Electricity - Thermistors and LDR's

Question 1

drifting delocalised electrons

Answer 1

Drifting delocalised electrons collide with the atoms of the component, doing work on them.

Question 2

These atoms

Answer 2

These atoms therefore vibrate with a greater amplitude, increasing the temperature of the component.

Question 3

The increased vibrational amplitude

Answer 3

The increased vibrational amplitude of the atoms also increases the probability (and hence rate) of further collisions, reducing the drift velocity of the electrons.

Question 4

Increasing the pd across a component that is increasing in temperature

Answer 4

Increasing the pd across a component that is increasing in temperature therefore produces a disproportionately smaller increase in current. The resistance therefore increases (as R = V/I).

Question 5

Thermistors

Answer 5

When the temperature of a thermal resistor increases, the greater atomic vibrational amplitude delocalises additional electrons (charge carriers), which more than compensates for their reduced drift velocity (due to collisions with atoms).

Question 6

Increasing the pd across a thermistor that is increasing in temperature

Answer 6

Increasing the pd across a thermistor that is increasing in temperature therefore produces a disproportionately larger increase in current. The resistance therefore decreases (as R = V/I).

Question 7

Light Dependant Resistors:

Answer 7

When the light intensity falling on an LDR increases, a variant of the photoelectric effect delocalises additional electrons: the resistance therefore decreases.