12.3-12.4 - Resistance, resistivity and component characteristics

State Ohm's Law

The p.d. across a metallic conductor is proportional to current provided physical conditions don't change.

What units are equivalent to an Ohm?

VA⁻¹

What are the units of resistivity

Ωm

Resistivity is equivalent to...

The resistance between opposite faces of a one metre cube of the material

What is a Superconductor

A material with a resistivity of zero below a critical temperature

State uses of superconductors

MRI scanners, particle accelerators, maglev trains.

Why does resistance of a metallic conductor increase with temperature?

Lattice ion vibrations increase, causing more frequent collisions between free electrons and lattice ions.

Why does current lead to heating in resistors?

Electrons collide with the lattice ions and transfer some of their kinetic energy to the ion's vibrations.

What is electrical resistance?

The opposition of an object to a flow of current through it.

What units are equivalent to an Ohm?

VA⁻¹

What is meant by the threshold pd of a diode?

The pd at which resistance starts to fall rapidly as pd increases.

What is ρ in R = ρL / A?

The resitivity of the material.

What is L in R = ρL / A?

The length of the wire.

What is A in R = ρL / A?

The cross-sectional area of the wire

As a metal gets hotter what happens to its resitivity?

It increases.

What is meant by a negative temperature coefficient component?

One in which the resistance drops as temperature increases.

What are thermistors commonly used in?

Temperature sensors.

What are LDRs commonly used in?

Light intensity sensors.

What happens to the resistance of an LDR as light intensity increases?

It decreases.