Think what would happen if you were to push a pin on its pointy end. Yes, you would hurt yourself, but why? The pin would push on your finger so much that it would break the bonds between the cells and become physically inserted into your finger. What would happen if you did the same, but this time with the flat end? The pin would go into the wall, this time breaking the bonds of the atoms in the wall and making a nice snug home for itself.
Assuming you applied the same force in both cases, the end result would be different, because of the difference in the pressure applied on your thumb. In the first case, the force is concentrated on a very small area, so the pressure is very high. In the second, the force is spread out over a larger area, so the pressure is lower.
The formula used to calculate pressure is:
pressure = force / area
When the force is in Newton (N) and the area in m2, the unit for pressure is Pascal (Pa) or N/m2. If the area is in cm2, then the pressure is measured in N/cm2.
Let’s think about this now, this means that if you increase the force then the pressure goes up. What will happen if you increase the area though? Well because pressure = force divided by area; this means that pressure is inversely proportional to pressure. This means that as area increases, the pressure will decrease.
Let’s not think about this in terms of gases and individual particles. Use your imagination to visualise how gas particles collide (crash) onto the walls of the containers in the diagram below. Each time they hit the walls of the container; they push it a little bit. Just like if you were to throw a ball at a book, when they collide the book will move back a little bit – it has been pushed. This is where our force comes from in the equation pressure = force/area.
In this example, the second container has a smaller volume, but the number of particles is the same, so the particles collide more, because there is not much space. Remember, every time there is a collision, there is a force small force – so if there are more collisions, what will happen to the total force?
Boyle's Law is used to calculate how much the pressure changes when the volume of a gas changes, provided the temperature and mass of the gas stay the same:
P1 × V1 = P2 × V2
P1 and V1 is the pressure and volume at the start, and P2 and V2 are the final pressure and volume of the gas. If you rapidly change the volume of the container, then you will change the pressure as well as there will be more collisions in a smaller area – both of these are conditions for a higher pressure.