Heat and Change of State

Heat

Change of State

• Occurs when a substance absorbs or releases energy, resulting in a change from one state (solid, liquid, gas) to another
• Types of change of state:
  • Melting: solid to liquid
  • Freezing: liquid to solid
  • Evaporation: liquid to gas
  • Condensation: gas to liquid
  • Sublimation: solid to gas (e.g., dry ice)
  • Deposition: gas to solid (e.g., frost)

Temperature Measurement

• Temperature scales:
  • Celsius (°C): water freezes at 0°C, boils at 100°C
  • Fahrenheit (°F): water freezes at 32°F, boils at 212°F
  • Kelvin (K): absolute temperature scale, 0 K = -273.15°C
• Temperature measurement devices:
  • Thermometers (mercury, digital, infrared)
  • Thermocouples
  • Resistance temperature detectors (RTDs)

Heat Transfer

• Methods of heat transfer:
  • Conduction: direct contact between particles
    • Example: metal spoon in hot soup
  • Convection: movement of fluids ( liquids or gases)
    • Example: heating a room with a radiator
  • Radiation: electromagnetic waves
    • Example: heat from the sun
• Heat transfer processes:
  • Heat conduction: heat flows from higher to lower temperature
  • Heat convection: heat flows with the movement of fluids
  • Heat radiation: heat flows through electromagnetic waves

Heat

• Definition: energy transferred from one body to another due to a temperature difference
• Units of heat:
  • Joules (J)
  • Calories (cal)
  • British thermal units (BTU)
• Heat capacity:
  • Specific heat capacity: amount of heat required to change the temperature of 1 kg of a substance by 1°C
  • Molar heat capacity: amount of heat required to change the temperature of 1 mole of a substance by 1°C

Heat

Change of State

• A substance changes state when it absorbs or releases energy, resulting in a phase transition (solid, liquid, gas)
• Melting occurs when a solid absorbs energy and becomes a liquid
• Freezing occurs when a liquid releases energy and becomes a solid
• Evaporation occurs when a liquid absorbs energy and becomes a gas
• Condensation occurs when a gas releases energy and becomes a liquid
• Sublimation occurs when a solid changes directly to a gas, e.g., dry ice
• Deposition occurs when a gas changes directly to a solid, e.g., frost

Temperature Measurement

• Celsius scale: water freezes at 0°C, boils at 100°C
• Fahrenheit scale: water freezes at 32°F, boils at 212°F
• Kelvin scale: absolute temperature scale, 0 K = -273.15°C
• Thermometers measure temperature using mercury, digital, or infrared technology
• Thermocouples measure temperature using the Seebeck effect
• Resistance temperature detectors (RTDs) measure temperature using changes in electrical resistance

Heat Transfer

• Conduction occurs when heat is transferred between particles in direct contact
• Convection occurs when heat is transferred through the movement of fluids (liquids or gases)
• Radiation occurs when heat is transferred through electromagnetic waves
• Heat always flows from higher to lower temperature
• Conduction is the primary method of heat transfer in solids
• Convection is the primary method of heat transfer in fluids
• Radiation is the primary method of heat transfer in gases

Heat

• Heat is the energy transferred from one body to another due to a temperature difference
• Joules (J) are the SI unit of heat
• Calories (cal) are a unit of heat, often used in nutrition and dietetics
• British thermal units (BTU) are a unit of heat, often used in engineering and HVAC
• Heat capacity is the amount of heat required to change the temperature of a substance
• Specific heat capacity is the heat capacity per unit mass of a substance
• Molar heat capacity is the heat capacity per mole of a substance