Resistance & Temperature

Metals (Positive Temperature Coefficient)

• The resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). of a metal increases with temperature.
• At higher temperatures, metal ions vibrate with greater amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m)..
• This increases the frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). of collisions with conduction electronsDelocalised electrons in a metal that are free to move through the lattice structure and carry electric currentThe rate of flow of charge. Measured in amperes (A)...
• More collisions mean less current per voltThe SI unit of potential difference and EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. One volt is one joule per coulomb.: higher resistanceThe opposition to current flow. The ratio of potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). to current. Measured in ohms (Ω)..
• The number of charge carriers stays roughly constant.

NTC Thermistors (Negative Temperature Coefficient)

• Higher temperature releases more charge carriersParticles that carry electric charge through a material, such as electrons in metals or ions in electrolytes. (conduction electronsDelocalised electrons in a metal that are free to move through the lattice structure and carry electric current.).
• More charge carriers means more current per voltThe SI unit of potential difference and EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. One volt is one joule per coulomb.: lower resistanceThe opposition to current flow. The ratio of potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). to current. Measured in ohms (Ω)..
• The change in resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). per kelvin is much greater than for metals.
• This makes NTC thermistors useful in temperature-sensing circuits.

Light-Dependent Resistors (LDRs)

• An LDRLight-dependent resistor. A component whose resistance decreases as light intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. increases.'s resistance decreases as light intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. increases.
• More light releases more charge carriers in the semiconductor.
• Used in light-sensing circuits (e.g. automatic street lights).

Superconductivity

• Below the critical temperatureThe temperature at or below which a material becomes a superconductor, with zero electrical resistance., resistance drops to exactly zero.
• Since R = 0, $V = IR = 0. No p.d. across$ the superconductorA material that has zero resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m). below its critical (transition) temperature..
• Current flows with no heating effect and no energy loss.
• Applications: MRI scanners, particle accelerators, powerThe rate of energy transfer. Measured in watts (W). transmission cables.
• Superconducting cables would make power transmission much more efficient.
• The critical temperature depends on the specific material.