Page 7 - CLASS VII Lesson Notes Forces and Energy

SUBTOPIC: 3.6 PRESSURE IN LIQUIDS AND GASES

Key Words:
pressure, depth, container, collide, atmospheric pressure, altitude, sea level

Pressure in Liquids:
• Liquids exert pressure because their particles are pushed by the
weight of the liquid above.
• The deeper you go, the more liquid is above, so the pressure
increases with depth.
• The pressure acts equally in all directions.
Example:
In a dam, the wall is made thicker at the bottom to withstand the greater pressure caused by deeper
water.

Pressure in Gases:
• Gases are made of particles that move randomly and collide with the walls of a container.
• Every collision exerts a small force.
• Many collisions create a large total force, which causes gas pressure.
• More gas particles or faster-moving particles mean higher pressure.
Example:
Blowing more air into a balloon increases the number of particles → more
collisions → higher pressure → balloon expands.

Pressure and Altitude:
• The atmosphere is made of gases that form a layer around Earth.
• Sea level is where the atmosphere is deepest, so air pressure is highest
there.
• At higher altitudes, there are:
o Fewer gas particles
o Less air above, so the pressure is lower

Atmospheric Pressure Effects:
• Pressure exerted by the particles present in the atmosphere on
us is atmospheric pressure.
• When air is pumped out of a container (creating a vacuum), the
outside pressure crushes it because there's no air inside to
balance it.

Pressure and Temperature in Gases:
• Heating a gas makes particles move faster.
• Faster particles collide more and with greater force → pressure
increases.
Example:
A pressure cooker builds up high pressure by trapping steam, which cooks food faster.

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