⇒ When current flows through the material of a circuit, such as the metal of the connecting wires, the material of the circuit gets in the way of the flow of the charge

• On a microscopic level, as the electrons flow through the metal they collide with the vibrating positive ion cores of the metal structures
• The collisions between the electrons and the positive ion cores transfer electrical energy from the electrons to the structure of the metal, causing the metal ion cores to vibrate more, thus heating up the wire

⇒ The opposition of a component to the flow of electric current through it is called resistance, symbol R, unit Ohms, Ω

⇒ The model of the metal shown above explains what happens to the resistance of the metal when it is heated

• The resistance increases as the temperature increases
• The vibrating, positively charged, ion cores move around much more as the temperature increases, thus getting in the way of electron flow
• This opposes the flow of the fas of electrons moving through the structure

⇒ Components with very high resistances let very ittle current through them, and are considered to be electrical insulators

• Some materials, at very low temperatures, have zero resistance - these materials are called superconductors

⇒ A current set up in a loop of superconducting materials carries on flowing indefinitely

⇒ Superconductors also exclude magnetic fields inside them - this llows a strong permanent magnet to be repelled and held above the superconductor

⇒ Superconducting materials completely lose their resistance below a temperature called the critical temperature, T_c

• Different superconducting materials have different critical temperatures; the metallic element tungsten, for example, has the lowest known criticial temperature of 0.015K (-273.135°C); and a type of ceramic copper oxide has the highest critical temperatureof 139K (-134.14°C), which is much higher than the boiling point of liquid nitrogen (77K)

⇒ As new materials are developed, so the superconducting critical temperature has risen substantially

• The ultimate goal is to develop superconductors with critical temperatures that are around room temperature

⇒ Imagine the world of possibilities with zero-resistance superconductors

• There could be, for example, electronic devices and computer units that don't generate any heat and don't need cooling fans; batteries that last for an extremely long time on one charge; cheap magnetic levitation; portable MRI scanners; super-strong electromagnets and electrical power transmission lines that don't waste any energy

⇒ Also see our notes on:

• Moving Charge and Electric Current
• Potential Difference and Electromotive Force
• Current/Potential Difference characteristics and Ohm's Law
• Thermistors
• Resistivity