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Heat Transfer Examples: Problems & Solutions

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  • 0:04 There Is Only Heat
  • 0:23 Thermal Energy
  • 2:50 Example
  • 4:59 Lesson Summary
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Lesson Transcript
Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Heat is energy and energy can be transferred. Heat spontaneously flows from warmer substances to cooler substances. In this lesson, we will explore how heat energy transfers from warmer objects to cooler objects.

There is Only Heat

Have you ever heard someone say, ''it's cold outside''? In terms of science, this is a fictitious statement, because there is no such thing as ''cold''. Technically, it is more correct to say an object is lacking heat energy. You can only measure the amount of heat in an object, not the amount of ''cold'' in an object. Let's go through how to analyze the transfer of heat.

Thermal Energy

Thermal energy stems from the friction generated between moving particles. Temperature is the measure of the average kinetic energy (energy of movement) of particles. The higher the kinetic energy, the more heat generated. Think about how warm you get when you run around. Your muscles are stretching and contracting, generating friction, which makes you warmer.

Thermal energy flows from warmer substances to cooler substances
coals

The second law of thermodynamics states that heat energy can only spontaneously flow from warmer objects to cooler objects. Imagine taking an ice cube out of the freezer and putting it on the counter. Since the counter contains more thermal energy than the ice cube, its energy moves into the ice cube. The flow of heat energy from the counter to the ice cube will continue until thermal equilibrium has been obtained. Thermal equilibrium is when all objects in a system are the same temperature.

The amount of thermal energy an object possesses can be calculated. Here is the equation:

Q = mcΔT

  • Q is thermal energy in joules (J)
  • m is the mass of the object in grams (g)
  • c is the caloric requirement for the phase of matter the object is in (J/g⋅°C) - for example, frozen water versus liquid water versus gaseous water
  • ΔT is the change in temperature (Tfinal − Tinitial)

Look at this table showing the specific heat of various objects:

Table 1: Specific Heat Values
Water Phases: Solid (≈ 2.1 J/g⋅°C) Liquid (≈ 4.19 J/g⋅°C) Gas (≈ 2.01 J/g⋅°C)
Water Latent Heat of Fusion: 334 J/g
Water Latent Heat of Vaporization: 2230 J/g
Iron: Solid (≈ 0.450 J/g⋅°C)
Copper: Solid (≈ 0.385 J/g⋅°C)
Aluminum: Solid (≈ 0.902 J/g⋅°C)

These values tell us how much energy is required to raise one gram of the substance 1° C. Notice water has three specific heat values for each phase water can exist, and two values with the term latent heat. Latent heat is essentially ''hidden heat''. Water molecules get arranged in a crystal lattice structure when ice is formed. As thermal energy is added to ice at temperatures below zero, the energy warms the ice to 0° C. If more thermal energy is added, it goes into breaking apart the crystal structure of the ice, not into raising the ice's temperature.

When ice is melting its temperature doesn
icecube

A similar process occurs at 100° C. Water molecules stick together similarly to how opposite ends of magnets stick together. At 100° C, the thermal energy added to water goes into vibrating the molecules enough that they detach from each other and enter the gas phase. Since there is no temperature change in the vaporization process, the heat being added is called the latent heat of vaporization.

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