Question

what is positive and negative pressure?

Answer 1

Realize that negative pressure can only exist in the gage pressure scale. Absolute pressure, like absolute temperature, has a minimum at zero.

Gage pressure is calibrated such that a zero reading on the gage corresponds to atmospheric pressure (1 atm, 101 kPa, 14.7 psi in the absolute scale).

Therefore, positive pressure is any pressure that is greater than atmospheric pressure and negative pressure is any pressure that is less than atmospheric pressure.

An example of negative pressure can be seen when using a vacuum cleaner. The negative pressure causes the air to rush into the hose. An example of positive pressure can be seen when using a leaf blower. The positive pressure causes the air to rush out of the hose.