Feeling the pressure: Investigating the effects of atmospheric pressure

 

Introduction
It is not always easy to get students to understand the effects of atmospheric pressure, but here are a couple of simple activities to challenge existing ideas and allow the development of a more sophisticated understanding of this concept.

 

Lesson organisation
Although these can readily be done as demonstrations, the simplicity of the equipment allows the activities to be done individually or in small groups as well.

Apparatus and materials
Each group/individual will need:

• Two straws
• A plastic cup of water
• A clear plastic bottle up to 1 litre in size
• A clear plastic bottle up to 1 litre in size, with a small hole on its base
• 2 well stretched balloons
• A drawing pin to make a hole in a straw

Technical notes and safety
Each student who tries the two straws activity should use fresh straws and used straws should be thrown away.

 

Click here to see a full list of Health and Safety procedures

Procedure
Activity 1 - atmospheric pressure and suction
1 Put a straw in the clear cup of water.
2 Hold a second straw outside the cup as shown.
3 Try sucking the water up through the straw.
4 Now make a small hole in one of the straws with the drawing pin about 3 cm from the top and try drinking through it.

Activity 2 - balloon in a bottle
1 Place a balloon inside each bottle; spread its neck over the top of the bottle.
2 Try blowing up the balloon in each case - only with the bottle with a hole in will it work.
3 Air will exit the bottle via the small hole in the base of the bottle. Quickly seal the hole with your thumb and the balloon will stay inflated.
4 By slowly allowing air to enter the bottle, the balloon will defl ate under your command.

Teaching notes
Both activities can be run after some discussion to encourage students to make predictions and attempt explanations that use the idea of a pressure difference to explain what happens.

Activity 1
The student will fi nd it impossible to drink if one of the straws is outside the glass.
If both straws are placed in the mouth it is difficult to maintain a sufficiently low pressure to cause the water to be sucked up, because air enters the mouth through the second straw. In order for the water to be forced into your mouth, the pressure outside (atmospheric pressure) needs to be greater than the pressure inside your mouth. This means that no matter how you suck, a straw won't work if air can get into your mouth. A similar effect is achieved by making a small hole in a straw about 3 cm from the top and putting this straw in the water.
Extension activities could include exploring how many straws put IN the water can drink be sucked through - increasing the surface area makes it harder.
By joining straws together find the longest straw it is possible to drink through.

Activity 2 Discuss why it is not possible to blow up the balloon without the hole in the bottle and why the balloon stays infl ated when the hole in the bottle is covered. Encourage students to use the idea of pressure differences in their answers. Putting the lid on the bottle, or tape over the hole, can leave the balloon inflated. By sucking air through the hole in the bottom of the bottle it is possible to inflate the balloon.

 

Click here to download this activity as a Microsoft Word document