What happens when the plunger is pushed down?

Answer:

If you press on the plunger, you increase the pressure of the air and thus the air in the balloon contracts or decreases its volume. ... The opposite happens when you close the opening of the syringe and pull the plunger back. This time you decrease the pressure of the air inside the syringe—and its volume increases.
The pushing of the plunger reduces the volume of the fluid in the barrel. This reduction in the volume causes a momentary increase in the pressure of the fluid, and the fluid is injected into the patient's body. In a similar way, the pulling of the plunger increases the volume of the fluid.

When you press the plunger down, it forces air into the drain and increases the atmospheric pressure on it. If the item is dislodged, the pressurized air is free to travel throughout the rest of the piping. When you then pull back up on the plunger, the vacuum created will force anything inside the tube to be forced upwards.

Boyle's law is: $$ p_1 V_1=p_2 V_2\\ 1 = \text{plunger not depressed}\\ 2 = \text{plunger depressed} $$

If the item does NOT get dislodged:
The $p_2$ (pressure after the plunger is pressed down) must increase since the volume that the air can take up decreases by forcing down the plunger. When it is pulled back up, the volume can then increase and the pressure decreases.

If the item DOES get dislodged:
The $V_2$ will increase greatly because the air is now free to move throughout the entire pipe. The $p_2$ is subsequently lowered. We now have: $$ p_3 V_3=p_4 V_4\\ 3 = \text{plunger depressed}\\ 4 = \text{plunger not depressed} $$ Now the rest state is the plunger depressed. When we decompress the plunger, we gain volume and the pressure then falls.

Does this make sense to you?

Scuba divers (ultramarinfoto, iStockphoto)

There are four laws, known as Gas Laws, which describe how gases behave. The four laws are Boyle’s Law, Charles’s Law, Gay-Lussac’s Law and Avogadro’s Law.

Boyle’s Law

Robert Boyle, a famous English chemist, discovered in 1662 that if you pushed on a gas, its volume would decrease proportionately. For example, if you doubled the pressure on a gas (increase the pressure two times), its volume would decrease by half (decrease the volume two times). The opposite is also true. If you reduced the pressure on a gas by 3.5 times, then its volume would increase by 3.5 times. This law is an example of an inverse relationship - if one factor increases, the other factor decreases.

Pulling up on the lid of a sealed container increases the volume and decreases the pressure. Pushing down on the lid of a sealed container decreases the volume and increases the pressure.

Boyle’s Law in Everyday Life

Here’s a story from British Airways. Back when British Airways was called British Overseas Airways Corporation (BOAC) (before 1974), female flight attendants in the airline were finding that their uniform skirts were fitting on take-off but once they reached cruising altitude, their skirts felt too tight. This tight-skirt mystery was solved using gas laws! A spokesman for BOAC used Boyle’s Law to explain what was going on. He explained that as the pressure in the cabin decreased at the higher altitude, the pressure in the flight attendants’ stomachs also decreased, thus causing the volume of their stomachs to increase (making their stomachs bulge). Since then, female flight attendants wear adjustable skirts.

The working of a syringe can also be explained using Boyle’s Law. When the plunger of a syringe is pulled out, the volume inside the barrel increases, resulting in a decrease in the pressure inside the barrel. Fluids (such as water) flow from a high pressure area to a low pressure area. This means that once the pressure inside a syringe is lower than the pressure outside the syringe, a fluid near the needle (e.g., water, medicine, etc.) will flow into the syringe.

Parts of a syringe (Let’s Talk Science using an image by Biggishben via Wikimedia Commons).

The opposite is also true. When the plunger is pushed back in, the volume decreases and the pressure increases. Once the pressure is greater than that outside the syringe, the fluid inside the barrel will flow out.

The operation of your lungs also can be explained using Boyle’s Law. When you inhale (breathe in), your diaphragm (a large muscle below your lungs) lowers, which increases the volume inside your lungs. This makes the air pressure inside your lungs lower than the air pressure outside your lungs (and your body); therefore, the outside air is drawn into your lungs (much like the syringe). When you exhale (breathe out), your diaphragm pushes upwards, reducing the volume inside your lungs, increasing the pressure and forcing the air outwards.

A: Inhalation (breathing in) and B: expiration (breathing out) (Let’s Talk Science using images by LadyofHats on Wikimedia Commons (A) and Wikimedia Commons (B)).

A weather balloon is a special type of high altitude balloon. These balloons can reach heights of 18 to 37 km above the Earth carrying instruments for measuring atmospheric pressure, temperature and wind among other things. When weather balloons are sent up, they are only partly filled with gas (typically with helium). Why don’t they fill them completely? Short answer – because they would pop! At higher elevations, the air pressure outside the balloon is lower than the pressure of the helium inside the balloon. As Boyle’s Law states, this causes the volume inside the balloon to increase. If the balloon was already full, this increase in volume could cause the balloon’s rubber to stretch beyond its breaking point.

A U.S. Navy weather balloon (Source: U.S. Navy photo [public domain] via Wikimedia Commons).