Work & Heat

Question 1

What are heat and work ?

Answer 1

Heat and work are the two forms of energy

They are not properties of the system, as they depend on the path of the process (path functions)

Question 2

Define heat ?

Answer 2

Heat is the transfer of energy due to a temperature gradient (energy in transit)

(Heat is a path function)

Question 3

Define work

Answer 3

Work is the transfer of energy due to a force moving a body over a distance

(Work is a path function)

Question 4

Which type of process (reversible or irreversible) does the greatest amount of work ?

Answer 4

A reversible process will do the most work.

|w(irr)| < |w(rev)|

Question 5

Assumptions for an ideal (perfect) gas

Answer 5

1. Molecules are independent of each other (no attractive forces between them).
2. Molecules don’t occupy any volume (free to move)
3. All collisions are perfectly elastic (no loss of KE during collision)
4. Temperature of gas is proportional to average KE of molecules.

Question 6

State Boyle’s Law

Answer 6

For a fixed amount of an ideal gas kept at a constant temperature, pressure and volume are inversely proportional

p~1/V (n, T constant)

pV=k

Question 7

State Charles’ Law

Answer 7

For a fixed amount of an ideal gas kept at constant pressure and volume is directly proportional to the absolute temperature.

V~T (n, p constant)

V/T=k

Question 8

What do most gases behave like when at low pressure.

Answer 8

Ideal gas

Question 9

Two special cases for calculation of displacement of work

Answer 9

Constant pressure process (Isobaric):

W12 = -p(V2-V1)

Constant volume process (Isochoric):

W12 = 0

Question 10

A process ran at constant temperature (polytropic process)

Answer 10

Isothermal process

Delta = 1

Question 11

Constant volume process (polytropic process)

Answer 11

Isochoric process

Delta = infinity

Question 12

What is an isobaric process ?

Answer 12

A (polytropic)process ran at constant pressure

Delta = 0

Question 13

What is an adiabatic process ?

Answer 13

A (polytropic) process in which there is no transfer of heat from the system to the surroundings. In this type of process energy is transferred to the surroundings as work (only).

Delta = Y(gamma)

(where Y = cp/cv)

Question 14

What happens when work is done on a fluid ?

Answer 14

The work done on the fluid is contained within the fluid in the form of internal energy.