Measuring Thermal Energy

1
Measuring Thermal Energy

• Critical Thinking Question
• Why do you add a coolant that includes water and
  ethylene glycol to the radiator in your car
  rather than just water?

2
Specific Heat

• Definition The amount of energy it takes to
  raise the temperature of 1 kg of the material 1
  Kelvin
• Measured in Joules per kilogram per Kelvin or
  J/(kg x K)
• What does does all of this mean?

3
How does the specific heat of water (4184 J/kg
x K) compare to the specific heat of iron (450
J/kg x K)?

• Water has a greater specific heat. So!!!!
• This means that it takes a lot more energy to
  heat up water than it does to heat up iron!
• or
• The water must absorb more heat than iron in
  order to change temperature

4
Calculating Changes in Thermal Energy

• Changes in thermal energy cannot be measured
  directly, but they can be calculated
• Q m x ?T x C
• Where Q change in thermal energy it is always
  
• m mass
• ?T Change in temperature
• ?T Tfinal - TInitial
• C Specific heat
• - Specific Heat is already known for
• most things

5
Calculate

• A .032 kg silver spoon (specific heat is 235
  j/(kg x K)) cools from 60 ºc to 20 ºc. What is
  the change in thermal energy?
• Q m x ?T x C
• Q .032 kg x (20 ºc - 60 ºc) x 235 j/(kgxK)
• Q -301 j (Remember Q is always positive so you
  need to take the absolute value)
• Q 301 j
• So, the spoon loses 301 j of thermal energy as it
  cools

6
Why do you add a coolant that includes water and
ethylene glycol to the radiator in your car
rather than just water?

• Answer The ethylene glycol has a higher
  specific heat than water so the mixture of water
  and ethylene glycol will absorb more heat from
  the engine than just water.