# PHYB5 - Matter Under the Microscope

Revision notes for AQA Physics B: Physics in Context. Covers a section of the unit 5 exam.

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• Created on: 26-06-11 18:57

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Matter under the Microscope
Molecules in Motion
Pressure is the force acting per unit area (p = F
A ) . A particular force will have a different
effect on an object depending upon how large an area it acts on. The standard unit is
Pascal's. In a fluid, force acts equally in all directions. A gas in a container exerts a pressure
on the container walls. This is because the gas particles are always hitting the walls. Each
time a particle does this, it exerts a force. The pressure that the gas causes depends on the
number of collisions per second and how hard the particles hit the wall.
Kinetic Theory of Gases and Pressure:
Evidence for the existence of molecules is based on observations of Brownian motion and
diffusion. The kinetic theory of gases is a statistical treatment of the movement of gas
molecules in which macroscopic properties such as pressure can be interpreted by
considering molecular movement
Assumptions:
A gas consists of such a large number of molecules that statistical rules can be used
with certainty
Each molecule has negligible volume when compared with the volume of the gas as a
whole
Molecules are in constant, rapid motion
At any instant as many molecules are moving in one direction as in any other
Molecules undergo perfectly elastic collisions with the walls of their containing vessel,
this reversing momentum
There are no intermolecular forces between the molecules except during collisions
The duration of a collision is negligible compared with the time between collisions
Each molecule produces a force on the wall of the container
The huge number of molecules will average out to produce a uniform pressure
throughout the gas
Gravitational effects on the molecules are negligible
1 Nm < c2 >
pV = 3
o p = pressure (Pa)
o = volume ( m3
V )
o N
= number of molecules in the gas
o m
= mass of each molecules (kg)
o < c2 >
= mean square velocity of the molecules ( m2s-2
)

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The equation for kinetic theory shows that there are three ways in which the pressure inside
a container can be increased:
1. More Molecules
- Putting more molecules inside the container with the volume staying
constant will increase the pressure, as pN
2. Decrease Volume
- Decreasing the volume will mean the same amount of molecules have less
space to move, thus hitting the walls and other molecules more frequently,
increasing pressure as p V1

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Boltzmann Constant = 1.38×10-23JK -1
From the ideal gas equation, there are three constants that can be changed. Assuming that
no molecules are added or taken away, the following relationship can be used.
p1V 1 p2V 2
T1 = T2
Temperature and Internal Energy:
The kinetic theory has been linked with the temperature of a gas above. Temperature,
pressure and volume are all macroscopic properties of a gas.…read more

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Absolute zero is 0K, or -273.15 C. When working with the ideal gas equation and related
equations, temperatures must be given in Kelvin, so 273 must be added to any quoted

### Page 5

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If the volume of a gas remains constant, we can say that there has been no work involved,
so W = 0 . This occurs in two instances:
U = Q
o Increase in internal energy as heat is supplied to the system
- U =- Q
o Decrease in internal energy as heat is taken from the system
Heat Engines:
A heat engine uses heat to do work, and then rejects any heat that cannot be used to do

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The Carnot cycle, as illustrated above, is composed of two isothermal paths and two
adiabatic paths. If thought of as a gas trapped in a cylinder by a piston, the piston will move
back and forth as the gas goes around the cycle.
It operates between a hot reservoir at temperature T H and a cold reservoir at temperature

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It can be shown that for an ideal gas undergoing a Carnot cycle, the thermal efficiency is
given by:
T H-T C
= TH
T H = Temperature of the hot reservoir
T C = Temperature of the cold reservoir
Internal Combustion Engine:
When a small quantity of high energy fuel is ignited in a small enclosed space, a large amount
of energy is released as the fuel vaporises and expands.…read more

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Carnot cycle) that is not continuous unless there is an input of energy (fuel).
During the cycle, work is done on the gas during the adiabatic compression and work is done
by the gas during the adiabatic expansion, meaning the area enclosed by the curve is the net
work done by the gas. The power of the engine is found by multiplying this work by the
number of cylinder and number of complete cycles per second.…read more

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Change in Temperature (K)
A material with a high specific capacity will require a lot of energy to raise its temperature. It
will also tend to retain the energy for a longer time
The main drawback for internal combustion engines is that they are extremely inefficient.
They typically convert around 20% of chemical energy into useful mechanical energy. Around
a third energy is released into the atmosphere in the exhaust gas' internal energy, and
another third being transferred to the surroundings via the cooling fluid.…read more

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The second law of thermodynamics can be stated as "the entropy of a system can never
decrease, the best it can do is stay the same but almost always increase". Entropy (S) is a
measure of the disorder of a system, or the number of ways in which a certain arrangement
can exist. E.g. ice has low entropy as the molecules are ordered and cannot move around,
therefore there are only a few ways in which it can be ordered.…read more