer results in a slower melting process.
a. When stirring the water and ice mixture in one of the cups vigorously with a nonmetallic stirrer until all of the ice has been melted, note the time it takes for all the ice to melt.
b. Set the second cup aside and observe it every 10 minutes or so until all the ice has melted. Note the time it takes for all the ice to melt.
c. Compare the times taken for the ice to melt in both cups.
In both cases, the energy required to melt the ice is primarily coming from the surrounding temperature. When stirring the mixture vigorously, the energy is transferred more quickly to the ice, accelerating the melting process. In the second cup, the energy transfer happens at a slower rate due to the absence of stirring, thus it takes longer for the ice to melt.