Overview
This lecture covers temperature and heat, their measurement, the concept of thermal equilibrium, temperature scales, thermal expansion, and heat capacity, with emphasis on engineering and industrial applications.
Temperature Measurement & Thermometers
- Temperature is measured using devices like glass, gas, platinum, Cernox, infrared, and bimetallic thermometers.
- Glass thermometers contain liquid (e.g., mercury or ethanol) that expands/contracts with temperature.
- Gas thermometers measure temperature via pressure changes in a fixed-volume gas.
- Platinum thermometers (resistance thermometers) use changes in metal resistivity to measure temperature, sensitive at high temperatures.
- Cernox (semiconductor) thermometers are highly sensitive at low temperatures.
- Infrared thermometers measure temperature from a distance using infrared detection.
- Bimetallic strip thermometers bend due to differential expansion of two metals.
Thermal Equilibrium & Zeroth Law
- Thermal equilibrium is when two objects in contact reach the same temperature.
- The Zeroth Law of Thermodynamics states: if A and B are each in equilibrium with C, then A and B are in equilibrium with each other.
- Thermal conductors facilitate heat transfer; insulators prevent it.
Temperature Scales & Conversions
- Celsius: 0°C = freezing, 100°C = boiling point of water.
- Fahrenheit: 32°F = freezing, 212°F = boiling point of water.
- Kelvin: absolute zero (0 K) is -273.15°C, no negative values.
- Convert: ( T_F = (9/5)T_C + 32 ), ( T_C = (5/9)(T_F - 32) ), ( T_K = T_C + 273.15 ).
Applications of Low Temperature
- Liquid helium (4.2 K) and liquid nitrogen (77 K) are used in scientific, industrial, and medical applications.
- Special insulation and safety measures are needed when handling cryogenic liquids.
Thermal Expansion
- Most materials expand when heated and contract when cooled (thermal expansion).
- Linear expansion: ( \Delta L = \alpha L_0 \Delta T ), where α is the linear expansion coefficient.
- Volume expansion: ( \Delta V = \beta V_0 \Delta T ), with (\beta) as the volume expansion coefficient.
- Holes in materials also expand with the surrounding material.
- Water is anomalous: between 0°C and 4°C, water decreases in volume as temperature increases.
Engineering Considerations & Thermal Stress
- Gaps in bridges, railroads, and buildings accommodate thermal expansion.
- Thermal stress arises when expansion is constrained: ( \text{Stress} = Y\alpha\Delta T ).
- Use materials with low expansion coefficients (e.g., Invar, quartz) for precision applications.
Heat and Specific Heat Capacity
- Heat (Q) needed to raise temperature: ( Q = mc\Delta T ), where c is specific heat.
- Calorie: heat required to raise 1g water by 1°C.
- Molar heat capacity: ( Q = nC\Delta T ), where C is molar heat capacity.
- Water has a much higher specific and molar heat capacity than metals.
Key Terms & Definitions
- Thermal Equilibrium — State when two objects in contact have equal temperatures.
- Zeroth Law of Thermodynamics — If two systems are each in equilibrium with a third, they are in equilibrium with each other.
- Specific Heat (c) — Heat required to raise unit mass of material by 1 K.
- Linear Expansion Coefficient (α) — Fractional change in length per unit temperature change.
- Thermal Stress — Stress produced in a material when thermal expansion is prevented.
- Kelvin Scale — Absolute temperature scale starting at absolute zero (0 K).
Action Items / Next Steps
- Review thermal expansion coefficients for common engineering materials.
- Practice temperature and heat calculation problems.
- Read next chapter on heat transfer: conduction, convection, and radiation.