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We know both the electrical resistance and the thermal resistance of a cylinder is inversely proprortional to the square of the radius of the cylinder. But the letters R and r are used sometimes for the radius and sometimes for either the electrical resistance or the thermal resistance. So how do we distinguish the radius from either the electrical or the thermal resistance? And how do we distinguish thermal resistance from electrical resistance?

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Authors have the responsability for clarifying the symbols they use in their texts.

Thus $R$ or $r$ can stand for a radius or a resistance (or even anything else!) but this should be made clear to readers. There is no agreed convention on how a radius or resistance should be noted, it's up to the author. This is true of other variables too.

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Context is king and the context should make it clear. There's nothing to prevent an author from using $R$, $r$, $R_0$, $\rho$ or whatever other symbol they want to use, provided the meaning of the symbol is made clear.

Indeed you don't want to be hostage to a single symbol simply because exhaustion of the alphabet means the same letter-symbol may have to appear more than once in completely different context. For instance $R$ might denote in a given problem the (constant) outer radius of a cylinder but $r$ might be a variable denoting the radial distance in cylindrical coordinates.

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