Exam (elaborations)
Exam (elaborations) Unit 5 - Principles and Applications of Science
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Revision NotesSection 1 Thermodynamics
SI Units
The SI Units used in the course are
Joules (J) the unit of energy, heat and work
Watts (W) the unit of Power
Pascals (Pa) the unit of Pressure (equivalent to Nm -2)
Kelvin (K) the unit of Temperatures (Kelvin = Celsius +273.15)
Newton (N) the unit of force
Metre (m) the unit of distance
SI Prefixes
k (kilo) x 103
M (Mega) x 106
G (Giga) x 109
c (centi) x10-2
m (milli) x10-3
μ (micro) x 10-6
Work Done
Work is done when energy is transferred
The formula for work done is
Work Done = Force x Distance moved or W = F x Δs
The units of work are Joules
When we compress a gas the work done is
Work Done = Pressure x change in volume or W = p x ΔV
This work will raise the temperature of the gas
If a gas expands it does work on the surroundings this means that the temperature of the system will
go down.
, Efficiency
We can work out efficiency by a number of equations
Efficiency = Work Out/Energy In
For a heat engine where heat is inputted and work and heat is outputted.
QOut
Efficiency = 1 - Q¿
For a reversible Heat Engine (i.e. Carnot Cycle) the Maximum Theoretical Efficiency is
T Cold
Efficiency = 1 - where TCold and THot are the temperature of the hot and cold sources.
T Hot
Conservation of Energy
The Law of Conservation of Energy can be expressed as:
Energy cannot be created or destroyed
For any process the total energy input = total energy output
For a heat engine:
The change in Internal Energy = Heat added from the surroundings – Work done on the surroundings
First Law of Thermodynamics
The change in Internal Energy = Heat added from the surroundings – Work done on the surroundings
ΔU = Q –W
U is the internal energy of the system which is the sum of the kinetic and potential energies of the
particle in the system.
Q is the heat added to the system form the surroundings
W is the work done by the system on the surroundings
Ideal Gas Law
An ideal gas is one where the force between the particles is assumed to be zero and the particles are
assumed to occupy no space. This is a fair assumption for normal pressures and normal pressures.
For an Ideal gas
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