Thermodynamics

1
Thermodynamics

• Ch. 21 to 24

2
Heat

• Heat is the energy that flows because of
  temperature differences.
• Temperature is a measure of the internal energy
  of a substance.

3
Thermal Contact

• When heat flows from one object to another that
  it is in contact with, this is called thermal
  contact.
• When thermal contact exists, heat flows from the
  higher temperature substance (warmer one) to the
  lower temperature substance (cooler one).

4
HEAT FLOWS ACCORDING TO TEMPERATURE DIFFERENCES
FROM HOT TO COLD
5
Thermal Equilibrium

• Equilibrium means balance.
• After objects in thermal contact with each other
  reach the same temperature, no heat flows between
  them.
• They are considered to be in thermal equilibrium.

6
Energy

• Potential energy represents the forces between
  molecules.
• Kinetic energy represents moving atoms within the
  molecules.
• The grand total of all energies inside a
  substance is called internal energy.

7
SUBSTANCES DO NOT CONTAIN HEAT, THEY CONTAIN
ENERGY.
8
Measuring Heat

• The calorie is the amount of heat required to
  raise the temperature of 1 gram of water by 1ºC.
• A kilocalorie or kCal is 1000 calories.
• When they use the term calorie on food labels,
  they actually mean a kCal.

9
Calories

• Again, this is a measurement only used in the
  United States.
• Everywhere else uses the Joule or the kilojoule
  (kJ).
• 1 calorie 4.184 Joule

10
Calories

• The energy value of food is determined by burning
  the food and measuring the energy that is
  released as heat.

11
Heat Capacity

• Water has a higher capacity for storing energy
  than most common materials.
• We say water has a high specific heat.
• This makes water a good cooling agent.
• Water takes longer to cool down.

12
Heat

• Gases expand and contract more than liquids.
  (Molecules are more free to move.)
• Liquids expand and contract more than solids for
  the same reason.

13
The math of thermo

• 1 liter of water _at_ 30C is mixed with 1 liter of
  water _at_ 20C . What is the final temperature of
  the mixture?

14
The equation.

• T1V1 T2V2 T3V3

15
The solution

• T1V1 T2V2 T3V3
• (30 x 1.0) (20 x 1.0) (T x 2.0)
• 30 20 2T
• 50 2T
• T 25C