# Resistance

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## Voltage and Resistance

• Resistance is a material's opposition to the flow of electric current; measured in ohms.
• Voltage, also known as potential difference. is a measure of the energy provided to the charge carriers. It can be defined as the amount of work done per unit charge
• V =W/Q
• R = V/I
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## Ohm's Law

• Ohm's Law: "The current in an ohmic calculator is proportional to the voltage across it, provided that the temperature and other physical conditions are kept constant".
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## The effect of temperature on resistance

• Electrical resistance is similar to friction in that it is a resistance to movement. Electrons drift slowly through a conductor when a voltage is put across the ends. The metal's atoms intefere with the motion of the electrons, causing resistance.
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## Resistivity

• Resitivity is a measure of the resisting power of a specified material to the flow of an electric current
• Resistivity = (Resistance x Cross-Sectional Area)/Length
• Resistivity = (RA)/L
• Resistivity is measured in ohm metres
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## Current and Drift Velocity

• I = nAve
• I = Current (amps)
• n = Charge Particles per unit volume
• A = Cross-Sectional Area (m^2)
• v = drift velocity (ms^-1)
• e = Charge of an electron (1.6x10^-19C)
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## Critical Temperature

• The temperature at which the electrical resitivity drops to zero is called the critical temperature
• The transition to superconduction is abrupt and complete. The resitivity of the superconducting material is at least 10^12 times less than that of the material at room temperature
• The materials are cooled using liquid nitrogen (77K) or liquid helium (4K)
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## Superconductivity

• A superconductor is defined as a material with zero electrical resistance
• At critical temperatures, free electrons couple to form Cooper pairs. These are more stable than a single electron.
• A passing electron attracts the lattice of the superconducting structure, drawing the positive nuclein inwards and causing a positive ripple in its path. Another electron is attracted to that area of increasing positive charge,
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## Superconducting Materials

• There are two types of superconductors: Type I and Type II
• Type I are majoritively metals whilst Type II are inorganic, ceramic solids
• Type II superconductors have critical temperatures of around 120K and are therefore called high temperature superconductors
• Superconductors are very expensive and have to be kept very cold
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## Uses of Superconductors

• Power transmission in overhead cables creates heating in the cables and hence a loss of energ. Superconductive cables would eliminate this problem.
• