Heat and Temperature Notes

Caloric Theory of Heat

  • Until the 19th century, heat was considered an invisible, weightless, self-repellent fluid called 'caloric'.
  • Caloric was believed to flow from hot objects to cold objects.
  • Hot objects were thought to possess more caloric than cold objects.
  • Caloric theory tenets:
    • Caloric flows more easily through metals than non-metals.
    • A body with more caloric expands due to caloric self-repulsion.
    • Heating a nail causes caloric to squeeze to the surface, making it hotter.
    • Smaller objects hold less caloric.
  • Shortcomings of the caloric theory:
    • Failed to explain melting.
    • Failed to explain evaporation.
    • Failed to explain heat produced by friction.

Count Rumford's Contribution

  • Count Rumford observed heat production during the boring of brass cannons using a blunt tool.
  • He recognized that heat was a result of the work being done.
  • Rumford's experiments discredited the caloric theory.
  • He established a link between heat and mechanical work.

Joule's Experiments and Kinetic Theory

  • James Prescott Joule proved that heat is a form of energy, not a material substance.
  • He discovered the Principle of Energy Conservation.
  • The unit of energy, the Joule (J), was named after him.
  • Key observations:
    • Heat flows/moves; it's not contained.
    • Temperature difference causes heat flow, not the amount of energy present.
  • Joule's work led to the abandonment of the caloric theory and the adoption of the "Kinetic Theory of Matter." *Kinetic Theory of Matter:
    • All matter is made up of tiny moving particles called molecules.
    • Molecules attract each other strongly when they are close.
    • Molecules possess kinetic energy due to their motion.
    • Molecules possess potential energy because their motion keeps them separate despite attractive forces.

Internal Energy

  • According to the kinetic theory, all materials (solid, liquid, or gas) have 'internal energy'.
  • Internal energy: The sum of kinetic and potential energies of all particles in an object or system.
  • Heating a substance increases the energy of its particles:
    • Solids: particles vibrate more rapidly.
    • Liquids/Gases: particles move more rapidly.
  • Heating increases the internal energy of the substance.

Evidence for the Kinetic Theory of Matter

  • Motion of Particles:
    • Brownian Motion
    • Diffusion
    • Osmosis
  • Intermolecular Force of Attraction:
    • Surface Tension
    • Cohesion and Adhesion
    • Capillary Action
  • Important Note: Internal energy is NOT heat.

Brownian Motion, Diffusion, and Osmosis

  • Brownian Motion:
    • Jerky, erratic movement of smoke particles.
    • Caused by collisions with invisible air particles in random motion.
  • Diffusion and Osmosis:
    • Substances initially separated become mixed due to the random motion of molecules.
    • Molecules move into each other’s spaces randomly.

Surface Tension, Cohesion, Adhesion, and Capillary Action

  • These phenomena support the existence of forces between molecules.
    • Cohesion: Force between molecules of the same kind.
    • Adhesion: Force between molecules of different kinds.
  • Cohesion of water gives water drops their shape.
  • Adhesion of water to glass creates a meniscus and causes capillary action.

Heat vs. Hot vs. Temperature

  • 'Hot' describes how an object or place feels.
  • Temperature indicates how hot an object or place is.
  • Both 'hotness' and temperature increase when something is heated.
  • 'Hotness' cannot be accurately measured because it's subjective.
  • Temperature is a measure of the average kinetic energy of the molecules.
  • Example: Boiling water
    • 150 ml of water takes longer to boil than 75 ml because there are more molecules to supply energy to.
  • It is important not to confuse the temperature of an object with the total quantity of thermal energy or heat it can give out.
  • A spoon of boiling water, for example, has the exact same temperature as a pan full of boiling water, however you would suffer far less burns from the spoon of boiling water if you were to trip over it.

Temperature

  • Temperature is a measure of the average kinetic energy of the molecules in an object or system.
  • Heat flows when there is a temperature difference.
  • Fast-moving, high-energy molecules collide with slow, low-energy particles.

