Chapter 4: Energy analysis of closed systems

Question 1

What is work transfer generated by in a closed system?

Answer 1

moving boundaries

Question 2

What is moving boundary work and what are the 2 types of processes?

Answer 2

form of mechanical work associated with moving boundary
2 types of processes: expansion & compression

Question 3

Why can we say moving boundary work is an inexact differential?

Answer 3

The work depends on the process path, the states don’t change but the path can

Question 4

What is boundary work equal to in a cycle?

Answer 4

area enclosed in P-v diagram

Question 5

What is the boundary work in an isochoric process?

Answer 5

there is no boundary work during a constant volume process

Question 6

What is the boundary work in a isobaric process?

Answer 6

W= mP(v2-v1), hence there is still work done even though pressure is constant (area under curve)

Question 7

What is the sign convention for boundary work for compression and expansion?

Answer 7

expansion = negative
compression = positive

Question 8

What is the boundary work during isothermal compression of ideal gases?

Answer 8

W = P1 V1 ln (V2/V1) = P2 V2 ln (V2/V1),
since PV = mRT = C (some constant)

Question 9

What is a polytropic process?

Answer 9

PV^n = constant
For expansion and compression of gases
Integrating Pdv gives expression for isothermal process

Question 10

what is specific heat?

Answer 10

energy required to raise the temperature of a unit mass of a substance by one degree
- measure of thermal energy storage capability
- property relations, hence independent of the type of process

Question 11

what is specific heat at constant volume?

Answer 11

Q = dU, cv= dU/dT change in internal energy with temperature at constant volume

Question 12

what is specific heat at constant pressure?

Answer 12

Q = dH, cp = dH/dT change in enthalpy with temperature at constant pressure

Question 13

what do u, h, cv, and cp all have in common?

Answer 13

they ONLY depend on temperature

Question 14

What do you do for the specific heat if there is small temperature intervals (less than 200°C)?

Answer 14

use constant average specific heats, hence assume to vary linearly with temperature

Question 15

what are the 3 ways of calculating u and h?

Answer 15

1) using cv/cp relations (Table A-2c) as a function of temperature and integrating
2) using tabulated u and h data
3) using average specific heats

Question 16

What is cp0 and cv0?

Answer 16

ideal-gas specific heats or zero-pressure specific heats, hence the specific heats of real gases at low pressure
- at low pressure they approach real-gas behaviour (hence why they depend only on temperature)

Question 17

what is the specific heat ration?

Answer 17

k = cp/cv
varies mildly with temperature

Question 18

what is an incompressible substance?

Answer 18

substance whose specific volume (or density) is constant
- solid and liquids are incompressible substances
- cv and cp values are identical and are denoted as c

Question 19

what is enthalpy changes in solids (incompressible substance) equal to?

Answer 19

change in internal energy, change in volume is zero and change in pressure is insignificant

Question 20

what are the 2 cases of enthalpy changes for liquids (incompressible substance)?

Answer 20

1) Constant-pressure processes (Ex: heaters) : only depends on internal energy
2) Constant-temperature processes (Ex: pumps) : only depends on change in pressure times volume
- in both cases change in volume is zero

Question 21

where are the 4 places the energy goes when our body loses internal energy?

Answer 21

1) heat transfer
2) doing work
3) stored fat
4) cell repair and growth (very very small portion)

Question 22

what helps restore the internal energy in our body?

Answer 22

food (heat & doing work takes it out, food puts energy back in)

Question 23

Why can we say food input is in the form of work?

Answer 23

the body metabolizes all the food we consume, so by eating, our metabolism will use an oxidation process to release chemical potential energy from food, thus increasing our internal energy

Question 24

how does cooling work for the human body?

Answer 24

when abeam temperature is above body temperature, the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish an outward flow rate of energy
we do so by sweating (evaporation of perspiration)