Physics Heat Transfer and Phase Changes Quiz

Heat Transfer

• Heat transfer is the rate at which thermal energy moves between a system and its surroundings.
• There are three main mechanisms: conduction, convection, and radiation.

Heat Conduction

• Conduction is the direct transfer of heat through a material.
• It's described quantitatively as the flow of heat energy through a material.
• This process involves an exchange of kinetic energy between the molecules. Less energetic particles gain energy from collisions with more energetic particles.
• Heat flow is directly proportional to the cross-sectional area and temperature difference, but inversely proportional to the length of the material.
• The constant of proportionality is the thermal conductivity (k).

Application of Heat Conduction

• Insulated windows use conductive properties of materials like air to reduce energy loss or gain for homes.
• Biological systems like counter-current exchange transfer heat efficiently.

Heat Convection

• Convection is the transfer of heat through a fluid (liquid or gas).
• Uneven heating leads to density differences in fluids, causing warmer parts to rise and cooler parts to sink.
• This flow of fluid carries heat throughout the system.
• Natural convection occurs due to density differences.
• Forced convection uses fans or pumps to move heated substances.

Heat Radiation

• All objects radiate energy in the form of electromagnetic waves (including visible light, infrared and ultraviolet).
• This energy transfer doesn't require a medium.
• The rate of radiated power depends on the object's surface area, emissivity (a measure between 0 and 1 of how effectively the object radiates), and temperature.
• Black objects have higher emissivity values than white or light-colored objects.
• Stefan-Boltzmann law quantifies radiated power by an object. The power of radiated body (P) is proportional to the surface area (A), emissivity (e), and fourth power of the absolute temperature (T^4) with a constant (σ).

Latent Heat

• Latent heat (L), is the thermal energy required to change the phase of a substance with no change in temperature.
• When a substance changes phase (solid to liquid, liquid to gas, etc.), latent heat is either absorbed or released.
• Different processes require different latent heats (latent heat of fusion, vaporization, sublimation).

Application of Latent Heat

• Adding salt to ice-water lowers the melting point of the ice, which is helpful in ice cream making.

Force in the Direction of Displacement

• Work (W) is calculated as the product of force (F) and the displacement (d) when the force is in the direction of the displacement.
• SI unit of work is joule (J). This is force multiplied by displacement.

Force at an Angle to the Displacement

• When force is applied at an angle to the displacement, the component of force in the direction of displacement is used to calculate work. Work is determined by the product of the force component (F cos θ) with the displacement(d).
• The angle (θ) between the force and displacement is crucial to the calculation. Positive work occurs in an angle range between -90° and 90°, Zero work occurs at angles of ±90° and negative work at an angle range between 90° and 270°.

The Power

• Power (P) is defined as the rate of doing work or the rate of energy transfer.
• calculated as work (W) divided by time (t).
• SI unit is watt (W).

The Energy

• Kinetic energy (K) is the energy an object possesses due to its motion, determined by the object's mass and velocity.
• Potential energy (U) is stored energy dependent on an object's position in a frame of reference (e.g., gravitational force).

Mechanical Energy

• Mechanical energy (E) is the sum of potential and kinetic energies within a system.
• Mechanical energy is conserved, meaning it remains constant if there's no input or output of energy to/from the system.

The First Law of Thermodynamics

• The first law of thermodynamics states energy principle of a system, stating the difference between the heat and work performed in a thermodynamic system.
• The change in internal energy (ΔU) of a closed system is equal to the heat added to the system (Q) minus the work done by the system (W). (ΔU = Q – W)

Metabolic rate

• Metabolic rate is the rate of change in internal energy.
• It can be measured by observing the rate at which a person uses oxygen in converting food into energy and waste materials.
• The metabolic rate per unit mass is calculated using the rate of oxygen consumption.
• The energy equivalent of oxygen helps understand how oxygen consumption is utilized to provide energy.

The Energy Content per unit mass

• Ratio of the energy released divided by mass, typically in units of kJ/g.

The efficiency of food utilization

• The percentage of energy transformed from food into mechanical work.