Resistivity, thermistors and superconductivityAQA A-Level Physics: Mind map

Exam questions on Resistivity, thermistors and superconductivity

1. A heating element is made from a nichrome wire of length 1.50 m and diameter 0.40 mm. The resistivity of nichrome at the operating temperature is 1.1 × 10⁻⁶ Ω m.
   The designer wants an element of resistance 20 Ω using the same wire. Calculate the length of wire required.2 marks
2. An incubator uses a negative temperature coefficient (ntc) thermistor as its temperature sensor. The resistance of the thermistor is 4.7 kΩ at 20 °C and 1.2 kΩ at 50 °C. A constant pd of 5.0 V is applied across the thermistor.
   The designer considers using a metal wire instead of the thermistor as the sensor. State how the resistance of a metal wire changes as its temperature rises, and explain why.2 marks
3. A power company is testing a 1.0 km cable made from a superconducting material with a critical temperature of 92 K. The cable is cooled by liquid nitrogen at 77 K and carries a constant current of 2.0 × 10³ A. For comparison, a copper cable of the same length would have a cross-sectional area of 5.0 × 10⁻⁴ m² and a resistivity of 1.7 × 10⁻⁸ Ω m.
   State what is meant by the critical temperature of a superconductor, and explain why cooling the cable with liquid nitrogen reduces the energy lost during transmission.3 marks