Lecture notes
Physics Unit 5
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- PEARSON (PEARSON)
All notes of physics chapters of student book 2 of unit 5
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UNIT 5P H YTHERMAL PHYSICS, MATERIALS AND FLUIDS
S I C S
THERMAL PHYSICS IN DOMESTIC AND INDUSTRIAL
APPLICATIONS
This subject builds up in small steps. When you can put it all together, it is surprisingly
powerful and answers important questions. So, try to make sure you grasp each definition and
law along the way.
Measurements
You will need to be able to recognise and use the SI (Systeme Internationale) units for the
following important quantities. Each unit starts with a capital letter because it is the name of a
scientist who did important work.
Energy: unit Joule (J)
Energy is closely related to work (see below) and they share the same unit.
Power, symbol P: unit Watt (W)
We commonly deal with large quantities of power, so you also need to be familiar with the
following multiple:
o Kilowatt (kW) = 1000 Watt
o Megawatt (MW) = 106 Watt
o Gigawatt (GW) = 109 Watt
1 Watt = 1 Joule per second (J s-1).
(Do not confuse with kilowatt-hour (kWh), which is a unit of energy, not power. As there are
3600 seconds in an hour and 1000 watts in a kW = 3,600,000 J.)
Temperature, symbol T: unit Kelvin (K)
In this subject it is often important to use the absolute temperature in equations. So, you need
to know how to convert from degrees Celsius (C) – also known as Centigrade – to Kelvin
(K). (Note that there is no degree sign before Kelvin.)
When you see the symbol T in an equation you should always use the absolute temperature in
Kelvin.
Pressure, symbol p: unit Pascal (Pa)
Notice that the letter “p” is used for a lot of things in thermodynamics; so, it is important to
be careful and consistent about how you write them. Capital P means power. Try to always
use a lower-case p for pressure. The unit symbol Pa has extra “a” to distinguish it from power
and pressure.
1 Pascal = 1 Newton per square metre (N m-2)
Work Done
When objects interact, the forces between them can lead to energy being transferred.
Sometimes the energy is stored in a useful form, e.g. in a spring or in a gravitational or
electric field (this is called potential energy). Sometimes energy is due to the speed of a
moving object (this is called kinetic energy). Alternatively, the thermal energy (heat) content
of an object may be increased.
,When you measure work, you are focussing on the amount of energy transferred during the
process.
Work done by a force
The most obvious way of doing work is to exert a force in order to move something, e.g.
pushing a child on a swing, or lifting a bag of shopping onto a table. Calculating the work
done in both those examples is straightforward:
Work done = force x perpendicular distance moved in the direction of the force
W = F x s
The symbol means “change in”. So, the “change in position”, s = s2 – s1, where s1 is the
initial value of the position (displacement) and s2 is its final value. Note that we always start
with the final value and take away the initial value.
Forces occur in opposing pairs: so, the force you exert to lift the shopping in opposed to the
weight of the shopping itself. When you lift the shopping and it moves up in the same
direction, you do positive work on it. But if you let it down again onto the floor, the work you
do is negative – or to put it another way, the shopping then does work on your arm. Some old
clocks operate in that way, with weights that slowly fall and drive the mechanism.
But sometimes the force exerted is in a different direction from the movement. For example,
when you push something up a slope, the force you are working against is gravity. So, for
calculating work done, it is only the vertical height gained that counts as the distance moved
– the part of the movement that is along the line that the force acts.
Work done by a gas expanding
Another important way that work is done is when a gas expands to take up a larger volume.
This is what happens in the cylinders of a steam engine or motor car engine (petrol or diesel).
Hot, high pressure vapour or gas pushes against a piston and moves it. To calculate the force
exerted by the gas you would need to multiply the gas pressure by the area of the piston head,
A. So, F = p A.
When the piston moves, the volume change for the gas is the same area, A, multiplied by the
distance moved by the piston. So, V = A s.
Putting those two facts together, you can now calculate the work done by the expanding gas:
Work done = pressure x volume change
(W = p x V)
Note that this definition of work saves you having to worry about directions. A gas exerts its
pressure in all directions on every surface of the container that defines its volume.
Law of Conservation of Energy
Because forces always occur in pairs that are equal in size but opposite in direction
(Newton’s First Law of Motion), the energy transferred to an object when work is done on it
is always equal in size to the amount of energy lost by the second object that is doing the
work. So, overall the total energy of the pair of objects remains unchanged. As this is true for
every pair of objects in the universe, you can assume that total energy of the whole universe
must also be constant, wherever goes on within it.
This is known as the Law (or principle) of Conservation of Energy. It is one of the most
fundamental laws of physics.
Thermodynamics is largely about simplifying things so that they can be studied and
measured. You cannot study whole universe all at once. So, scientists define a system by
drawing an imaginary boundary around what they are studying. They can talk about the
system and its surroundings, i.e. the rest of the universe. (you will often study two systems
and measure the interactions between them).
The energy contained in a system can be in many different forms, including those shown in
table 5.1.
, Type of energy Nature of the energy
Mechanical energy Either potential energy, due to its mass and
position in a gravitational field, or kinetic
energy, due to an object’s mass and speed of
motion.
Electrical energy Can be associated with static electric charge
and potential, or with moving charges,
current and magnetism
Chemical energy Intrinsic to the microscopic structure of the
material and chemical bond energies.
Nuclear energy Due to the binding subatomic particles
(protons and neutrons) in the nuclei of
atoms – changes in that are what cause
radioactive decay, and also where nuclear
power comes from.
Thermal energy Due to the microscopic vibrations and
movements of atoms and molecules in a
material – movements that you cannot
directly measure, but they give rise to a
measurable quantity, temperature.
Table 5.11 – Types of energy and their nature.
The principle of energy conservation means that when, in some process, you observe a
reduction in one kind of energy, you will always find an equivalent increase in other forms of
energy. Energy is transferred, but never lost. (Nevertheless, sometimes when energy is
transferred to the surroundings in a form that is not “useful”, people do talk about energy
losses. You will think more about “useful” and “wasted” energy when calculation efficiency.)
Heat and Temperature
When two systems interact, there may be a flow of heat between them due to a temperature
difference. This is a transfer of thermal energy from one system to the other. Flow of heat
between two bodies in thermal contact (i.e. they are not completely insulated from one
another) will continue until they reach thermal equilibrium and have the same temperature.
Temperature is such a commonly experienced quantity that scientists did not realise they
need to define it until after they and developed the first two laws of thermodynamics. So, the
definition of temperature is often called the Zeroth Lay of Thermodynamics. That simply
states that there exists a property called temperature will be in equilibrium with one another,
and indeed also in equilibrium with a third object – a thermometer – used to measure each of
their temperatures – see Figure 5.22.
At a microscopic level, temperature is linked
to the average kinetic energy of vibration and
of motion of the atoms and molecules. You
can picture that particles continually bump
against one another and so exchange thermal
energy, with the result that the energy
gradually gets spread more and more evenly.
At equilibrium, the rate of energy transfer in
one direction will exactly match that
transferred back in the other direction. So,
overall there will be no net transfer – no heat
flow.
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