Temperature Scales

  • Science uses Celsius (℃) or Kelvin (K).
  • Fahrenheit is not commonly used.
  • Celsius Scale:
    • Two fixed points:
      • Ice Point: Melting point of pure ice (0 ℃).
      • Steam Point: Boiling point of pure water under standard atmospheric pressure (100 ℃).
  • Absolute Zero:
    • Theoretically, molecules would have no energy at around −273 ℃.
    • This temperature is called absolute zero.
    • No object can reach a temperature lower than absolute zero.
  • Kelvin Scale:
    • Absolute temperature scale with zero at absolute zero.
    • SI unit for temperature.
    • Each Kelvin (K) is the same size as a degree Celsius (℃).
  • Conversion: K=℃+273K = ℃ + 273
  • Example: 15 ℃ = 15 + 273 = 288 K

Thermometric Properties and Thermometers

  • Thermometric properties: Physical properties that vary with temperature.
    • Expansion of mercury and alcohol.
    • Resistance of wires.
    • Pressure of a gas (at constant volume).
    • Volume of a gas (at constant pressure).
    • Heat emitted by hot objects.

Types of Thermometers

  • Laboratory Thermometers:
    • Mercury-in-glass thermometer
    • Alcohol-in-glass thermometer
  • Clinical Thermometers
  • Industrial Thermometers:
    • Thermocouple thermometer
    • Resistance thermometer

Thermometer Comparison

*Mercury-in-glass thermometer:
* Use: Measure the temperature of things in the laboratory.
* How it works: When the temperature rises the liquid in the bulb expands by moving up the tube next to the scale. Because the tube is narrow a small increase in the volume causes the ‘thread’ to move a long way up the tube.

  • Does not wet sides of tube
    • Thread is easy to see
    • Conducts heat well
    • Responds quickly to temperature changes
    • Not suitable for low arctic temperatures
    • Poisonous [if broken]
    • Expensive
  • Range: −39℃to357℃-39℃ to 357℃ *Alcohol-in-glass thermometer:
    • Use: SAME AS MERCURY
    • How it works: When the temperature rises the liquid in the bulb expands by moving up the tube next to the scale. Because the tube is narrow a small increase in the volume causes the ‘thread’ to move a long way up the tube.
    • Suitable for low Arctic temperatures
    • Expands greater than mercury
    • Has to be colored to be seen easily
    • Clings to sides of tube
    • Thread has a tendency to break
    • Has a narrow range
  • Range: −117℃to79℃-117℃ to 79℃ *Clinical thermometer:
    • Use: Used to measure the temperature of the human body
    • How it works: SAME AS MERCURY
    • Has a constriction which keeps the reading from changing till it is read.
  • Range: 35℃to42℃35℃ to 42℃ *Thermocouple Thermometer:
    • Use: Used for measuring temperature in industrial work
    • Two metal wires are joined together at two junctions. When a temperature difference exist between the junctions a small current is produced in the wire.
      *Advantages:
      *For industrial work it is usually more convenient for the operator to read the temperature on a meter or digital display some distance away from the source of heat.
      *Electrical methods of measure temperature give readings which can be recorded automatically or fed directly to a computer controlling the heating process.
    • Wire junction can be very small allowing it to respond very quickly to changing temperatures
    • Suitable to use in furnaces
  • Range: −200℃to1600℃-200℃ to 1600℃ *Resistance thermometer *Advantages:
    • Use: Used for measuring temperature in industrial work
    • How it works: As temperature rises, the resistance of metal increase and the amount of current flowing decreases.
      *Electrical methods of measure temperature give readings which can be recorded automatically or fed directly to a computer controlling the heating process.
  • Range: −20℃to1200℃-20℃ to 1200℃

Objectives

  • Differentiate between the caloric and kinetic theories of heat as they existed in the eighteenth century.
  • Discuss the role of Joule’s experiments in establishing the principle of conservation of energy
  • Relate temperature to the direction of net energy transfer.
  • Identify physical properties which vary with temperature, and which therefore may be used as the basis for measuring temperature
  • Relate use of a thermometer to its design
  • Define fixed points on the Celsius scale.
  • Relate temperature of a body to the kinetic energy of molecules
  • Distinguish amongst solids, liquids and gases.
  • Use kinetic theory to explain the different macroscopic properties of solids, liquids and gases