Properties of Matter

Air pressure due to suction

Teaching Guidance for 11-14 Supporting Physics Teaching

Pressure vs. suction

Wrong Track: If you pull the air out of the can with that pump, the pump sucks the walls of the can in. It's all to do with suction.

Right Lines: When the air is removed from inside the can the forces on the outside, from the air pressure, collapse the walls inwards. This air pressure is due to the bombardment of air molecules. We don't notice air pressure until air is removed from one side of a surface (remove air from a coke can and the air on the outside crushes the walls inwards).

What's doing the sucking?

Thinking about the learning

Even when you emphasise the air pressure explanation for familiar phenomena, problems can arise.

Thinking about the teaching

This kind of thinking can be a big problem. Pupils need to make the move from thinking about the can being sucked in from the inside to accepting that the can collapses because of the air pressure on the outside. A useful starting point in challenging suction ideas is to ask:

Teacher: What is doing the sucking inside the can? There's nothing there. All of the air had been taken out.

A common reply to this is that the vacuum does the sucking. In which case we return to the question, how can nothing do the sucking? At this point you need to re-emphasise the pressure explanation.

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