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?

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?

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

Calculating Changes in Thermal Energy

• Changes in thermal energy cannot be measured directly, but they can be calculated

• Q = m x ΔT x C– Where Q = change in thermal energy; it is always +

m = mass

ΔT = Change in temperature

ΔT = Tfinal - TInitial

C = Specific heat - Specific Heat is already known for

most things

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 ΔT 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

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.