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Expert-verifiedAn electronic device designed to operate at any temperature in the range from \(-10.0°{\rm{C}}\) to \(55.0°\;{\rm{C}}\) contains pure carbon resistors. By what factor does their resistance increase over this range?
The resistance is increased by the factor \(1.03\).
The resistance of a conductor can be given in terms of the temperature as:
\(R = {R_0}\left( {1 + \alpha \left( {T - {T_0}} \right)} \right)\)
The value of \({R_0}\) is the resistance at some reference temperature, and the value of \({T_0}\) and the value of \(\alpha \) is the temperature coefficient of resistivity.
Resistance of the carbon resistors corresponding to the lower limit of the temperature range is found from the above equation as:
\({R_{lower}} = {R_0}\left( {1 + \alpha \left( {{T_{lower}} - {T_0}} \right)} \right)\) ………………(I)
The value we take here is: \({T_0}{\rm{ }} = {\rm{ }}{20^\circ }\;{\rm{C}}\) which has to be the reference temperature.
Resistance of the carbon resistors corresponding to the upper limit of the temperature range is found from the above equation as:
\({R_{Upper}} = {R_0}\left( {1 + \alpha \left( {{T_{Upper}} - {T_0}} \right)} \right)\) …………….(II)
Dividing the first equation with the second equation, we get:
\(\begin{align}\frac{{{R_{lower}}}}{{{R_{upper}}}}{\rm{ }} &= {\rm{ }}\frac{{{R_0}\left( {1 + \alpha \left( {{T_{lower}} - {T_0}} \right)} \right)}}{{{R_0}\left( {1 + \alpha \left( {{T_{Upper}} - {T_0}} \right)} \right)}}\\ &= {\rm{ }}\frac{{1 + \alpha \left( {{T_{lower}} - {T_0}} \right)}}{{1 + \alpha \left( {{T_{Upper}} - {T_0}} \right)}}\end{align}\)
Entering the values and we obtain is:
\(\begin{align}\frac{{{R_{lower}}}}{{{R_{upper}}}}{\rm{ }} &= {\rm{ }}\frac{{1 + ( - 0.5 \times {{10}^{ - 3}}^ \circ \;{{\rm{C}}^{ - 1}})( - {{10.0}^ \circ }\;{\rm{C}} - {{20.0}^ \circ }\;{\rm{C}})}}{{1 + ( - 0.5 \times {{10}^{ - 3}}^ \circ \;{{\rm{C}}^{ - 1}})({{55.0}^ \circ }\;{\rm{C}} - {{20.0}^ \circ }\;{\rm{C}})}}\\ &= \frac{{1.015}}{{0.9835}}\\ &= {\rm{ }}1.03\end{align}\)
Therefore, the factor by which the resistance increases is: \(1.03\).
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