1aTemperature and Thermal Energy
Explore temperature scales, unit conversions, absolute zero, and the zeroth law of thermodynamics.
Temperature and thermal energy are the foundation of all thermodynamics. Every heat engine, refrigerator, weather system, and living cell operates because of temperature differences — understanding the scales and what temperature really means unlocks the rest of Unit 3.
Lesson Overview
Temperature measures the average kinetic energy of particles in a substance, while thermal energy is the total kinetic energy of all particles. In this lesson you will master the three temperature scales, convert between them, understand absolute zero, and apply the zeroth law of thermodynamics to thermal equilibrium.
Key Concepts
Temperature
Average kinetic energy per particle; measured in °C, K, or °F
Thermal Energy
Total kinetic energy of ALL particles in a substance
Kelvin Scale
Absolute scale; 0 K = absolute zero; K = °C + 273.15
Absolute Zero
0 K (−273.15 °C) — minimum possible temperature; no thermal motion
Thermal Equilibrium
Two objects at the same temperature; no net heat flow between them
Zeroth Law
If A is in equilibrium with C, and B is with C, then A is in equilibrium with B
Convert 25 °C to Kelvin.
Convert 310 K to Celsius.
Convert 98.6 °F (body temperature) to Celsius.
A cup of hot coffee and a large pot of hot coffee are both at 80 °C. Which has more thermal energy?
A metal spoon is placed in a bowl of warm soup. After several minutes, both are at 60 °C. What has occurred?
Convert −40 °C to Kelvin.
Hint: Use K = °C + 273.15. Note: −40 °C = −40 °F — a fun coincidence!
A gas is cooled to 150 K. What is this temperature in Celsius?
Hint: Rearrange the Kelvin formula: °C = K − 273.15.
Two identical iron blocks — one at 200 °C and one at 50 °C — are placed in contact. What happens to their temperatures over time?
Hint: Think about the direction of heat flow and what equilibrium means.
Why is the Kelvin scale preferred in scientific calculations over Celsius?
Hint: Consider what happens to gas laws when temperature reaches 0 on each scale.
Object A is in thermal equilibrium with object C. Object B is also in thermal equilibrium with object C. What can you conclude about A and B?
Hint: State the zeroth law of thermodynamics.
Key Vocabulary
Temperature
A measure of the average kinetic energy of the particles in a substance.
Example: Water boils at 100 °C (373.15 K) at standard pressure.
Thermal Energy
The total kinetic energy of all particles in a substance; depends on both temperature and the number of particles.
Example: A bathtub of warm water has more thermal energy than a cup of boiling water.
Absolute Zero
The lowest possible temperature, 0 K (−273.15 °C), at which particles have minimum kinetic energy.
Example: Liquid helium approaches absolute zero at about 4 K.
Thermal Equilibrium
The state in which two objects in contact have the same temperature and no net heat flows between them.
Example: A thermometer reads correctly once it reaches thermal equilibrium with the patient.
Interactive Practice — 5 Questions
What does temperature measure?
What is the Kelvin equivalent of 0 °C?
Absolute zero is defined as:
The zeroth law of thermodynamics states that:
A large swimming pool at 30 °C vs. a small cup at 80 °C — which has more thermal energy?
Independent Practice
Convert the following temperatures to Kelvin: (a) 100 °C, (b) −196 °C (liquid nitrogen), (c) 20 °C.
Explain the difference between temperature and thermal energy using two objects of different sizes at the same temperature.
A thermometer is placed under a patient's tongue. Explain, using the zeroth law, why the thermometer gives an accurate reading.
Why is it impossible to reach absolute zero in practice? What happens to particle motion as temperature approaches 0 K?
★ The surface of the Sun is approximately 5778 K. Convert this to Celsius and Fahrenheit. Then explain why scientists prefer Kelvin for stellar temperatures.
ChallengeCommon Mistakes
Using Celsius in gas-law formulas (e.g., PV = nRT).
Always convert to Kelvin first. Using °C gives wrong answers because 0 °C ≠ zero thermal energy.
Confusing temperature and thermal energy — assuming the hotter object always has more thermal energy.
A large cold object can have more thermal energy than a small hot one because thermal energy depends on particle count too.
Math Tips
Three conversion formulas to memorize: K = °C + 273.15 | °C = (°F − 32) × 5/9 | °F = °C × 9/5 + 32. For quick estimates, use 273 instead of 273.15.