Heat

A. Heat vs Temperature:

• Heat ($Q$): The form of internal energy transferred between two bodies due to a temperature difference. It is the sum total of kinetic and potential energy of all molecules in the body. SI Unit: Joule ($\text{J}$) ($1\text{ cal} = 4.186\text{ J}$).
• Temperature ($T$): The physical quantity that measures the degree of hotness or coldness of a body, determined by the average kinetic energy of its molecules. SI Unit: Kelvin ($\text{K}$).

B. Thermometric Scales & Master Conversion Equation:

• Celsius Scale: Lower Fixed Point (Ice point) $= 0^\circ\text{C}$; Upper Fixed Point (Steam point) $= 100^\circ\text{C}$. (100 divisions).
• Fahrenheit Scale: Ice point $= 32^\circ\text{F}$; Steam point $= 212^\circ\text{F}$. (180 divisions).
• Kelvin Scale: Absolute zero $= 0\text{ K} = -273.15^\circ\text{C}$. $T(\text{K}) = t(^\circ\text{C}) + 273$.
• Classic Discovery: At what temperature do Celsius and Fahrenheit scales read the same value? $$\frac{x}{5} = \frac{x - 32}{9} \implies 9x = 5x - 160 \implies 4x = -160 \implies \mathbf{x = -40^\circ}$$ !

A. Clinical Thermometer:

• Designed specifically to measure human body temperature ($37^\circ\text{C}$ or $98.6^\circ\text{F}$).
• Range: $35^\circ\text{C}$ to $42^\circ\text{C}$ (or $94^\circ\text{F}$ to $108^\circ\text{F}$).
• The Kink (Constriction): Contains a narrow bend or constriction just above the bulb. When removed from the patient's mouth, the mercury column contracts and breaks at the kink, preventing mercury from falling back into the bulb before reading! It must be vigorously jerked before subsequent use.

B. Laboratory Thermometer:

• Used in science labs for general experiments. Range: $-10^\circ\text{C}$ to $110^\circ\text{C}$.
• Has NO constriction (kink). Temperature must be read while the bulb is still immersed in the liquid!

1. Conduction (Solids):

   Heat transfer through matter from particle to particle by molecular vibration without any actual macroscopic movement of the particles themselves. Metals (copper, aluminum) are excellent conductors; wood, air, and glass are insulators.

2. Convection (Fluids: Liquids & Gases):

   Heat transfer by the actual bulk bodily movement of heated, less dense fluid molecules upward, while cooler, denser fluid moves downward, creating Convection Currents.

   • Sea Breeze (Daytime): Land heats faster than sea $\to$ warm air over land rises $\to$ cool air blows from sea to land.
   • Land Breeze (Nighttime): Land cools faster than sea $\to$ warm air over sea rises $\to$ cool air blows from land to sea.
3. Radiation (Vacuum & Transparent Media):

   Heat transfer by electromagnetic infrared waves traveling at the speed of light without requiring any material medium (e.g., Sun's heat reaching Earth across 150 million km of empty space). Dull black surfaces are the best absorbers and emitters; shiny silver surfaces are the best reflectors.

A. The Thermos Flask (Dewar Flask):

• Double-walled Glass Vessel: Glass is a poor conductor of heat.
• Vacuum between Walls: Eliminates heat transfer by Conduction and Convection!
• Silvered Inner Surfaces: Minimizes heat loss or gain by Radiation (reflects infrared rays back).
• Cork/Plastic Stopper: Insulating material preventing convection loss through the neck.

B. Thermal Expansion in Solids:

• Railway Tracks: Small gaps are intentionally left between consecutive steel rails to accommodate linear expansion during hot summer days, preventing track buckling.
• Bimetallic Strip: Two different metal strips (e.g., brass and iron) welded together. Because brass expands more than iron when heated, the strip bends, acting as an automatic circuit breaker switch in thermostats (irons, fire alarms).