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Introduction to Heat Transfer - How Does Heat Transfer?

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Question: Introduction to Heat Transfer - How Does Heat Transfer?
What is heat? How does heat transfer take place? What are the effects on matter when heat transfers from one body to another?
Answer: Heat transfer is a process by which internal energy from one substance transfers to another substance. Thermodynamics is the study of heat transfer and the changes that result from it. An understanding of heat transfer is crucial to analyzing a thermodynamic process, such as those that take place in heat engines and heat pumps.

Forms of Heat Transfer

Under the kinetic theory, the internal energy of a substance is generated from the motion of individual atoms or molecules. Heat energy is the form of energy which transfers this energy from one body or system to another. This heat transfer can take place in a number of ways:

  • Conduction is when heat flows through a heated solid.
  • Convection is when heated particles transfer heat to another substance, such as cooking something in boiling water.
  • Radiation is when heat is transferred through electromagnetic waves, such as from the sun. Radiation can transfer heat through empty space, while the other two methods require some form of matter-on-matter contact for the transfer.
In order for two substances to affect each other they must be in thermal contact with each other. If you leave your oven open while turned on and stand several feet in front of it, you are in thermal contact with the oven and can feel the heat it transfers to you (by convection through the air).

Normally, of course, you do not feel the heat from the oven when you're several feet away and that is because the oven has thermal insulation to keep the heat inside of it, thus preventing thermal contact with the outside of the oven. This is of course not perfect, so if you stand nearby you do feel some heat from the oven.

Thermal equilibrium is when two items that are in thermal contact no longer transfer heat between them.

Effects of Heat Transfer

The basic effect of heat transfer is that the particles of one substance collide with the particles of another substance. The more energetic substance will typically lose internal energy (i.e. "cool down") while the less energetic substance will gain internal energy (i.e. "heat up").

The most blatant effect of this in our day-to-day life is a phase transition, where a substance changes from one state of matter to another, such as ice melting from a solid to a liquid as it absorbs heat. The water contains more internal energy (i.e. the water molecules are moving around faster) than in the ice.

In addition, many substances go through either thermal expansion or thermal contraction as they gain and lose internal energy. Water (and other liquids) often expands as it freezes, which anyone who has put a drink with a cap in the freezer for too long has discovered.

Heat Capacity

The heat capacity of an object helps define how that object's temperature responds to absorbing or transmitting heat. Heat capacity is defined as the change in heat divided by the change in temperature.

Laws of Thermodynamics

Heat transfer is guided by some basic principles which have become known as the laws of thermodynamics, which define how heat transfer relates to work done by a system and place some limitations on what it is possible for a system to achieve.
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