Atmospheric pressure — KCSE Physics

A mercury barometer is an instrument used to measure atmospheric pressure. It consists of a glass tube, about 1 meter long, filled with mercury and inverted into a dish of mercury. The functioning of the barometer can be described in the following steps:

1. Inversion: When the tube is inverted, some mercury flows out into the dish, creating a vacuum at the top of the tube.
2. Pressure Balance: The weight of the mercury column in the tube is balanced by the atmospheric pressure acting on the surface of the mercury in the dish.
3. Measurement: The height of the mercury column, usually measured in millimeters (mm), indicates the atmospheric pressure. Standard atmospheric pressure at sea level is 760 mmHg.

Thus, the barometer measures atmospheric pressure by the height of the mercury column, which varies with changes in atmospheric conditions.

Key Points:

• A mercury barometer consists of a sealed glass tube filled with mercury.
• Atmospheric pressure pushes down on the mercury in the dish.
• The height of the mercury column reflects the atmospheric pressure.
• Standard atmospheric pressure is 760 mmHg at sea level.
• Changes in weather can cause fluctuations in mercury height.

Atmospheric pressure is the force exerted by the weight of air above a surface. As altitude increases, atmospheric pressure decreases. This is because there is less air above you, resulting in fewer air molecules pressing down.

Key points to remember:

• At sea level, atmospheric pressure is approximately 101.3 kPa.
• As you ascend a mountain, pressure drops about 12 kPa for every 1,000 meters.
• Lower pressure means that the boiling point of water is lower at higher altitudes.

For example, at 2,000 meters above sea level, water boils at around 93.4°C instead of the standard 100°C. This is significant, as it affects cooking times and methods at high altitudes.

In summary, as you go higher, atmospheric pressure decreases, leading to a lower boiling point of liquids due to less pressure on the liquid surface.

Atmospheric pressure is the force exerted by the weight of air above us. It plays a crucial role in various everyday applications. Here are some key applications:

• Siphon: A siphon uses atmospheric pressure to move liquids from one container to another. When the liquid is higher in one container, the pressure difference allows it to flow downwards, overcoming gravity.

• Straw: When you sip through a straw, you reduce the pressure inside the straw. Atmospheric pressure pushes the liquid up into the straw, allowing you to drink.

• Syringe: A syringe operates by changing the volume inside the barrel. Pulling the plunger creates low pressure, causing atmospheric pressure to push liquid into the syringe.

• Bicycle Pump: When you push down on a bicycle pump, you compress the air inside, increasing its pressure. When you open the valve, the high-pressure air forces the lower-pressure air in the tire to fill it up.

Understanding these applications helps us grasp how atmospheric pressure influences our daily lives.