PHYSICS LECTURE TEMPERATURE

PHYSICS LECTURE – TEMPERATURE AND RELATED CONCEPTS

1. Temperature

Temperature is a measure of the degree of hotness or coldness of a body. It is a physical quantity that determines the direction of heat transfer between bodies.

Illustration:
Temperature gradient: Hot body 🔥 → Cold body ❄

2. Measurement of Temperature

Temperature is measured using:

• Liquid-in-glass thermometer: Mercury or alcohol in a glass tube expands with heat.
• Thermocouple: Junction of two different metals generates voltage depending on temperature.
• Resistance thermometer: Resistance of wire changes with temperature.

Liquid-in-glass thermometer:
|
| Mercury/Alcohol rises with heat ↑
|___________

3. Temperature and Gas Pressure

The pressure of a gas is directly proportional to its absolute temperature if the volume is kept constant (Gay-Lussac’s law):

P ∝ T or P1/T1 = P2/T2

Pressure-Temperature relation:
Cylinder with piston: Heating → Gas molecules move faster → Pressure ↑

4. Pressure Laws

• Boyle’s Law: At constant temperature, P × V = constant
• Charles’ Law: At constant pressure, V ∝ T
• Gay-Lussac’s Law: At constant volume, P ∝ T

5. Thermometer and Thermocouple

• Thermometer measures temperature; liquid expands with heat.
• Thermocouple generates a voltage difference between two metals, giving temperature readings.

Thermocouple setup:
Metal A + Metal B → Junction → Voltage ∝ Temperature → Display

6. Molecular Expansion of Temperature

As temperature increases:

• Molecules vibrate faster.
• Solids expand (linear, area, volume expansion).
• Liquids expand (e.g., mercury in thermometers).
• Gases expand and pressure rises if volume is fixed.

Linear Expansion:
|---------| → |---------| (length increases)
Material expands with heat

7. Summary

• Temperature measures hotness/coldness.
• Thermometers and thermocouples are primary instruments.
• Pressure of gases rises with temperature (Pressure Law).
• Molecular expansion explains solids, liquids, gases behavior on heating.