How would you calculate #DeltaE# of a gas for a process in which the gas absorbs 35 J of heat and does 8 J of work by expanding?

1 Answer
Jul 10, 2016

This is asking you to apply the first law of thermodynamics:

#stackrel("otherwise known as")stackrel(DeltaU)overbrace(\mathbf(DeltaE))\mathbf( = q + w)#

where:

  • #DeltaE# is the change in internal energy.
  • #q# is the heat flow in the thermodynamic process.
  • #w# is the expansion/compression work performed by the gas or on the gas.

Since the work is done by the gas (rather than on the gas) through the expansion, #w < 0#. Work is also defined as:

#w = -PDeltaV = -P(V_2 - V_1)#

where final volume #V_2# is larger than initial volume #V_1# in an expansion, i.e. when work is negatively-signed. If work were to be positively-signed, then #V_2# would be smaller than #V_1#.

Since the gas absorbs heat, heat flows into the gas, i.e. #q > 0#.

Therefore:

#color(blue)(DeltaE) = ("35 J") + (-"8 J")#

#=# #color(blue)("27 J"#