Explain the term thermal resistance. State its SI unit and dimensions.

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Thermal resistance $R_T$ (§7.9.1.3) is defined by direct analogy with electrical resistance $V/I$ : just as electrical resistance is potential difference divided by current, thermal resistance is temperature difference divided by the RATE of heat flow, $R_T = (T_1-T_2)/P_{cond}$ , which from Eq. (7.37) works out to $R_T = x/(kA)$ for a slab of thickness $x$ , area $A$ and conductivity $k$ . It measures how strongly a particular slab or rod OPPOSES the flow of heat through it -- the lower the material's thermal conductivity $k$ (and the thicker or narrower the piece), the higher its thermal resistance, and materials/objects with high $R_T$ are good thermal insulators. Its SI unit is degree Celsius-second per joule (°C s/J), and …

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$Q. Explain various modes of heat transfer.$
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Heat travels by conduction (molecule-to-molecule, no bulk motion), convection (bulk fluid motion), or radiation (electromagnetic waves, no medium needed) -- each mechanism operates differently and dominates in different situations.

Conduction: Heat is transferred through a material by direct collisions between vibrating/energetic molecules (or free electrons in metals) passing kinetic energy to their neighbours, without the material itself moving from place to place. It is the dominant mode of heat transfer in solids, especially metals (good conductors). Example: one end of an iron rod placed in a fire becomes hot at the other end after some time.

Convection: Heat is transferred by the actual bulk movement of a fluid (liquid or gas) -- the heated fluid becomes less dense, rises, and is replaced by cooler fluid, setting up convection currents that carry thermal energy along with the moving fluid itself. This requires a fluid medium and cannot occur in solids or vacuum. Example: water boiling in a pot, air circulation in a heated room, sea breezes.

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$Q. Discuss various modes of transfer of heat.$
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Heat transfer occurs by three modes: conduction (through a stationary medium via molecular collisions), convection (via bulk fluid motion), and radiation (via electromagnetic waves, no medium needed).

Conduction: In this mode, heat is transferred from the hotter to the colder part of a body (typically a solid) through the vibration and collision of neighbouring molecules/free electrons, without any actual bulk movement of the material itself. Metals are good conductors due to free electrons; heat conducted through a rod is a classic example.

Convection: Here heat is carried by the physical, bulk movement of a fluid (liquid or gas). The heated fluid, becoming less dense, rises and is replaced by cooler fluid, setting up convection currents that transport heat from one region to another. Examples include boiling water in a vessel, land and sea breezes, and heating of a room by a radiator.

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Q.Explain the term thermal resistance. State its SI unit and dimensions.

Concept understanding — Heat Transfer

Q.Explain the term thermal resistance. State its SI unit and dimensions.

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