Heat and Thermodynamics
Track where thermal energy goes, interpret heating evidence, and solve calorimetry and energy-balance problems efficiently.
Heat and Thermodynamics
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Heat is energy crossing a boundary; temperature tells how energetic particle motion is on average
Heat flows spontaneously from a higher-temperature object to a lower-temperature object until thermal equilibrium. Temperature does not measure the total energy of an object: a large amount of warm water can contain more internal energy than a tiny, hotter drop.
In the first-law convention above, Q is heat added to the system and W is work done by the system.
Temperature change and phase change use different equations
Use mcΔT while temperature changes within one phase. Use mL during melting, freezing, boiling, or condensation at essentially constant temperature.
Heat transfer has three mechanisms
Conduction needs contact, convection involves bulk fluid motion, and radiation travels as electromagnetic waves—even through a vacuum.
Follow the energy before choosing a formula
Temperature changes? Use Q = mcΔT.
Phase changes? Use Q = mL; do not invent a ΔT across a flat heating-curve segment.
Objects mix? Set energy lost by the warmer object equal to energy gained by the cooler object.
A system absorbs heat and performs work? Subtract work done by the system: ΔU = Q − W.
Why it works
Thermal problems become confusing when every energy transfer is forced into one formula. Identifying the physical stage first prevents that error.
Five forms you should recognize
Problem: Raise 200 g of water by 5°C; c = 4.2 J/(g·°C).
Q = 200(4.2)(5) = 4200 JProblem: Melt 50 g of ice already at 0°C; Lf = 334 J/g.
Q = 50(334) = 16,700 JThe temperature stays near 0°C while the phase changes.
Problem: A hot metal is placed in cooler water in an insulated cup.
|Qmetal lost| = |Qwater gained|Use each material’s own mass, specific heat, and temperature change.
Problem: A gas absorbs 500 J and does 200 J of work.
ΔU = 500 − 200 = +300 JMetal spoon: conduction. Rising warm air: convection. Sun warming Earth: radiation.
Check before you commit
- Calling temperature the total heat stored
- Using Celsius values where only a temperature difference is needed but changing the difference incorrectly
- Using mcΔT during a constant-temperature phase change
- Forgetting that evaporation removes high-energy molecules
- Assuming a vacuum stops radiation
- Adding work instead of subtracting work done by the system
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Heat and Thermodynamics FAQ
Why can temperature stay constant while heat is added?
During a phase change, energy changes intermolecular potential energy rather than average kinetic energy.
Does “cold” flow into an object?
No. Net thermal energy flows from the warmer object toward the cooler one.
Why does water moderate climate?
Its high specific heat lets it absorb and release large amounts of energy with smaller temperature changes.
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