Resistivity

Question 1

What does the resistance of a wire depend on ?

Answer 1

The length of a wire
The cross sectional area of a wire
The material at which the wire is made

Question 2

What is resistivity?

Answer 2

Numerically equal to the resistance of a piece of material of length one metre and cross sectional area of 1m

Question 3

What does a good conductor have ?

Answer 3

Low resistivity

Question 4

Simple model for resistivity in metals

Answer 4

Atoms is a solid share of one or more electrons with other atoms, free to move around. They are able to move when an emf is applied to the metal, this implies huge number of conduction electrons have a certain amount of thermal energy which increases temp so they move around randomly within the metal

Question 5

Simple model for resistivity in semiconductors

Answer 5

Electrons are held in bonds between the atoms some electrons have enough thermal energy to escape from these bonds so become conduction electrons and are free to carry a current
Much higher R and p than metals
As temp increases thermal energy increases so more electrons have sufficient energy to escape from their bonds
Number of conduction electrons increases as temp increases
Semiconductor becomes a better conductor with a lower p as temp rises

Question 6

Simple model for resistivity in insulators

Answer 6

Energy required to free electrons from their bonds is higher not enough thermal energy so no conduction electrons

Question 7

What is the effect on temp on resistors ?

Answer 7

R of a pure metal increases as temp increases
For pure metals the change in R with temp is linear at greater temps the ions within the metallic lattice have greater KE and vibrate more causing more R to the current
Negative temp coefficient semiconductors have a resistance that decreases with temp can be used in thermistors to measure temp or design circuits that are temp controlled. The extra conduction electrons freed at higher temps decreases the resistance

Question 8

Some values for resistivity

Answer 8

Metal: copper= 1.710^-8
Aluminium=2.710^-8
Alloys: constantan=4910^-8
Nichrome=11010^-8
Semiconductors: germanium= 0.6
Sillicon= 2300
Insulators: glass= 10^10
Polystyrene=1.0*10^15