Description

Week 3: work and heat

State function
• Definition: a property whose value doesn’t depend on the path taken to reach that specific value.
• Examples: energy E, volume V , temperature T, pressure P, number of particles n, entropy S
• Counterexamples: work W, heat Q
1

State function
• Definition: a property whose value doesn’t depend on the path taken to reach that specific value.
• Examples: energy E, volume V , temperature T, pressure P, number of particles n, entropy S
• Counterexamples: work W, heat Q
• Intuitively, imagine the following scenario: during weekdays, you will come to campus to take courses.
• You can commute between two locations by car, bike, walk, and by different path.
• Location is a state function, independent from path you take. 9500 Gilman Dr, La Jolla, CA 92093.
• Way & path to commute is not a state function.
Energy change of a system
• Energy change of a system equals to the work done on the system ∆W and heat ∆Q absorb by the system.
∆E =∆W +∆Q (1)
• For an ideal gas system, one has ∆W = −P∆V and ∆Q=C∆T, where C is the heat capacity at constant volume.
∆E = −P∆V +C∆T (2)
| ∆{Wz } |∆{zQ}
Sign of energy/work/heat change
• If ∆E/∆W/∆Q>0, the system gain energy/work/heat from the outside.
• If ∆E/∆W/∆Q<0, the system lose energy/work/heat to the outside.
• Recall ∆W = −P∆V
Week 3, problem 4

Temperature change caused by heat transfer
Heat capacity
• Recall ∆Q=C∆T ⇒ ∆T = ∆CQ.
• Exercise: let the heat capacity of a 10g substance be C =10J · K−1.
• How would the temperature change if this substance gain 100J heat?
• How would the temperature change if this substance lose 100J heat?

Temperature change caused by heat transfer
Specific heat capacity
• Now define specific heat capacity by c=C/m ⇔ C =mc, where m is the mass of the substance. The physical meaning is the heat absorbed per unit mass.
• From ∆Q=C∆T one can then derive
∆Q
∆Q=C∆T ⇒ ∆Q=mc∆T ⇒ ∆T = (3)
mc
• Exercise: let the specific heat capacitiy of a substance be c=5J · g−1·K−1
• How would the temperature change if 10 gram of such substance absorbs 1000J heat?
Week 3, problem 5

Week 3, problem 6