Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Boyle's law

Demonstration

Apparatus and materials

Boyle's law apparatus

Foot pump and adaptor

Kinetic theory model kit (transparent cylinder with small steel balls)

Health & Safety and Technical notes


It has been known for the glass tube to fly upwards when the gas is at maximum pressure. To prevent this, check the compression joint holding the tube and any tube supports before use. (The apparatus is filled and emptied by removing the pressure gauge.)

Boyle's law apparatus

The apparatus has been specially designed to give quick, clear readings which the class can see. 
 
A sample of dry air is confined in a tall, wide glass tube by a piston of oil. The volume is found from the length of the air column, which should be clearly visible at the back of the class. 
 
The pressure is read from a Bourdon gauge connected to the air over the oil reservoir. This is calibrated to read absolute pressure and is also visible from the back of the class.

The foot pump is attached to the oil reservoir and is used to change the pressure. The gauge reads up to 3 x 105 N m-2 and the pressure can safely be taken up to this value but must not be taken beyond. 
 
To fill the apparatus with oil, unscrew the Bourdon gauge with a spanner and fill the chamber with a low vapour pressure oil. Tilt the apparatus in the final stage of filling in order to get enough oil into the main tube. When refixing the gauge, tighten the nut to get a good seal, but not so much that the thread is damaged.

 

Procedure


a Give a quick demonstration to show that doubling the pressure halves the length of the air column, and so its volume. 

b Increase the pressure to its maximum value, and then record it and the (minimum) length of the air column. 
 
c Next, disconnect the pump and release a little air using the valve on the oil reservoir, so that the oil level in the tube falls a few centimetres. 
 
d Before taking the next pair of readings, wait a while so that the air temperature recovers and the oil left behind has fallen down the wall of the tube. 
 
e Keep repeating step c until the gauge returns to atmospheric pressure.


Teaching notes


1 It is important to ensure that students have grasped that the volume of the air column is directly proportional to its length, so that the way the length changes tells us how the volume alters. (It is not hard to get a good estimate of the internal diameter of the tube, if finding the cross-sectional area of the air column would help.) 

2 It is helpful if students plot a graph of pressure (P) against lengths of air columns (V). This can lead them to see that trying a graph of P against I/length (I/V) might be a good idea. 
 
3 Students might then also use a spreadsheet to find how the product of pairs of values P x length (P x V) compare. 

This experiment was safety-checked in January 2006

 

Related guidance


A simple theory of a gas

Equi-partition of energy