Physci text 06 Heat

06 Temperature and heat

Heat energy, moving heat energy, and temperatures

Heat is another form of energy. Previously we worked with potential energy PE, the energy an object of mass mass m possesses for being a height h above a surface in gravitational field g. GPE = mgh. We also worked with kinetic energy, the energy of motion. Kinetic energy KE = 05mv².

There are three way heat energy is transferred (moved).

•     Radiation: heat carried as wave of light. Use a flat cardboard square covered with aluminum foil to reflect solar radiation. One can feel the heat of the sun reflected by the foil.

•     Convection: heat carried upward by rising fluids. Warm air rises. Warm water rises to the surface of the lagoon. Water vapor generated by a pot of boiling water rises. Convection is important in the fields of weather and oceanography. Convection in the atmosphere generates clouds. In the tropics, convection is almost the exclusive generator of low level clouds.

•     Conduction: the transfer of heat through a solid substance or between two substances.

Temperature is not the same thing as heat. Temperature is a measure of the kinetic energy within the molecules of a substance or fluid. Temperature is related to heat. Heat energy moves from places at a higher temperature to places at a lower temperature.

Temperatures

The following are some of the temperatures encountered locally or which were particularly relevant in 2021.

•     Storage temperature Pfizer Covid-19 vaccine: -70℃

•     Storage temperature Moderna Covid-19 vaccine: -20℃

•     Inside a kitchen freezer in a home: -3℃

•     Melting ice water temperature: 0℃

•     Storage temperature Johnson & Johnson Covid-19 vaccine: 2℃

•     Recommended temperature in a refrigerator: around 4℃

•     Melting "frozen" coconut oil temperature: around 23℃

•     Night time low temperatures:  around 26℃

•     Annual average temperature and temperature of the ocean around the islands: 27℃ to 28℃

•     Room temperature daytime:  around 29℃

•     Outdoor temperature sunny day: 30℃ to 32℃

•     Human body temperature: 37℃

•     Asphalt road in the midday sun: up to 50℃

•     Sauna: around 75℃ depending on the thermostat setting

•     Boiling water temperature: 100℃

•     Oil for cooking french fries: 170℃ to 200℃

Laboratory 06: Newton's law of cooling

Have the student pour boiling water into an uncovered cup. Record the temperature for up to an hour. The exploration is whether Newton's law of cooling well accounts for the cooling behavior. In the following formula t₁ is the time in minutes, T₁ is the temperature in Celsius, and T₀ is the final temperature of the cooling water. b is the base for the exponential and will be less than one. T₀ is theoretically room temperature.

T₁=(100−T₀)b^(t₁)+T₀

Where  is the starting temperature and T₁ is the ending temperature. Note that the model fits best if the formula is entered as seen and Desmos is permitted to optimize both T₀ and the base b. Discussion of results can be enhanced by being able to display all of the cooling curves on large monitor.

While Newton's law does not strictly apply to convective cooling, the system is reasonably well modeled by the law.

Use the following structure for the equation. Subscripts on mobile are accessed from the alpha keyboard.