heat transfer

Examples of heat transfer in the following topics:

• Overview of Heat

  • This module defines and explores heat transfer, its effects, and the methods by which heat is transferred.
  • After defining and quantifying heat transfer and its effects on physical systems, we will discuss the methods by which heat is transferred.
  • So many processes involve heat transfer, so that it is hard to imagine a situation where no heat transfer occurs.
  • Yet every process involving heat transfer takes place by only three methods:
  • Convection is the heat transfer by the macroscopic movement of a fluid.

• The First Law

  • The 1st law of thermodynamics states that internal energy change of a system equals net heat transfer minus net work done by the system.
  • Heat engines are a good example of this—heat transfer into them takes place so that they can do work.
  • Q represents the net heat transfer—it is the sum of all heat transfers into and out of the system.
  • Q is positive for net heat transfer into the system.
  • Explain how the net heat transferred and net work done in a system relate to the first law of thermodynamics

• Heat Engines

  • Heat transfer from the hot object (or hot reservoir) is denoted as Qh, while heat transfer into the cold object (or cold reservoir) is Qc, and the work done by the engine is W.
  • Thus the net work done by the system equals the net heat transfer into the system, or
  • We define the efficiency of a heat engine (Eff) to be its net work output W divided by heat transfer to the engine Qh:
  • (b) A heat engine, represented here by a circle, uses part of the heat transfer to do work.
  • Qh is the heat transfer out of the hot reservoir, W is the work output, and Qc is the heat transfer into the cold reservoir.

• Work

  • For closed systems, energy changes in a system other than as work transfer are as heat.
  • Heat transfer, a less organized process, is driven by temperature differences.
  • Heat transfer and work are both energy in transit—neither is stored as such in a system.
  • An isobaric expansion of a gas requires heat transfer during the expansion to keep the pressure constant.
  • Analyze the necessity to exclude energy transferred between system as heat from mechanical work

• Heat Pumps and Refrigerators

  • A heat pump is a device that transfers heat energy from a heat source to a heat sink against a temperature gradient.
  • Heat pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot.
  • Heat transfer (Qc) occurs from a cold reservoir and into a hot one.
  • Thus the heat transfer to the hot reservoir is Qh=Qc+W.
  • As with heat pumps, work input is required for heat transfer from cold to hot.

• Conduction

  • Conduction is the transfer of heat through stationary matter by physical contact.
  • Heat transfer from the left side to the right side is accomplished by a series of molecular collisions.
  • The thicker the material, the more time it takes to transfer the same amount of heat.
  • The rate of conductive heat transfer through a slab of material, such as the one in the figure above is given by
  • The rate of heat transfer is inversely proportional to the thickness d.

• The Second Law

  • The second law of thermodynamics states that heat transfer occurs spontaneously only from higher to lower temperature bodies.
  • For example, heat involves the transfer of energy from higher to lower temperature.
  • A cold object in contact with a hot one never gets colder, transferring heat to the hot object and making it hotter.
  • (a) Heat transfer occurs spontaneously from hot to cold and not from cold to hot.
  • (b) The brakes of this car convert its kinetic energy to heat transfer to the environment.

• Specific Heat

  • For the same substance, the transferred heat also depends on the phase (gas, liquid, or solid).
  • The quantitative relationship between heat transfer and temperature change contains all three factors:
  • (a) The amount of heat transferred is directly proportional to the temperature change.
  • (b) The amount of heat transferred is also directly proportional to the mass.
  • (c) The amount of heat transferred depends on the substance and its phase.

• Convection

  • Example: Calculating Heat Transfer by Convection: Convection of Air Through the Walls of a House.
  • Calculate the heat transfer per unit time in watts needed to warm the incoming cold air by 10.0 ºC, thus replacing the heat transferred by convection alone.
  • The rate of heat transfer is then Q/t, where t is the time for air turnover.
  • Thus, an overall transfer of heat from the ocean to the atmosphere occurs.
  • Convection plays an important role in heat transfer inside this pot of water.

• Heat as Energy Transfer

  • Heat is the spontaneous transfer of energy due to a temperature difference.
  • This observation leads to the following definition of heat: Heat is the spontaneous transfer of energy due to a temperature difference .
  • The total amount of energy transferred as heat is conventionally written as Q for algebraic purposes.
  • We use the phrase "heat transfer" to emphasize its nature.
  • Heat transfer occurs due to the difference in temperatures.