THERMODYNAMICS

THERMAL ENERGY

Heat is what allows molecules to constantly be in motion. An increase in heat will result in an increase in motion, and vice versa. Through each state (solid, liquid, gas), the transfer of heat varies in method.

All methods of energy transfer are forms of work being done - including thermal energy transfer.

Thermal/heat energy - the energy produced when a rise in temperature causes the molecules of matter to collide at faster speeds

Heat energy can be transferred from bodies of masses in three ways:

Conduction - the transfer of internal energy within a body via the collisions of particles and movement of electrons; heat will from a hotter region to a colder region via conduction
Convection - the movement of thermal energy (heat) through fluids, such as gases or liquids
Radiation - the emission of electromagnetic waves from all matter with a temperature above absolute zero, regardless of medium; occurs when thermal energy converts to electromagnetic energy

Absolute zero - the temperature at which a thermodynamic system has the lowest possible energy; −273.15 °C

Nothing in the universe (as far as we know) has reached absolute zero.

CONDUCTION

Through solids, the transfer of heat is called conduction. Solids are heat conductors because of the close proximity between particles. Metals are some of the best thermal conductors.

Thermal expansion - when a rise in temperature causes molecules to begin to move faster and further apart

Thermal equilibrium - the condition under which two substances in physics contact exchange no heat energy

Conduction occurs when two objects with varying temperatures come into contact with one another. As the heat energy in molecules increases, the mass will undergo thermal expansion. Thermal energy is constantly in motion, so in the case of two objects in contact (one with greater heat energy), heat will move out of the warmer object into the cooler object, until the system reaches thermal equilibrium. Heat always moves out of warmer objects/regions and into cooler objects/regions.

Conduction occurs due to the fact that warmer objects have a greater kinetic energy, therefore, they vibrate at a greater speed. When the masses make contact, the object with faster moving particles (in other words, the object with more thermal energy) will cause the other object's particles to move faster as well, via transfer of heat energy.

CONVECTION

Convection describes the movement of heat through fluids (gases and liquids).

Convection current/convection heat transfer - the circulating path of fluid particles that results from the process of continuous heating up of liquids or gases via convection

As heat is applied to a space, the air molecules near the source of heat begin to gain energy, colliding with one another and spacing apart. As they spread out, the heated gas becomes less dense, and rises. When warm air rises, the cooler air has the opportunity to move towards the heat source and obtain thermal energy. Meanwhile, the warm air cools down due to distance from the heat source, becomes dense, and descends back to the level as the previously cool air. The previously cool air, now heated, rises, and the cycle thus continues.

This cycle can also occur in water bodies (or other liquids), as convection currents occur in all sorts of fluids.

RADIATION

Radiation is the process by which thermal energy is transferred through electromagnetic waves.

While both convection and conduction require particles to transfer energy, it is also possible that energy be transferred through a vacuum and not require any matter via which the transfer occurs. We know this because the sun transmits thermal energy to the Earth. All objects with mass transmit thermal radiation via infrared waves.

Infrared radiation - the electromagnetic radiation that can transfer heat energy; the wavelengths are longer than those of visible light but shorter than radio waves

If an object’s rate of absorption and emission of infrared radiation is equal, the temperature of the object remains the same. If an object absorbs more radiation than it emits, the temperature of the object will rise. If an object emits more radiation than it absorbs, the temperature of the object will fall.

Emission of infrared radiation occurs when the molecules of an object change their rotational-vibrational movements; as an object heats up, the kinetic energy of its particles increases and thus the emission of IR also increases. Absorption of infrared radiation is dependent on the frequency at which the atoms of the object in question are vibrating - if they vibrate at the same frequency as IR radiation, the radiation will be absorbed. Substances like water vapor, carbon dioxide, and ozone can efficiently absorb IR radiation.

Vibrational motion - the movement of a body moving back and forth in its general position

MEASURING TEMPERATURE

Apparatus used to measure temperature:

Clinical thermometer (mercury)
Thermistor
Thermopole
Laboratory thermometer (alcohol)

Heat causes molecules in mercury to expand, thus the mercury goes ‘up’ the thermometer (the mercury is contained inside the hollowness of the thermometer, known as the bore), and when the temperature goes down, it contracts and the height decreases. The meniscus of mercury is convex (for comparison, water is concave).

Kelvin, Celsius and Fahrenheit - units of measurement of temperature

Calibrating a thermometer:

Ice point is determined (the temperature at which ice forms)
this is done by putting the thermometer in crushed ice and wait for the thermometer level to stabilize. Where the meniscus rests is the ice point.
Steam point is determined (the temperature at which water becomes vapor)
This is done by putting the thermometer in a sealed jar of steam and wait for the temperature to stabilize. Where the meniscus rests is the steam point
Find the length of one division of celsius degree and then divide the length of the thermometer by this