Pressure behaves differently in solids, liquids and gases, and IGCSE Physics requires students to understand and apply separate formulas and concepts for each state of matter.
1. Pressure in Solids
Pressure exerted by a solid is calculated using P = F/A, where F is force in newtons and A is the contact area in square metres, giving pressure in pascals. A smaller contact area results in greater pressure for the same force, which explains why sharp knives cut more easily than blunt ones.
2. Pressure in Liquids
Pressure in a liquid increases with depth and is calculated using P = hρg, where h is depth, ρ is the liquid's density, and g is gravitational field strength. This explains why water pressure is greater at the bottom of a swimming pool than near the surface.
3. Pressure in Gases
Gas pressure results from gas particles colliding with the walls of their container. According to the kinetic theory, increasing temperature increases particle speed and collision frequency, which increases pressure if volume remains constant.
4. Atmospheric Pressure
Atmospheric pressure is caused by the weight of air above the Earth's surface and decreases with altitude, which is why it becomes harder to breathe at high altitudes such as on mountains.
Frequently Asked Questions
What is the SI unit of pressure?
The SI unit of pressure is the pascal (Pa), equivalent to one newton per square metre.
Why does pressure increase with depth in a liquid?
Pressure increases with depth because there is more liquid above pressing down, increasing the weight and therefore the force acting on a given area.
Conclusion
Recognising which pressure formula applies to solids, liquids or gases is essential for accurately answering IGCSE Physics questions across this varied topic.