5.2 Resistance & Resistivity

How does materials have _________ to the flow of charge?

What is the relationship between free electrons as they move through the wire and ions?

Materials have resistance to the flow of charge: Current flow of charge, ions resisting flow causes resistance

Free electrons move through metal wire, collides with ions in the way so transfer some/all of their kinetic energy on collision causing electrical heating

What does resistance depend on?

Equation of Resistance and units =

Resistivity equation: The longer the wire = ,The thicker the wire (cross-sectional area) =

Resistivity (Ω m) is property describes….

Resistance depends on length of wire, cross-sectional area through which current passing, resistivity of the material

Equation of Resistance = R=ρL/A. A = Area = π × r/d²,

R = resistance ( Ω), ρ = Resistivity (Ωm) , L = length (m), Cross-sectional area (m²)

Resistivity equation: The longer the wire, the greater its resistance, The thicker the wire, the smaller its resistance

Resistivity (Ω m) is property describes extent a material opposes flow of electric current through it

Property of material, dependent on ______

Higher resistivity of material so it has…

Copper, relatively ______resistivity room temperature, used for electrical wires — ________ flows through very ______

Insulators have _____ resistivity, ______ current will flow through them

Cross-sectional area of a wire is proportional to the diameter ______ so if diameter doubles, area ______ causing resistance to ______ by a ________.

Property of material, dependent on temperature

Higher resistivity of material, higher its resistance

Copper, relatively low resistivity room temperature, used for electrical wires — current flows through very easily

Insulators have high resistivity, no current will flow through them

Cross-sectional area of a wire is proportional to the diameter squared so if diameter doubles, area quadruples causing resistance to drop by a quarter.

Solids: vibrating atoms

Higher the temperature =

What happens when electric current flows of free electrons in a material?

Current decreases, resistance will ________ (from _ = _ _)

What does metallic conductors obey and if temperature increases or decreases what does this cause?

In a thermistor what happens when increases in temeperature?

Solids: vibrating atoms

Higher the temperature, the faster these atoms vibrate

Electric current flow of free electrons in a material so electrons collide with vibrating atoms which impede their flow, so current decreases

Current decreases, resistance will increase (from V = IR)

Metallic conductor obeys Ohm's law: Increase in temperature causes increase in resistance, Decrease in temperature causes decrease in resistance

Thermistor, increase in temperature causes decrease in resistance

What is a thermistor?

What is ntc?

Thermistors temperature sensors uses:

• Thermistor hotter, resistance _______

• Thermistor cooler, resistance ______

Thermistor non-ohmic conductor, sensory resistor resistance varies with temperature

Most thermistor negative temperature coefficient (ntc) components so if temperature increases, resistance of thermistor decreases

Thermistors temperature sensors uses: circuits in ovens, fire alarms and digital thermometers

• Thermistor hotter, resistance decreases

• Thermistor cooler, resistance increases

Do all material have resistivity?

Resistance: ________ flows through material, ______ up and electrical energy _______ as _______ energy

How is resistivity of material lowered?

All materials have some resistivity - even good electrical conductors e.g. copper and silver

Resistance: electricity flows through material, heats up and electrical energy wasted as thermal energy

Resistivity of material lowered by lowering its temperature

Superconductor:

Critical temperature/temperature threshold/transition temperature:

Common superconducting material:

Are all materials superconductors?

Superconductor: A material with no resistance below a critical temperature.

Critical temperature: The temperature at which a material becomes superconducting

Common superconducting material mercury has a critical temperature of 4.2 K

Superconductivity is property only certain materials have the characteristics above

Disadvantages of superconductors:

What are superconductors useful for?

Why is superconductors used in strong magnetic fields:

Applications:

What and where are maglev trains?

Getting it having a low temperature cold is difficult and expensive - uses lots of power

Superconductors useful for applications that require large electric currents

Useful for production of strong magnetic fields, reduction of energy loss / dissipation in the transmission of electric power

Applications: MRI scanners, Transformers & generators - for fewer fire risks, Motors, Monorail trains, Maglev (magnetic levitation) trains, Particle accelerators - need large magnetic fields to accelerate particles, Fusion reactors, Electromagnets, Power / electrical cables, Microchips,

Maglev trains require extremely strong electromagnets to levitate the train due to such a large mass so cna travel at extremely high speeds up to 603 km / h

Maglev train systems currently only exist in Japan, South Korea and China