Thermal Energy Transfers(2025)

Thermal Energy (heat energy, is the energy that comes from the temperature of matter) Transfers

  • Guiding Questions

    • How do macroscopic observations provide a model of the microscopic properties of a substance?

    • How is energy transferred within and between systems?

Molecular Theory of States of Matter

Solid

  • Strong intermolecular forces (attraction or repulsion) hold particles in fixed positions.

  • Particles oscillate (lúc lắc) but cannot move.

  • There is no freedom for particles to move about the solid.

Liquid

  • Weaker intermolecular forces compared to solids.

  • Particles are free to move throughout the liquid.

  • Attractive forces prevent particles from escaping the liquid.

Gas

  • Very little to no attractive forces between particles.

  • Particles only exert force on one another during collisions.

Density

  • Density (rho) is defined as ( \rho = \frac{m}{V} ) where:

    • ( m ) is mass

    • ( V ) is volume

Temperature and Absolute Temperature

  • Temperature

    • Related to the average kinetic energy (energy the object possesses during its motion) of particles.

    • Average kinetic energy can reach zero, termed absolute zero ((-273 \degree C)).

  • Absolute Temperature

    • Measured in kelvin (K).

    • Defined as 0 K at absolute zero.

    • Change in temperature is equivalent in Celsius and Kelvin.

    • Relation for conversion: degrees Celsius to Kelvin = °C + 273.15

Heat and Temperature

  • Heat (Thermal) Energy

    • Energy transferred between materials causing temperature or phase change.

    • Methods of energy transfer:

      • Conduction

      • Convection

      • Thermal radiation

  • Kinetic Energy

    • Related to particle speed; higher speed = more kinetic energy.

    • Only average kinetic energy affects temperature (temperature is a measure of the average kinetic energy of the particles within a substance).

  • Potential Energy (separation)

    • Related to separation between molecules; more separation = higher potential energy.

    • Increases without changing temperature (as distance increases, potential energy increase)

Internal Energy

  • Defined as total potential and kinetic energy a substance possesses:

    • ( \text{Internal Energy} = \text{Kinetic Energy} + \text{Potential Energy} )

    • Q = (mass)(specific heat capacity of ice)(change in time) + mLf + (mass)(specific heat capacity of water)(change in time)

Thermal Equilibrium

—> is the state in which two objects or systems in contact with each other reach the same temperature, resulting in no net (total) heat transfer between them.

  • Temperature determines heat transfer direction.

    • This occurs because the particles in the hotter object have higher average kinetic energy, which causes them to transfer energy to the cooler object, thereby raising its temperature until both reach the same temperature.

  • Heat flows from hotter to cooler objects until thermal equilibrium is reached.

    • Hotter objects have higher kinetic energy because temperature is directly related to the average kinetic energy of the particles within that object. As the temperature of a substance increases, the particles within it move faster and collide more energetically, leading to an increase in average kinetic energy.

Specific Heat Capacity

  • Specific Heat Capacity (c): Energy needed to raise 1 kg of substance by 1 K.

    • Relation: ( Q = mc\Delta T ) where ( Q ) is heat in joules.

    • Different materials and states of matter have distinct heat capacities.

Values for Specific Heat Capacity

Substance

Specific Heat Capacity (J/kg K)

Aluminum

900

Copper

390

Glass

840

Iron (Steel)

450

Silver

230

Marble

860

Wood

1700

Mercury

140

Alcohol

2400

Water (ICE)

2100

Water (LIQUID)

4200

Water (STEAM)

2010

Phase Changes and Internal Energy

  • Internal Energy increases with phase changes:

    • Solid -> Liquid -> Gas

Specific Latent Heat

  • Specific Latent Heat of Fusion (Lf): Energy required to convert 1 kg of solid to liquid at constant temperature.

  • Specific Latent Heat of Vaporization (Lv): Energy required to convert 1 kg of liquid to gas at constant temperature.

  • Melting Point: Temperature solid changes to liquid.

  • Boiling Point: Temperature liquid changes to gas.

Energy Changes During Phase Change

  • Energy added to melt or vaporize is equal to that released during freezing or condensation.

  • Flat regions on phase change graphs indicate potential energy increases, while slopes indicate kinetic energy increases.

Equation for Specific Latent Heat

  • Energy equation:

    • ( Q = mL ) where:

      • ( Q ) is energy supplied (J)

      • ( m ) is mass (kg)

      • ( L ) is specific latent heat (J/kg)

Practical Applications of Heat Transfer

Electrical Method

  • Electrical heating elements define heat transfer over time.

    • Power of a heating element indicates transfer rate.

    • Example: 10W heater transfers 600 J in 1 minute.

Method of Mixtures

  • Thermal equilibrium can find specific heat capacities through mixing.

    • Example: Metal in boiling water transfers heat to water of different temperature.

Example Problem

  • Calculation for specific heat capacity using the method of mixtures can relate energy exchanged between substances.

  • For example, using the equation ( Q_{metal} = Q_{water} ) where:

    • ( C_{metal} m_{metal} \Delta T_{metal} = C_{water} m_{water} \Delta T_{water} )

    • Values and mass specific to the experiment involved.