University:
Kentucky State University
Course:
PHY 212 | General Physics II
Academic year:

2019
Views:

164

Pages:

5
Author:

hacedoras1kxy

Temperature Dependence of Resistivity Many materials have resistivities that depend on temperature. We can model this temperature dependence empirically:   0 1    T  T0  , where 0 is the resistivity at temperature T0*, and  is the temperature coefficient of resistivity. Quiz: what is the dimension of ?   0.0005 C-1 *T0 is a reference temperature, often taken to be 20 °C. This approximation can be used if the temperature range is “not too great;” i.e. up to 100 °C or so. Example 5 : a carbon resistance thermometer in the shape of a cylinder 1 cm long and 4 mm in diameter is attached to a sample. The thermometer has a resistance of 0.030 . What is the temperature of the sample? This is the starting equation:   0 1    T  T0  We can look up the resistivity of carbon at 20 C and its coefficient: 0  3.519 105  m   0.0005 C-1 Notice that the coefficient is –ve: the resistance is reduced with increasing temperature. We use the thermometer dimensions to calculate the resistivity when the resistance is 0.03 , and use the above equation directly. Example: a carbon resistance thermometer in the shape of a cylinder 1 cm long and 4 mm in diameter is attached to a sample. The thermometer has a resistance of 0.030 . What is the temperature of the sample? L R A RA (R)  L (R  0.03)  2 0.03  0.002     0.01  3.7699 105   m Example: a carbon resistance thermometer in the shape of a cylinder 1 cm long and 4 mm in diameter is attached to a sample. The thermometer has a resistance of 0.030 . What is the temperature of the sample?   0 1    T  T0     T  T0    1 0  1  T  T0    1   0  Folklore; Ms. Sahar, 5th grade!  3.7699 105  1 T  20   1  122.6 C  5 0.0005  3.519 10  Electric Power • Power is the time rate of work done by a force: dWF PF  dt • Work done by electric force moving element of charge dq through potential difference V is dq V • Instantaneous power: dW dq P  V  IV. dt dt