Wednesday, Feb 20, 2008

Collected Oleic Acid Lab Reports, RA 14.3

Introductory lecture on Heat and Temperature

Temperature: Measure of how "hot" or "cold" an object is. On a microscopic scale, it is proportional to the random kinetic energy of the particles.
This idea is powerful. In a room full of air, all the molecules, on average, have the same KE. Since an oxygen molecule has more mass than a nitrogen molecule but they have, on average, the same KE, you can compare the average speeds of the molecules. The nitrogen molecule is moving faster by a factor of the square root of the ratio of their atomic masses.

Internal Energy: This is the total random energy of all the particles in a substance. It includes the random KE (jiggling, rotation, vibration) as well as associated PE due to vibration and intermolecular forces.

Heat: Energy that is transferred from one object to another due to a difference in temperature. Note that the Atlantic Ocean has way more internal energy than a cup of hot coffee but if you mix the two, energy will be transferred from the coffee to the ocean.

Temperature scales: Discussed Fahrenheit, Rankin, Celsius, Kelvin - what the temperatures are for each at absolute zero, freezing point of water, boiling point of water, and how you can sus out how to convert from one scale to another.
Showed a picture of the range of temperatures.
Discussed how absolute zero can be determined using a constant volume thermometer and extrapolating to zero pressure.

Heat Capacity, Specific Heat Capacity: This is a type of thermal inertia - a measure of how much the substance resists a change in temperature for a given amount of heat energy. The greater the heat capacity, the greater the thermal inertia and the smaller the change in temperature for a given amount of heat energy. Water has a large specific heat capacity = 4180 J/kg-K. Demo of heat capacity.

Lead had a much smaller specific heat capacity than aluminum since, for the same mass, there are many more atoms of aluminum. If you supply a quantity of heat energy, it has to be spread among the many atoms of aluminum, so each atom gets less energy and thus a smaller increase in temperature.

Showed how degrees of freedom affect specific heat capacity and showed a graph of when (at what temperatures) the rotational and vibration modes kick in for hydrogen gas. Because a diatomic gas has more degrees of freedom than a monoatomic gas, it has a greater heat capacity.

Discussed Heat Capacity Lab that students will do on Friday.

Linear expansion: Most substances increase in size when heated since the particles are jiggling more and take up more space. If you have a rod of length L, for a change in temperature delta T it will increase in length by delta L. If you have two identical rods, each will increase in length the same amount. If you put them together to make a rod twice a big, the expansion is twice as much. Showed how to derive the equation for linear expansion: delta L = alpha * L * delta T where alpha is the coefficient of linear expansion that depends on the material.

Demonstrated expansion using bimetallic strip (brass and steel) and ball and ring.

Derived area and volume expansion equations.

If you heat a hole, it also expands.

Demoed ivory soap and CD in microwave.

Homework: work on WebAssign problems.