What happened to the pressure and volume of air inside the syringe when the plunger is pushed out?

Understanding pressure-related phenomena involves being aware of different variables and the relationships between them, and reasoning about these variables systematically. Scientists analyze the behavior of a system to see what rules govern its behavior. They have discovered several laws that nature consistently follows in relation to air pressure. One such law is explored in this lesson. Boyle's Law states that at constant temperature (T), the pressure (P) times the volume (V) of an enclosed gas remains constant. In formulaic terms:
PV = k, where k equals some constant value and T is unchanging. Thus, when one increases, the other decreases. A relational causal model nicely demonstrates this. According to Boyle's Law, if the volume increases then the air pressure should decrease to maintain the constant value, k. Let's look at the relationship between force and area more closely. Remember that

P=F/A

If the original force is 4 and the area is 2, then the air pressure is 2 (4/2 = 2). We know that if the volume of an enclosed gas increases, then the area must also increase (for example, to 4). Therefore the air pressure does indeed decrease, to 1 (4/4 = 1), when the volume increases.

Using a Syringe to Demonstrate Boyle's Law

Boyle's Law can be demonstrated with a syringe, the device with a plunger and a barrel to which doctors attach a needle to draw blood samples or give injections. When the plunger is drawn back on the syringe, the volume inside the barrel increases. This decreases the pressure of fluids (such as air or liquids) on the inside of the syringe. The atmospheric pressure outside of the syringe remains unchanged and therefore is greater. The pressure differential that is created forces fluid to enter the syringe. Pushing in the plunger of the syringe decreases the volume inside the barrel, thus increasing the inside pressure. This makes the internal pressure greater than the outside atmospheric pressure. The pressure differential pushes fluids out of the syringe. However, if you hold one of the variables (such as force) constant, you can experience Boyle's Law first-hand. If you cover the opening of the syringe with your finger and then pull the plunger back, the molecules in the fluid inside the syringe spread out to fill the available space. To accommodate the increase in volume, pressure decreases proportionally, and you can feel your finger being "pulled" into the syringe a bit. Conversely, if you draw back the plunger, then cover the opening with your finger, and depress the plunger, the same amount of fluid must fit in a smaller space. Therefore the molecules are pushed closer together, the pressure increases to accommodate the decrease in volume, and you feel the resulting "push" on your finger.

Boyle's Law is in Action Around Us Everyday

There are many examples of Boyle's Law in action around us everyday. For instance, if we step on an inflated balloon or push in on a bubble in a piece of bubble wrap, we decrease the volume and thus increase the pressure inside until it is too great for the outer membrane, and the balloon or bubble pops! When we pump up a bicycle tire, we force air into the tire and the volume of the tire increases to accommodate the additional air without increasing the pressure (until the tire cannot expand any farther and then the pressure increases). Every time we take a breath, the muscle located just below the lungs (called the diaphragm) moves downward, increasing the volume in the lungs. This results in decreased air pressure inside the lungs relative to the atmospheric pressure, which forces outside air into the lungs. Exhaling moves the diaphragm upward, decreasing the volume of the lungs and correspondingly increasing the air pressure inside the lungs as compared to the air pressure outside the lungs. This imbalance in pressure pushes waste gases from respiration out of the lungs. In order to avoid being eaten, a Puffer Fish takes on water (or sometimes air), which increases its volume to maintain a constant pressure, despite the added fluid. If you begin to fill your cheeks with air, your cheeks will expand until they have as much volume as they can accommodate. If you continue to add air beyond this point, you will feel increased pressure. As these examples illustrate, Boyle's Law explains many everyday phenomena. Try to come up with a few examples of your own.

25th Dec 2019 @ 4 min read

Boyle's law is a pressure versus volume relationship. The law was discovered by Robert Boyle in the 17th century. It states the pressure of a fixed amount of a gas is inversely proportional to its volume at a constant temperature. The law can be empirically proven. The article discusses an experimental method to verify the law using a syringe.

Experiment: Sealed syringe

The experiment is very simple. It can be performed at home. When the tip of a syringe is sealed with a cap, the air inside the syringe is isolated from the atmosphere. This will fix the amount of the gas. The weights (books) are added upon the plunger of the syringe. It will push the plunger downwards; in other words, the air in the syringe is compressed. By recording the weights of the books added and the volume reading from the syringe, we can establish the pressure-volume relationship.

Objective

To verify Boyle's law and to plot the pressure-volume graph

Materials

1. A 140 mL disposable syringe
2. A seal cap
3. Two wooden blocks: one with the central hole on which the syringe will be mounted and the other which will be attached to the plunger
4. Books that can comfortably place on the wooden block
5. A lubricant
6. A wooden split or tongue depressor

Nomenclature

1. Vi is the volume reading.
2. wi is the weight on each book.
3. w0 is the initial weight, which is the sum of the weight of the wooden piece resting on the plunger and the weight of the plunger.
4. Wi is the total weight on the air inside the syringe.

Procedure

1. Take the syringe and paste a thin layer of the lubricant to the rubber gasket of it with the help of a wooden split or tongue depressor. This will reduce friction.
2. Pull the plunger of the syringe upwards—around 110 mL.
3. Now, attach the seal cap to the syringe.
4. When a small amount of downward force is applied to the plunger, it should revert to the original position. If not, the more lubrication is necessary or the seal cap is not properly attached.
5. Mount the tip of the syringe to the cavity of the wooden block and place it in the upside-down position as shown in the above figure.
6. Fix the other block to the plunger of the syringe such that the syringe is perpendicular to the blocks.
7. Measure the initial volume reading.
8. Place a book on the wooden piece and record the volume reading.
9. Repeat the previous step for two books, three books, four books, and five books.
10. Remove all the books and weigh each. Also, weigh the wooden block with the plunger; it will give w0.
11. Reset the apparatus. Repeat all the above steps twice. Take the average of all three sets.

Precautions

1. The proper lubrication is necessary to eliminate friction.
2. The end of the syringe should tightly fix by a sealed cap. Otherwise, the experiment will fail.
3. The syringe must be properly fixed, so it can firmly withstand the weights.

Observation

The initial weight (w0) is 92 g.

The total weight is

The observation table is as follows:

Calculation

The pressure on the air inside the syringe is the pressure exerted by the weights plus atmospheric pressure.

The pressure exerted by the weights is the force exerted by the weights divided the inner area of the syringe.

Now, Force (Fw) is mass (Wi) times acceleration (a).

Here, r is the inner radius of the syringe, which can be measured; r = 0.005 m. a is the acceleration due to gravity; a = 9.81 m s−2.

For Wi = 92 g,

Assume atmospheric pressure (Patm) as 101.325 kPa.

Similarly, we can calculate the total pressure for the rest.

The calculation table is as follows:

We have to plot the graph of Pi vs Vi and PiVi vs Vi.

Results

The Pressure vs volume graph is as follows:

The pressure-volume vs volume graph is as follows:

Conclusion

The PV curve from the above figure is satisfactory. As the pressure of the air increases, its volume decreases. The air obeys Boyle's law. Also, the product of pressure and volume approximately constant and its value is independent of volume or pressure.

Also, check a laboratory method: To verify Boyle's law»

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Thanks for your response!

Zephaniah Lapa
07th Jun 2021

Very helpful, Thankyou so much..

Henry
30th Jun 2020

Awesome! work, i like your examples, thank you sir.