PHYS 1405 – Conceptual Physics I

Boyle’s Law

Introduction

In this lab we will investigate Boyle’s Law. Boyle’s law relates the pressure and volume of a gas if you keep the amount of gas and the temperature constant.

Procedure for Boyle’s Law

1. Set Up

We want to have a known volume of air in this experiment. We will read the volume from a syringe marked in cc, directly.

To set up the experiment, connect the the pressure sensor (PS) to the tip of the syringe. Position the valve on the top of the PS so that it is open to the atmosphere. This will make the pressure inside the syringe the same as that of the atmosphere. Pull the syringe so that the tip sits at the 20 cc mark. The syringe should move easily, otherwise the valve is not in the correct position. Now change the position of the valve on the top of the PS so that pressure sensor reads the pressure in the syringe.

Connect the PS to CH 1 on the LabPro. Double click on the LoggerPro 3.1 icon, click on the open button and and open the experiment file called Boyle’s Law by following the path "Probes&Sensors"=>"Pressure Sensors"=>"Pressure Sensor"=>"Boyle's LAW-PS".

2. Try out the Apparatus

Make sure that the valve on top of the PS is closed. Push on the plunger of the syringe.

1) What happens to the volume of air contained when you push on the syringe?

2) Can you push equally hard and make the volume smaller? Try it.

3) How hard do you have to push to make the volume smaller?

4) When you push on the syringe, how do you change the pressure of the air in the syringe?

5) Complete the following, to make the volume smaller at you have to __________ the pressure.

This is the germ of Boyle's law which we will now investigate quantitatively.

3. Data Acquisition

Note: A useful relation in the following is that 1 ml = 1 cm³ = 1 cc

Open the valve on top of the pressure sensor and return the volume to 20 cm³. Close the valve on top of the pressure sensor again.

Click on the collect button to start acquiring data. Click on the keep button and record the volume of the syringe and the tube in cm³ in the box that appears.

Now decrease the volume in the syringe to 17.5 cm³, and repeat the data acquisition procedure. Click on the Keep button when you are holding the syringe steadily at 17.5 cm³. Repeat this procedure each time decreasing the volume by 2.5 cm³ more, until you reach a volume of 5 cm³. Do not go beyond about 5 cm³ as this can damage the pressure sensor. Click on the stop button at this point.

Data Analysis and Questions

We now wish to analyze our data.

6) Do your data appear to lie on a straight line?

7) Add a best fit line through your data. Do you see any systematic trends in the data?

8) May some other type of curve fit the data better?

It turns out that the answer to the last question is yes. Let us try and determine what type of curve will fit the data better. Our first volume was approximately 20 cm³ with its corresponding pressure. A later data point had a volume of approximately 10 cm³ with the corresponding pressure. Note here that the 10 cm³ volume is approximately 1/2 of the 20 cm³ volume.

9) How do the pressures for these two volumes compare?

Compare the volume of approximately 5 cm³. Here the volume is 1/4 as much as the initial volume.

10) What is the ratio of the pressure at this volume to the initial pressure?

You should have noticed that the ratios of the pressures and of the volumes are approximately the reciprocals of each other. This is the essence of Boyle’s law. Boyle’s law states that if the temperature and the amount of gas are held constant, that the pressure is inversely proportional to the volume.

Click on the curve fit button . Choose Inverse from the list of types of curves that can be fit and then click on the Try Fit button. Click OK to accept the fit.

11) Does the inverse relation fit the data more closely?

12) Based on your data and fits is the relationship between Pressure and Volume better described by a linear relationship or an inverse relationship?

13) State in your own words Boyle’s Law.