2.3 Thermodynamics

Question 1

Name the 3 types of thermometers

Answer 1

• Bulb thermometer
• Bimetallic thermometer
• Thermistor

Question 2

What are thermocouples used for

Answer 2

Measure high temperatures

Question 3

Where would a Bimetallic Thermometer be used

Answer 3

Fire detection system & OAT

Question 4

What is a Thermistor

Answer 4

A device to measure temperature that changes its electrical resistance with changes in temperature. Most common device

Question 5

3 scales of tempeature

Answer 5

Deg Celcius (C)
Kelvin (K)
Deg Farenheit (F)

Question 6

Formula to convert Deg F to Deg C

Answer 6

deg C = 5 / 9 (F - 32)

(50 - 32) x 5 / 9 = 10 deg C

Question 7

Formula to convert Deg C to Deg F

Answer 7

deg F = 9 / 5 C + 32

9 / 5 x 50 + 32 = 122 deg F

9/5 = 1.8

Question 8

Temperature at which deg C & deg F are the same

Answer 8

-40 deg

Question 9

Formula to convert Kelvin to deg C

Answer 9

K = deg C + 273.15

Question 10

What is zero Kelvin otherwise known as

Answer 10

Absolute Zero. Whare all molecular movement would stop

Question 11

Formula for Specific Heat

Answer 11

c = Q / m x /\T or Heat added Q = m x c x /\T

• c = specific heat {j/kg K}
• Q = heat energy added {J}
• m = mass {kg}
• /\T = change in temp {K}

Question 12

Formula for Heat Capacity

Answer 12

C = Q / /\T or Haet added Q = C x /\T
* C = heat capacity (J/K)
* Q = heat energy added (J)
* /\T = change in temp (K)

Question 13

Name the 3 types of Heat transfer

Answer 13

Conduction, Radiation, Convection

Question 14

The only method of heat transfer in solids is

Answer 14

Conduction, however it also occurs in liquids

Question 15

Heat transfer from one part of the fluid to another, is called?

Answer 15

Convection

Question 16

Formula for Linear Expansion

Answer 16

/\L = aLo/\T
* /\L = change in length (m)
* a = coefficient of linear expansion
* Lo = original length
* /\T = change in temperature

Question 17

Formula for Area Expansion

Answer 17

/\A = 2aAo/\T
* Ao = original area of body (m^2)
* a = coefficient of linear thermal expansion
* /\A - change in area of the body (m^2)
* /\T = change in temperature (deg C)

Question 18

Formula for Volumetric Expansion

Answer 18

/\V = 3aVo/\T
* Vo = original volume of the body (m^3)
* a = coefficient of linear thermal expansion
* /\V = change in volume of the body(m^3)
* /\T = change in Temperature (deg C)

Question 19

Describe the first law of thermodynamics?

Answer 19

Energy can neither be created nor destroyed but can only be transformed from one form to another. In simple terms: Energy in = Energy out

Question 20

Formula for Thermodynamics first law

Answer 20

Q = /\U + W or W = Q - /\U or /\U = Q - W
* Q = heat energy added (J)
* W = work done by the gas (J)
* /\U = change in internal energy (J)

Question 21

2nd Law of Themodynamics

Answer 21

The second law of thermodynamics states that the total energy output (as produced by a machine) is equal to the amount of heat supplied.

Question 22

Formula for change in Entropy

Answer 22

/\S = Q / T
* /\S = change in entropy (J/K)
* Q = heat energy absorbed (J)
* T = temperature (K)

Question 23

There are two types of thermodynamic cycles: open and closed. Each cycle has the same five basic elements. What are they?

Answer 23

Working substance – a medium that receives, stores and transports the energy.
Heat source – supplies thermal energy to the working fluid.
Heat receiver – absorbs the heat (or thermal energy) from the working substance.
Pump – moves the working substance from the low-pressure side to the high-pressure side of the cycle.
Engine – converts the thermal energy to work in mechanical energy.

