2.1: Types of Heat Transfer

Heat Transfer

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

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

    

Figure \(\PageIndex{1}\): The net direction heat travels is from warm object to 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 2.1.1 and 2.1.2 illustrate the general nature of heat transfer. When two objects of different temperatures are next to one another in Figure 2.1.1, heat travels from the hot object to the cold object until the two objects have the same temperature (in Figure 2.1.2). There are three 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 types of heat transfer is provided in Figure 2.1.3. 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 serves as an intermediate object that allows heat to transfer from the flame to the person's hand. Wearing a thick glove or having a wooden handle on the skewer can insulate the hand from the heat transfer. Air itself is a poor conductor. However, when air comes into direct contact with the flame, it heats up, becoming less dense than the surrounding air and causing it to rise. This, in turn, forces the cooler air surrounding the warm air to sink to the flame, where it then gets heated. This transfer of warm air rising and cold air sinking is called Convection. This distributes heat throughout the atmosphere and is key to warming our atmosphere. Finally, the heat that radiates from the flame is referred to as 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, and it can also travel through empty space, known as a vacuum.

 

Figure \(\PageIndex{2}\): Different Types of Heat Transfers. (CC BY 4.0; Cmglee Kmecfiunit via WikiMedia Commons) Alternative description of image

Reflect and Respond Question

Instructions: Consider the following processes. Are they conduction, convection, or radiation, and why?

1. Steam rising off of a cup of coffee
2. Touching your computer, and feeling warmth as you touch it
3. Burning your feet when walking on a hot concrete surface
4. A hot air balloon
5. Being warmed by standing out in the sun.