S4 H Physics Work for P5 Thursday 29 August
Kinetic Theory and Gas Laws – now for some maths!
Task 1: Copy this note down:
We should now be able to describe in words what happens to the pressure, volume and
temperature of a gas if one of these variables is altered in some way.
What we now need to look at is the maths behind the words. The maths concerned with how the
pressure, volume and temperature of a gas are related is called THE GENERAL GAS EQUATION:
𝑃1 𝑉1
𝑇1
=
𝑃2 𝑉2
𝑇2
State of gas before change:
State of gas after change:
P1 = pressure before
P2 = pressure after
V1 = volume before
V2 = volume after
T1 = temperature before
T2 = temperature after
The formula deals with the pressure, volume and temperature of a gas before a change and after a
change has been made. That change could be to do with either the pressure, volume or
temperature. It doesn’t matter what is being changed. In fact the formula can cope with changes to
all three variables. Also, if one of the variables isn’t changed, say temperature, you can just take it
out of the formula altogether, eg.,
P1 V1 = P2 V2
Pressure must be measured in PASCALS (Pa)
Temperature must be measured in KELVIN (K) ** IMPORTANT **
Volume can be in either mm3, cm3, or m3. It doesn’t matter, as long as you are consistent on both
sides of the formula.
Pto., for example…
Example:
A fixed mass of gas at 200C has a pressure of 30Pa. Calculate the pressure of the gas if it is heated to
450C. Assume the volume and mass remain constant.
P1 = 30Pa,
P2 = ?
T 1 = 200C = 20 + 273 = 293K
T2 = 45 + 273 = 318K
Since volume is constant, formula becomes P1/T1 = P2/T2
Rearranging this, P2 = P1 x T2/T1 = 30 x 318/293 = 32.6Pa
Task 2: Purple ‘ Revsion Questions for Higher Physics’ book page 37 questions 5 and 8.
Task 3: ‘Higher Core Physics’ textbook, page 67, questions 1, 2, 5, 6, 7, 8, 9, 12, 13
(There are answers in the back of this book – page 212 for these questions)