Heat & Calorimetry

Heat and Calorimetry

Heat always flows from a hotter object to a colder one, never the other way, until both reach the same temperature. How much energy that takes depends on three things: how much stuff you're heating, what it's made of, and how big a temperature change you want.

Q = m · c · ΔT

• Q — heat energy (J): the energy transferred into or out of an object as heat
• m — mass (kg): how much of the substance is being heated or cooled
• c — specific heat (J/(kg·K)): the energy needed to raise one kilogram of the material by one kelvin
• ΔT — temperature change (K): the difference between the final and initial temperature

How much heat energy is needed to warm 2 kg of water by 5 kelvin? (specific heat of water ≈ 4000 J/(kg·K))

• m = 2 kg
• c = 4000 J/(kg·K)
• ΔT = 5 K

1. Q = m · c · ΔT
2. Q = 2 kg × 4000 J/(kg·K) × 5 K

Q = 40,000 J

Instead of 2 kg, you now have 4 kg of water and still want to raise it by 5 kelvin. How much heat energy is needed?

• 40,000 J
• 160,000 J
• 80,000 J

Back to the 2 kg of water, but now you want to raise its temperature by 10 kelvin instead of 5. How much heat energy is needed?

• 80,000 J
• 40,000 J
• 20,000 J