Thermal Energy

Question 1

Change in thermal energy

Answer 1

ΔQ = CΔT
or
ΔQ = mcΔT

Question 2

Heat capacity

Answer 2

C= mc

with c = specific heat in J/K per kg

Question 3

Thermal energy during phase change

Answer 3

During a phase change the temperature stays constant

Question 4

Thermal energy during phase change equation

Answer 4

ΔQ = mL, where L = latent heat of evaporation or fusion

Question 5

Three ways of thermal energy and heat transfer

Answer 5

• Conduction
• Convection
• Radiation

Question 6

Conduction

Answer 6

transfer of heat and energy through an object

Question 7

Parameters that play a role in conductivity

Answer 7

• Temperature difference (T1-T2)
• Area (cross-section) (A)
  Thickness or length (d or l)
• Type of material
  -> Thermal conductivity k (W/m*K)

Question 8

Fourier’s law of heat conduction:

Answer 8

P (W) = kA(T1-T2)/d

Question 9

thermal conductivity k:

Answer 9

Is high for for highly conductive materials
Low for not so conductive materials
k<0.2 for insulating materials (wood, paper, glass)

Question 10

Convection

Answer 10

Movement of energy through a fluid based on density differences due to temperature gradients

Question 11

Convection equation

(Heat transfer due to convection at boundary of body
)

Answer 11

P/A = Nu*k(T1-T2)/L

Question 12

nu =

Answer 12

Nusselt number

Question 13

Radiation

Answer 13

Heat transfer by electromagnetic waves

Question 14

Radiation equation

Answer 14

Pe/A = εσT^4

ε = emissivity
σ ~ 5.67 x 10-8 W/(m2K4) (Stefan-Boltzmann constant)

Question 15

Example of forced convection

Answer 15

an oven

Question 16

Main factors influencing heat conductivity

Answer 16

• thermal conductivity
• thickness
• area

Question 17

Ways to improve heat transfer

Answer 17

• use high emessivity materials
• use high thermal conductvity materials
• thin walls, large area
• optimize design to stimulate convection

Question 18

reduce heat transfer

Answer 18

• low emissivity
• apply insulation to reduce conduction
• thick walls, small area
• limit or control ventilation

Question 19

Thermal insulation equation

Answer 19

Q = UAΔT

Question 20

Thermal conductance equation

Answer 20

U = 1/R = k/d

Question 21

Thermal mass

Answer 21

use the thermal capacity of the building to maintain steady interior atmosphere

Example: During the day a large concrete floor absorbs extra heat and radiates it during the night/ cools down during the night.

Question 22

First law of thermodynamics

Answer 22

The heat input in a system (Q) and the work output of a system (W) are equal to the change in internal energy ΔU

(mass conservation)

Question 23

Firts law of thermodynamics eqaution

Answer 23

ΔU = Q-W

Question 24

Specific enthalpy equation

Answer 24

h = u + p*v = u + (p/ρ)

Question 25

Second law of thermodynamics

Answer 25

• The entropy of an isolated system never decreases
• Heat flows from a high to a low temperature
• Not all heat can be converted into work

Question 26

Second law thermo equation

Answer 26

ηc = 1 - (T2/T1)

Question 27

adiabatic

Answer 27

no heat transfer with the surroundings

Q = 0

Question 28

isotherm

Answer 28

tempreature stays constant

ΔT = 0

Question 29

Isobaric

Answer 29

pressure stays constant

ΔP = 0

Question 30

Isochoric

Answer 30

volume stays constant

ΔV = 0

Question 31

enthalpy is useful for describing

Answer 31

1) heat transfer at constant pressure

2) Adiabatic compression or expansion

Question 32

formula for heat transfer at constant pressure

Answer 32

h1 - h2 = Q

Question 33

Adiabatic compression equation

Answer 33

W = h1 - h2

Question 34

Work in thermodynamics

Answer 34

W = p * v

Question 35

When there is adiabatic compression or expansion

Answer 35

there is no change in entropy

Question 36

entropy equation

Answer 36

Δs = ΔQrev/T

Question 37

Two types of closed cycle power plants

Answer 37

• carnot cycle

- rankine cycle

Question 38

The carnot cycle

Answer 38

Work goes in at the compressor and leaves at the turbine

Heat comes in at the boiler and leaves at the condensor

Question 39

Carnot cycle power plant

Answer 39

Q1 - Q2 = Wt - Wcomp