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Temperature and Heat: Definitions & Scales

CSEC Physics: Thermal Physics Foundation

Essential Understanding: Students often confuse heat and temperature. In CSEC Physics, distinguishing between internal energy (Heat) and the measure of average kinetic energy (Temperature) is crucial for understanding thermodynamics, heat transfer, and changes of state.

🔥 Heat: Energy in transit (Joules)
🌡️ Temperature: Avg. Kinetic Energy (Kelvin/Celsius)
❄️ Absolute Zero: 0 K = -273°C

The Core Distinction: Heat vs. Temperature

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Heat (Internal Energy)

Definition: Heat is a form of energy that is transferred from a region of higher temperature to a region of lower temperature.

  • Symbol: Q
  • SI Unit: Joule (J)
  • Depends on: Mass, material, and temperature change.
  • Analogy: The total amount of “money” (energy) a bank account has.
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Temperature

Definition: A measure of the degree of hotness or coldness of a body. It determines the direction of heat flow.

  • Symbol: T or θ (theta)
  • SI Unit: Kelvin (K)
  • Physical Meaning: The average kinetic energy of the molecules.
  • Analogy: The “value” of a single bank note, regardless of how many you have.

🔬 Interactive Lab: Kinetic Theory Visualization

Observe how particle motion (speed) changes with Temperature. Higher Temperature = Higher Kinetic Energy.

300 K

Temperature Scales

In Physics, we primarily use two scales. The Celsius scale is common in daily life, but the Kelvin scale is essential for scientific calculations and equations (like the Ideal Gas Law).

Feature Celsius (°C) Kelvin (K) Fahrenheit (°F) – Context
Freezing Point of Water 0 °C 273 K 32 °F
Boiling Point of Water 100 °C 373 K 212 °F
Absolute Zero -273 °C 0 K -460 °F
SI Unit? No (Derived) Yes No

Visualizing the Scales

Conversions and Calculations

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The Kelvin-Celsius Relationship

The size of 1 degree Celsius is exactly the same as 1 Kelvin. They only differ in their starting point (zero).

Conversion Formula

\[ T(\text{K}) = \theta(^\circ\text{C}) + 273 \] \[ \theta(^\circ\text{C}) = T(\text{K}) – 273 \]

Note: CSEC typically uses 273 for simplicity, though the precise value is 273.15.

1
Identify the unit: Check if the value is in °C or K. Look for the symbol.
2
Apply the offset: To go to Kelvin, add 273. To go to Celsius, subtract 273.
3
Check for validity: You cannot have negative Kelvin. If you get a negative value in Kelvin, check your calculation!

CSEC Exam Practice

Test Your Understanding

Question 1: Distinguish between heat and internal energy.

Answer: Internal Energy is the total potential and kinetic energy of the molecules of a substance (stored energy). Heat is the transfer of that internal energy from a hotter body to a colder body due to a temperature difference. Heat is energy in transit.

Question 2: Convert 25°C to Kelvin.

Answer: \( T = 25 + 273 = 298 \text{ K} \).

Question 3: Explain what is meant by ‘Absolute Zero’.

Answer: Absolute Zero (0 K or -273°C) is the theoretical temperature at which particles have the minimum possible kinetic energy. It is the lowest possible temperature; molecular motion virtually stops.

Question 4: A cup of tea at 80°C is poured into a large pot of water at 30°C. In which direction does heat flow?

Answer: Heat flows from the cup of tea (80°C) to the pot of water (30°C). Heat always flows from higher temperature to lower temperature until thermal equilibrium is reached.

🎯 CSEC Exam Tip

Kelvin vs Celsius: When using formulas like the Gas Laws (\( \frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2} \)), you must convert all temperatures to Kelvin first. Never use Celsius in gas law equations unless the formula explicitly accounts for the offset (which is rare in CSEC).

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