# 13.2: Gas Pressure

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### How are hot air balloons able to move smoothly in the air?

Many people enjoy riding in hot air balloons. Some use them for romantic picnics and marriage proposals. Others race in competitions. Being above the earth gives a whole new perspective on the world around us. As the beginning of a hot air balloon ride, the balloon is flat because the pressure inside the balloon equals the pressure outside. When the air inside the balloon is heated, the speed of movement of those air molecules increases and the pressure goes up. After a while the balloon is completely expanded and the flight is ready to take off.

## Gas Pressure

Pressure is defined as the force per unit area on a surface.

$\text{Pressure} = \frac{\text{force}}{\text{area}}\nonumber$

When a person stands on the floor, his feet exert pressure on the surface. That pressure is related to both the mass of the person and the surface area of his feet. If the person were holding a heavy object, the pressure would increase because of a greater force. Alternatively, if the person stands on his toes, the pressure also increases because of a decrease in the surface area.

Gas molecules also exert pressure. Earth's atmosphere exerts pressure because gravity acts on the huge number of gas particles contained in the atmosphere, holding it in place. Pressure is also exerted by a small sample of gas, such as that which is contained in a balloon. Gas pressure is the pressure that results from collisions of gas particles with an object. Inside the balloon, the gas particles collide with the balloon's inner walls. It is those collisions which keep the balloon inflated. If the gas particles were to suddenly stop moving, the balloon would instantly deflate. The figure below is an illustration of gas particles exerting pressure inside a container.

The pressure inside the hot air balloon is affected by temperature. As the molecules heat up, they move faster and strike the inside wall of the balloon harder. This increased motion of the gas particles increases the force on an area of the balloon, producing a rise in the pressure.

## Summary

• Pressure is defined as $$\frac{\text{force}}{\text{volume}}$$.
• Gas pressure is the result of collisions between gas particles and an object.
• An increase in temperature will produce an increase in pressure of a gas.

## Review

1. What is pressure?
2. How does a gas create pressure on an object or container?
3. What would happen to the pressure if gas particles suddenly stopped moving?
4. How does temperature affect pressure?

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