A2 Physics Gas LAws

FLASH BACK Explain Kirchoff’s Laws and how a thermistor and LDR function (6 marks)

FLASH BACK Conservation of charge: The charge entering a junction must be equal to that leaving. Aka: Sum of charges at a junction=0 Conservation of energy: The supply EMF of a circuit is equal to the sum of the potential differences Semiconductors have a low charge carrier density Their charge carrier density increases with the absorption of energy As this occurs, the overall resistance of the material decreases, therefore increasing the current A thermistor decreases the resistance according to an increase in temperature and an LDR with light

Fluid Pressure The pressure at any particular point in a fluid is defined as the force per unit area acting on a very small area round the point. Let’s consider that we have a column of water, and that this water will be causing some pressure on the bottom circular area. The pressure upon that area would equate to force/area and as the force in this instance is the weight of the water above, this would be ‘mg’. TASK: Using your knowledge of density, write an equation for this pressure in terms of density, gravitational field strength and the height of the column

Fluid Pressure As the weight is = mg, Then as the mass, m, is equal to the density x volume then this becomes pVg But as the volume is being divided by the area, it becomes the height of the column. As such, pressure is now equal to pgh! So it has nothing to do with mass or the surface area of the water in the pipe! Question on page 217 Don’t forget however, that there is an additional source of pressure acting upon the fluid and that is air pressure (standard value= 101kPa) so the equation becomes Pressure = PA + pgh NOTE: if stationary, the pressure at any point in the liquid must act equally in all directions, if not a resultant force would exist and the fluid would flow!

Boyle’s Law A.k.a: PV= Constant Robert Boyle was an Irish scientist who explored the relationship between the pressure and volume for a fixed mass of gas at a constant temperature. He concluded that: “The pressure of a fixed mass of gas is inversely proportional to its volume, provided that the temperature of the gas was kept constant” Therefore: A.k.a: PV= Constant

Ideal Gas LAw We have shown that pV= constant and that P/T=constant. So combining this (for a fixed mass of gas) we get the equation pV=constant x T If we take the ‘fixed mass’ as one mole of gas, we will find that the constant is the same for all gases and is known as the universal molar gas constant (R). ‘R’ has the value of 8.31JK-1mol-1 If we use this constant to give the mole in terms of the number of molecules in the gas (utilising Avagadro’s number, NA), we get a new constant, that of Boltzmann (k) which is equivalent to k=R/NA This gives us a new and more succinct equation that includes all the necessary quantities, this is the Ideal gas law

Ideal Gas LAw This gives us a new and more succinct equation that includes all the necessary quantities, this is the Ideal gas law Option 1: Technically there’s no ‘ideal gas’ but the equation works for most gases in normal circumstances. Just remember, T must be in Kelvin Option 2: Number of molecules of the gas PV= NkT Boltzmann Constant (1.38x10-23JK-1)

Kinetic Theory and Brownian Motion