Ch.10.2 The First Law of Thermodynamics
Warm Up
Heat and Work
Work can be used to create heat
Heat and Work
The opposite is also true: heat can be used to create work
→ As the heat boils the water, it will do work on the balloon, making it expand
Heat and Work – �Combustion Engines
Fuel is ignited, creating heat which expands the fuel into gas, and raises the piston
As the gas cools/escapes, the piston lowers again
[piston animation]
Heat and Work – �Combustion Engines
Q > 0
W > 0
Q < 0
W < 0
Work done by system
Work done on system
Energy added as heat
Energy lost as heat
[table on pg. 343]
Check for Understanding
Is heat being added (Q>0) or removed (Q<0) from system?
Is work being done by system (W>0) or on system (W<0)?
Baking bread in the oven
Q>0, W>0
Using sandpaper on wood
Q<0, W<0
Putting coal in a steam engine
Q>0, W>0
Internal Energy
Heat added to system, Q
Work done by system, W
Change in internal energy of system: ∆U (Joules)
First Law of Thermodynamics
The change in the internal energy of a system is equal to the amount of heat supplied to the system minus the amount of work done on its surroundings
First Law of Thermodynamics
∆U = Q – W
First Law of Thermodynamics for Special Processes
→ no work done (W = 0)
→ no change in internal energy (∆U = 0)
→ no energy transferred as heat (Q = 0)
→ no energy transferred as heat, no work done by system, no change in internal energy (W = ∆U = Q = 0)
First Law of Thermodynamics
The first law of thermodynamics is a form of conservation of energy which takes into account a system’s internal energy
A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transformed as heat? Has energy been added to or removed from the refrigerant as heat?
Textbook Example pg.345
U
D
W = -135 J
∆U = 114 J
Q = ?
F
A
S
∆U = Q – W
Q = ∆U + W
Q = 114 J – 135 J
Q = -21 J
Since Q<0, energy has been removed from the system
Homework
Pg.346 #1-5