Physics O Level/ IGSCE

Pressure

Pressure is the force exerted on the object per unit area (of the object).

P = F/A

Where P is the Pressure, F is the force (at right angles to the area) and A is the area on which the force is exerted.

Pressure is measured in Pascals(Pa).

Thus alternatively, the pressure at a surface produces a force in direction at right angles to the surface.

Solids

Solids exert pressure on the object beneath them (like ground for example) due to their weight.

Thus the greater the mass of the object, the greater the weight and the greater the pressure.

Also the greater the surface area of the solid, the less pressure it exerts.

Liquids

The pressure in a liquid:

• At any point, is equal in all directions

• Depends only on the height of the surface of the liquid.

• Pressure due to a liquid is give by: p =hρg

• The total pressure inside a liquid will also include the atmospheric pressure

1. The greater the density of a liquid, the greater the pressure exerted by the liquid.

2. The greater the gravitational acceleration, the greater the pressure exerted.

3. The greater the height of the liquid column the greater the pressure exerted.

A manometer is used to measure the atmospheric pressure. An apparatus is set up.

Atmospheric air exerts pressure on the open sides and causes the mercury inside the column to rise up. The mercury that rises up then in turn starts to exert pressure due to its height above the liquid level. It rises until the pressure exerted by mercury is equal to the pressure exerted on the open ends by the atmospheric air.

At this point, the pressure exerted by the mercury column is equal to the atmospheric pressure. The pressure of the mercury column is then found from the equation: P = Δhρg.

Atmospheric pressure is equal to 101,325 Pa or 101 kPa which is then rounded off to 100 kPa. This equals about 760mm of the height of the mercury column in a barometer.

Gases

In gases, the molecules are constantly moving. When placed inside a container, they end up exerting a force on the walls of containers. This force results in pressure being applied on the walls of the container.

The pressure of a gas mainly depends on:

• The frequency of collisions of molecules with the walls of containers.

• The force with which the molecules collide with the walls of the container.

• The number of molecules present in the gas.

Factors Affecting Frequency of Collision:

• Speed of the molecules (which is dependent on the temperature)

• Volume of the container

Factors affecting Force of Collision:

• Speed of molecules (which is dependent on its temperature)

As a result we get the following relationships:

1. Pressure ∝ 1/Volume

Increasing the volume decreases the frequency of collisions resulting in increased pressure and vice versa.

Relation is valid provided:

• Temperature is constant

• Number of Molecules is constant.

2. Pressure ∝ Temperature

Increasing the temperature increases the frequency of collisions as well as the force of collisions resulting in increased pressure and vice versa.

Relationship is valid provided:

• Volume is constant
• Number of Molecules is constant

3. Volume ∝ Temperature

Increasing the temperature increases the volume of gas and vice versa.

Relationship is valid provided:

• Pressure is constant
• Number of molecules are constant

Using the first relationship, we get the equation:

• P ∝ 1/V - P = k/V - PV = constant

If the volume or the pressure of a gas at constant temperature changes, the initial and final pressure can be related via equation: P1V1=P2V2