4.1: Heat Transfer

Types of Heat Transfer

Heat is energy that travels between two objects due to differences in their temperature. Imagine you're cooking a pizza in the oven: you put a cold pizza in a warm oven, and heat from the air inside the oven transfers to your pizza, thus cooking it. This example demonstrates the fundamental facts of heat transfer:

• Heat travels from a warm object to a cold object.
• Once the two objects have the same temperature (i.e., thermal equilibrium), heat transfer ceases.

Figure \(\PageIndex{1}\): The net direction heat travels is from a warm object to a cold object. The figure indicates heat transfer from a "Hot Object" in red to a "Cold Object" in blue, and this process is shown via a black arrow with the words "Net Heat Transfer" (CC BY 4.0; Alicia Mullens)

Figure \(\PageIndex{2}\): Once everything has the same temperature, net heat transfer ceases. (CC BY 4.0; Alicia Mullens)

Figures \(\PageIndex{1}\) and \(\PageIndex{2}\) illustrate the general nature of heat transfer. When two objects of different temperatures are next to one another in Figure \(\PageIndex{1}\), heat travels from the hot object to the cold object until the two objects have the same temperature in Figure \(\PageIndex{2}\). There are three primary ways that heat can travel between the warm object and the cold object.

• Conduction: The transfer of heat through direct contact/via an intermediate object.
• Convection: The vertical transfer of heat through a fluid.
• Radiation: The transfer of heat through waves of electromagnetic energy called photons.

An example of the three main types of heat transfer is provided in Figure \(\PageIndex{3}\). When roasting a marshmallow at a bonfire, heat can travel from the fire's flame through the skewer to the person holding the skewer. This is Conduction, where the metal skewer acts as an intermediate object that allows heat from the flame to transfer to the person's hand. Wearing a thick glove or using a wooden-handled skewer can insulate the hand from heat transfer. Air itself is a poor conductor. However, when air comes into direct contact with the flame, it heats up, becomes less dense than the surrounding air, and rises. This, in turn, forces the cooler air surrounding the warm air to sink toward the flame, where it is heated. This transfer of heat via warm air rising and cold air sinking is called Convection. This distributes heat throughout the atmosphere and is key to warming our atmosphere. However, heat can sometimes travel horizontally as well. Warm ocean currents travel horizontally from the equator to the poles, transporting warm water to cooler regions of the Earth. This horizontal physical transport of a fluid and its stored heat from one place to another is called Advection. Since convection contributes to most of the heat transfer on our planet, advection isn't considered a primary mode of heat transfer, but a secondary one. Finally, the heat radiated by the flame is called radiation. If you’ve ever been near a warm object, such as a flame or a heater, and felt heat without touching it or being directly above it, the heat you experienced is radiation. Radiation is the transfer of heat through electromagnetic particles, known as photons. The benefit of radiation is that, unlike conduction, which requires either direct contact or an intermediate object, and convection, which requires a fluid, radiation can travel through nothing. Radiation can travel through various media, such as water, air, and glass, as well as through space, known as a vacuum.

Figure \(\PageIndex{3}\): Different types of heat transfer processes (CC BY 4.0; Cmglee Kmecfiunit via Wikimedia Commons). Alternative description of the image.

Based on this information, let's try to answer the following questions:

1. Provide and describe an example of how each type of heat transfer is present in your everyday life (not counting boiling a pot of water). Please provide one example for each type of heat transfer (conduction, convection, and radiation), and explain how it is an example of that type of heat transfer.

2. When you step out in the sun and feel warmth, the sun is heating you via _________.
   1. conduction
   2. convection
   3. radiation