First Law of thermodynamics

Question 1

What is thermodynamics?

Answer 1

Thermodynamics studies relationships involving heat, mechanical work, energy, and energy transfer.

Question 2

How does the First Law of Thermodynamics extend conservation of energy?

Answer 2

It includes internal energy (of particles) alongside kinetic and potential energy from mechanics.

Question 3

What is an example of thermodynamics in action?

Answer 3

A steam engine: heat flows into water, raising its temperature, boiling it, and expanding steam does work.

Question 4

What is a thermodynamic system?

Answer 4

A collection of objects that can exchange energy with its surroundings.

Question 5

What variables describe the state of a thermodynamic system?

Answer 5

Pressure (p), volume (V), temperature (T), and mass (m).

Question 6

What happens in a thermodynamic process?

Answer 6

Changes occur in the state of the system, with energy exchanged as heat (Q) or work (W).

Question 7

When is work positive in thermodynamics?

Answer 7

When it leaves the system

Question 8

When is work negative in thermodynamics?

Answer 8

When it enters the system

Question 9

What type of work is focused on in thermodynamics?

Answer 9

Work that causes the state of the system to change (e.g., altering temperature or pressure).

Question 10

How is mechanical work calculated for constant force?

Answer 10

W=F⋅x (force times displacement).

Question 10

What does positive work mean in mechanics?

Answer 10

Force and displacement are in the same direction.

Question 11

What does negative work mean in mechanics?

Answer 11

Force and displacement are in opposite directions.

Question 12

How is work calculated for a variable force?

Answer 12

W=∫ F(x)dx, the area under the force-displacement graph.

Question 13

How is force related to pressure in a gas-piston system?

Answer 13

F=pA (pressure times piston area).

Question 14

What is the small amount of work done in a gas-piston system?

Answer 14

dW=Fdx=pAdx=pdV

Question 15

How is total work calculated for a volume change?

Answer 15

W=∫ ^_(V initialV final)
​pdV, the area under the pV graph

Question 16

Why can’t work be simply
p(Vf−Vi)p(Vf−Vi) for a gas?

Answer 16

Pressure changes as volume changes, requiring an integral.

Question 16

What does the area under the pV graph represent?

Answer 16

The work done during a volume change.

Question 17

When is work positive on a pV diagram?

Answer 17

When volume increases (V_initial < V_final), work is done by the system.

Question 18

When is work negative on a pV diagram?

Answer 18

When volume decreases (V_initial < V_final), work is done on the system.

Question 19

What is the work for a constant pressure process?

Answer 19

W=p(Vf−Vi)

Question 20

What is an isothermal process?

Answer 20

A process at constant temperature.

Question 21

Quiz: For a pV diagram from a to b (path 1) and back to a (path 2), which path has W>0?

Answer 21

Path 1 only (expansion), assuming path 1 increases volume and path 2 decreases it.

Question 22

How much work does an ideal gas do in isothermal expansion from V_1 to V_2

Answer 22

W = nRT ln(V2/V1)

Question 23

Why is work positive in isothermal expansion?

Answer 23

V2 > V1, so ln(V2/V1) >0

Question 24

What is the ideal gas law used in isothermal work calculation?

Answer 24

p = (nRT)/V

Question 24

What is the First Law of Thermodynamics?

Answer 24

ΔU=Q−W, where ΔU is internal energy change, Q is heat added, W is work done by the system.

Question 24

What does internal energy depend on?

Answer 24

Only the system’s state, not the path taken.

Question 25

Do heat and work depend on the path?

Answer 25

Yes, they depend on the path between initial and final states.

Question 26

What is an adiabatic process?

Answer 26

No heat transfer Q=0, ΔU = −W

Question 27

What is an isochoric process?

Answer 27

Constant volume W=0, ΔU=Q

Question 28

What is an isobaric process?

Answer 28

Constant pressure W=p(V2−V1)

Question 29

What is an isothermal process for an ideal gas?

Answer 29

Constant temperature ΔU=0, Q=W

Question 30

What does internal energy depend on for an ideal gas?

Answer 30

Only temperature, not pressure or volume.

Question 30

What is the relationship between molar heat capacities for an ideal gas?

Answer 30

Cp = C_V + R, where R is the gas constant.

Question 31

What is γ in thermodynamics?

Answer 31

γ = Cp/Cv

Question 31

What is constant in an adiabatic process for an ideal gas?

Answer 31

pV ^γ and TV ^(−γ-1) .

Question 32

What is the Second Law of Thermodynamics?

Answer 32

When all systems in a process are included, entropy (disorder) either remains constant or increases.

Question 33

What does a sandcastle example illustrate?

Answer 33

Spontaneous processes increase disorder (e.g., destruction), not order (e.g., building).

Question 33

Why doesn’t the First Law explain process direction?

Answer 33

It holds for both spontaneous and non-spontaneous processes; the Second Law (entropy) determines direction.

Question 33

Can mechanical energy be fully converted to heat?

Answer 33

Yes, with 100% efficiency (e.g., friction in brakes).

Question 34

Can heat be fully converted to mechanical energy?

Answer 34

No, it’s impossible to build a 100% efficient engine.

Question 35

What is needed to transfer heat from cold to hot?

Answer 35

Work (e.g., in refrigerators or heat pumps).

Question 35

How does heat flow spontaneously?

Answer 35

From a hot body to a cold body, without work.

Question 36

What is the Zeroth Law of Thermodynamics?

Answer 36

Two systems are in thermal equilibrium if they have the same temperature.

Question 37

What is the Third Law of Thermodynamics?

Answer 37

It’s impossible to reach 0 K in a finite number of steps

Question 38

What can a thermodynamic system exchange with its surroundings?

Answer 38

Energy via heat transfer or mechanical work.

Question 39

What is the work formula for constant pressure?

Answer 39

W=p(V2−V1).

Question 40

What happens to internal energy in an isolated system?

Answer 40

It remains constant.

Question 40

The First Law of Thermodynamics, in words

Answer 40

The change in the internal energy of a system is equal to the heat added to the system minus the work done by the system

Question 41

When is work done

Answer 41

if and only if volume changes