Resistance changes with temperature and lightEdexcel A-Level Physics: Flashcards

What these 12 flashcards ask

Why does a metal's resistance increase with temperature?
Does the number of conduction electrons in a metal change much on heating?
Why does an NTC thermistor's resistance fall as it is heated?
What does NTC stand for?
What happens to the lattice vibrations in a thermistor on heating?
Why does the resistance of an LDR fall in bright light?
Write the equation linking current to charge carriers.
Why is the resistance change in a thermistor much larger than in a metal?
A metal wire's resistance increases when heated. What happens to the drift velocity for a fixed p.d.?
How do you explain a resistance change using I = nqvA?
What is a common wrong answer for why a hot metal has more resistance?
An LDR carries 2.0 mA in bright light and 40 μA in the dark at fixed p.d. By what factor does n change?

Exam questions on Resistance changes with temperature and light

A platinum resistance thermometer uses a coil of platinum wire. The resistance of the coil is 100 Ω at 0 °C and increases steadily as the temperature rises.
The same p.d. is applied across the coil at a higher temperature. Using I = nqvA, explain what happens to the mean drift velocity of the conduction electrons.2 marks
A light-dependent resistor (LDR) has a resistance of 1.5 kΩ in bright light and 90 kΩ in darkness. It is connected across a 3.0 V cell of negligible internal resistance.
Explain, in terms of conduction electrons, why the resistance of the LDR is lower in bright light.2 marks
An NTC thermistor has a resistance of 8.0 kΩ at 15 °C and 1.2 kΩ at 60 °C. It is connected across a 5.0 V supply of negligible internal resistance.
Calculate the power dissipated in the thermistor at 15 °C and at 60 °C.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).