Heat and Temperature: A Comprehensive Study

Posted on Nov 30, 2024 in Physics

Temperature:

Concept: Temperature is the observable manifestation of the kinetic energy of molecules. It measures the thermal state of a body. The greater the kinetic energy of molecules, the higher the temperature.

Effects: Expansion of a body, changes in electrical resistance or pressure.

Thermometers:

A thermometer is a device used to measure temperature. It functions on the principle of thermal equilibrium, meaning the thermometer reaches the same temperature as the object it contacts.

Clinical thermometers have a bottleneck that prevents mercury from dropping back down. They are graduated between 35°C and 42°C, recording the maximum measured temperature.

Temperature Scales:

• Celsius (Centigrade): Measured in degrees Celsius (°C).
• Kelvin: The absolute scale, where 0K corresponds to -273.15°C (the lowest thermal state). To convert from Celsius to Kelvin, add 273.15.
• Fahrenheit: 0°C = 32°F and 100°C = 212°F.

Thermal Expansion:

One effect of temperature change is a change in an object’s size. This phenomenon is called thermal expansion.

It depends on:

• Temperature change
• The volume of the body
• The body’s chemical composition (e.g., metal vs. plastic)

The coefficient of expansion expresses the change in volume per unit of initial volume for each degree of temperature change (linear, volumetric, or superficial).

Liquid Expansion:

The volume thermal expansion of a liquid follows a similar principle to that of solids. A change in liquid volume is directly proportional to the initial volume and the temperature variation. If a bottle filled with liquid to the brim is heated, some liquid will spill out. The displaced liquid does not represent the actual expansion of the liquid, as the container also expands. The displaced liquid represents the apparent expansion. Therefore:

Expansion of Water (Anomaly):

• If 1 kg of water at 0°C is heated, it contracts until it reaches 4°C (maximum density) and then expands.

Anomaly of Water in Lakes:

1. Lake water is initially at 14°C.
2. Air cools, causing the surface water temperature to decrease.
3. When surface water reaches 4°C, it sinks because it is denser than the warmer water below.
4. This process continues: surface water cools to 4°C, sinks, and is replaced by warmer water, which then cools and sinks.
5. Once all the water reaches 4°C, the surface temperature can drop to 0°C and freeze without sinking. Due to this anomaly, lakes freeze from the surface downwards, allowing aquatic life to survive in the liquid water below the ice.

Heat:

Heat is the energy transferred between objects due to a temperature difference.

Thermal Equilibrium:

Two bodies in thermal contact are in thermal equilibrium if there is no heat flow between them.

Calorimetry:

Calorimetry is the quantitative measurement of heat exchange. A calorimeter is used for these measurements, allowing for the calculation of specific heats. A bomb calorimeter measures the heat released during combustion. Applications include determining the caloric content of food and the heat of combustion of substances.

Amount of Heat:

The amount of heat (Q) needed to change the temperature of a substance depends on its mass (m), the temperature change (Δt), and the substance’s specific heat (Ce):

One calorie is the amount of heat required to raise the temperature of one gram of water by 1°C (from 14.5°C to 15.5°C). These specific values are used because the required heat varies at different temperatures.

Specific Heat:

Specific heat is the amount of heat needed to change the temperature of one gram of a substance by 1°C.

It is measured in:

Fundamental Theorem of Calorimetry:

Latent Heat:

Latent heat is the amount of energy required to change the state of one gram of a substance. It depends on the substance and the type of state change.

The Six State Changes:

• Gas to Liquid: Condensation
• Liquid to Gas: Vaporization
• Solid to Liquid: Melting (Fusion)
• Liquid to Solid: Solidification (Freezing)
• Solid to Gas: Sublimation
• Gas to Solid: Deposition

Heat Transfer:

• Conduction: Heat transfer through a material (e.g., a metal rod heated at one end).
• Convection: Heat transfer through fluid movement due to density differences (e.g., heating water).
• Radiation: Heat transfer through electromagnetic waves (e.g., a quartz oven or the sun).

Change of State:

During a change of state, all energy supplied to a system is used to overcome intermolecular forces, resulting in a constant temperature.

The amount of heat required for a change of state is given by:

Cl represents latent heat.