Question 24

There are three main processes in the engine working cycle during which changes in temperature, pressure, or density occur:

Answer 24

• Compression
• Combustion
• Expansion

Question 25

Boyle’s Law formula - (Isothermal)

Answer 25

PV = k or P1V1 = P2V2

(Temp remains Constant)

• P = pressure (N/m^2)
• V = volume (m^3)
• • k = a constant (pa-m^3)
    or
• P1 = initial pressue (N/m^2)
• V1 = initial volume (m^3)
• P2 = final pressure
• V2 = final volume

Question 26

When real gases are compressed at a constant temperature, what happens?

Answer 26

changes in the relationship between pressure and volume occur. ie. Volume increase, Pressure decreases and vice versa

Question 27

Charles’s Law (Isobaric Process)

Answer 27

At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature increases or decreases. This is called an isobaric process.
V / T = k
* V = volume (m^3)
* T = temp (K)
* k = a conctant (m^3 / K)

Question 28

Gay-Lussac’s Law (Isochoric Process)

Answer 28

At constant volume, the pressure of a given mass of gas varies directly with the absolute temperature of the gas. i.e Temp increases, Pressure increases. This is called an isochoric process.
P / T =k
* P = pressure (Pa)
* T = temp (K)
* k = a constant (Pa/K)

Question 29

An isothermal process is one in which?

Answer 29

the temperature remains constant. Boyles Law

Question 30

Specific Heat at constant volume

Answer 30

cv = Q / m x /\T or Q = m x cv x /\T
cv of air is given as 718 J/KgK

Specific heat at Constant Pressue = Cp

Cp for air is given as 1005 J/KgK

Question 31

An adiabatic process is one in which?

Answer 31

no heat is transferred to or from the system.

Question 32

What is used to remove heat during expansion & compression

Answer 32

A heat sink

Question 33

Expansion and Compression, name the different types of processes

Answer 33

• Isochoric: - Constant volume - Guy Lussacs
• Isobaric: - Constant pressure - Charles
• Isothermal: - Constant temperature - Boyles
• Adiabatic: - No heat transfer between system and surroundings

Question 34

Name the two main types of thermodynamic processes:

Answer 34

Reverisble & Irreversible

Question 35

What is an Otto cycle

Answer 35

The Otto cycle is an idealised thermodynamic cycle
Piston Engine

Question 36

There are four thermodynamic processes making up a complete Otto cycle:

Answer 36

Process 1–2 Adiabatic compression: Piston moves from bottom dead centre (BDC) to top dead centre (TDC). No heat transfer takes place, temperature and pressure increase and the volume decreases.
Process 2–3 Reversible isochoric heating: Piston is at TDC, ignition of the fuel-air mixture takes place. Temperature and pressure increase, volume is constant.
Process 3–4 Adiabatic expansion: The increased pressure exerts a force on the piston and pushes it towards BDC. Air expands and does work on the piston. Pressure and temperature decrease. No heat transfer takes place during the process.
Process 4–1 Reversible isochoric rejection (cooling): The piston is at BDC. Pressure and temperature decrease to original value.

Question 37

What is a Brayton Cycle

Answer 37

The Brayton cycle is a thermodynamic cycle describing the processes in a constant‑pressure heat engine. Three compunants Gas Compressor, Combustion Chamber & Expansion Turbine

Question 38

There are four thermodynamic processes making up a complete Brayton cycle in gas turbines:

Answer 38

Process 1–2 Adiabatic compression: Fresh air is drawn into the compressor, pressurising it. No heat transfer takes place, temperature and pressure increase and the volume decreases.
Process 2–3 Isobaric heating: The compressed air runs through a combustion chamber, where fuel is burned, heating the air. Pressure is constant, volume and temperature increase.
Process 3–4 Adiabatic expansion: The heated, pressurised air gives up energy when it expands through a turbine. Some of the work extracted by the turbine is used to drive the compressor. Pressure and temperature decrease. No heat transfer takes place during the process.
Process 4–1 Isobaric cooling: The air cools down in the atmosphere. Volume and temperature decrease to their original values.