Learning

Doing, Talking, Writing and Drawing

Apprenticing Students into the World of Science

2014 John PoliasLexis Education and Department of English, The Hong Kong Polytechnic University

cience

S32.06

–2+4+6

16

© John Polias 2014

When we switch on the vacuum pump, the can collapses or we can say the can gets crushed.

Why does the can get crushed?

The can gets crushed because of a big difference in pressure between the inside of the can and the outside.

to the vacuum pumpopen canclosed can

Explaining why a can gets crushed

But why is there a big difference in pressure between the inside

and the outside of the can ?

© John Polias 2014

air particlesinside the can

air particles outside the can

to the vacuum pump

Before we switch on the vacuum pump, the number of particles (above a square centimetre) outside the can is the same as inside the can. So, we can say that the pressure inside the can and the pressure outside the can are the same.

Before we switch on the vacuum pump

© John Polias 2014

to the vacuum pump

the wall of the can

The air pressure inside the can is the same as outside the can

Before we switch on the vacuum pump

© John Polias 2014

air particlesinside the can

air particles outside the can

to the vacuum pump

When the vacuum pump is switched on, it sucks up air particles from inside the container so the number of particles inside the can decreases.

After we switch on the vacuum pump

© John Polias 2014

to the vacuum pump

the wall of the can

The air pressure inside the can has decreased because of the vacuum

pump

The air pressure outside the can is now much greater than the pressure inside the can

After we switch on the vacuum pump

© John Polias 2014

When the vacuum pump is switched on, the number of air particles inside the can decreases greatly. So, the air pressure inside the can is much less than the air pressure outside the can. This large difference causes the greater outside air pressure to crush the can.