Chapter 5: Problem 20

On a cool night with no wind, people facing a campfire feel a breeze on their backs. Why?

Short Answer

Expert verified

The breeze felt by people near a campfire on a cool, windless night is the result of convection currents generated by the heat from the fire. The fire heats the air above it, which expands and rises. This creates a low-pressure area near the ground, drawing in cooler, denser air from the surrounding areas – in this case, from behind the people near the fire. The movement of this cool air towards the fire, replacing the rising warm air, creates a breeze that people can often feel on their backs when near a campfire. This phenomenon demonstrates how heat transfer and fluid dynamics work in tandem to influence our physical world.

Step by step solution

Identify the relevant physical principles

Heat transfer and fluid dynamics are the relevant principles to consider. We should focus on how the heat from the campfire affects the air around it, ultimately causing a breeze felt by the people nearby. Convection is the main type of heat transfer responsible for the breeze.

Understand convection currents

Convection currents occur when a fluid (such as air) is heated, causing its temperature and density to change. As air is heated, it expands and becomes less dense; this causes the warmer air to rise. Meanwhile, cooler, denser air sinks to replace the rising warm air, creating a continuous circulation of air known as a convection current.

Relate convection currents to the campfire situation

In the case of a campfire, the heat generated by the fire warms the air directly above it. The warm air rises, and the cooler air from the surrounding areas rushes in to replace it. As the cool air comes in contact with the fire, it, too, becomes warmer and rises, perpetuating the convection current.

Explain the breeze felt by people near the campfire

The people facing the campfire have cool air behind them. As the warm air rises above the fire, it creates a low-pressure area near the ground. The cooler, denser air from behind the people is drawn towards the low-pressure area, resulting in a breeze felt on their backs even on a windless night. This movement of cooler air towards the fire is a direct consequence of the convection currents generated by the heat from the campfire.

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On a cool night with no wind, people facing a campfire feel a breeze on their backs. Why?

Short Answer

Step by step solution

Identify the relevant physical principles

Understand convection currents

Relate convection currents to the campfire situation

Explain the breeze felt by people near the campfire

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