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

Pressure of air at constant volume

Demonstration

Measuring the change in pressure when air is heated at constant volume.

Apparatus and materials

Crushed ice

Bourdon gauge

Round-bottomed flask, 250 ml

Rubber bung and tubing

Aluminium container or water bath

Tripod

Electric kettle to provide hot water OR Bunsen burner and tripod

Stand and clamp to hold the flask down

Heatproof gloves

Rotary vacuum pump (optional)

Thermometer -10°C to 110°C

Health & Safety and Technical notes


Heatproof gloves will be needed to handle the flask and water bath after the experiment unless they can be left to cool down. 
 
If a rotary vacuum pump is used, remember that it is too heavy for one person to lift or carry.

You can improve the accuracy of measurements in this demonstration by ensuring that the neck of the flask is in the water. This probably means using a clamp to hold the flask in the water.

 

Procedure


Apparatus set-up for measuirng air pressure

a Connect the Bourdon gauge to the flask. 

b Note the gauge reading when the flask is first immersed in cold water (preferably at or near the freezing point). Then note it with the flask in hot water (preferably at or near the boiling point).


Teaching notes


1 As the air inside the flask is cooled, its molecules move more slowly. Collisions with the walls become less frequent and less violent, meaning pressure falls. When the air is warmed, molecules move faster. Collisions with the walls become more frequent and more violent, meaning pressure rises. 

2 If you measure the water temperature, you can take a set of pressure readings against temperature and plot a graph. The Bourdon gauge scale will probably have to be interpolated because it may not be sensitive enough. About 40 kN m-2 pressure change should be obtained between 0 and 100°C. If you start counting temperature at minus 273°C the line will pass through the origin, showing that pressure is proportional to temperature. 
 Apparatus set-up with rotary vacuum
 
3 If you attach a T-piece and tap or clip to the flask, you can pump out about two-thirds of the air. Then you can try the experiment at another density. The change in pressure should be in the same proportion. 
 
4 Having noticed that the pressure falls as the temperature decreases, ask whether we could predict a temperature at which the pressure would be zero. (To get students to see this is not a daft idea, get them to consider what happens to the motion of the molecules as the temperature falls.) 
 
5 Either by calculation or - better - by drawing onto an extra sheet of graph paper, get them to extrapolate to find values of the temperature at which the pressure would be zero. Discuss its significance. 
 
4 The Bourdon gauge may be calibrated in a variety of units, lb in-2, kg m-2, N m-2, etc. Check that you can translate these into units your students are used to. (See CLEAPSS Laboratory Handbook, section 20.24.) 

This experiment was safety-checked in August 2007

 

Related guidance


Theoretical throughts: extrapolation

From the pressure law to the Kelvin scale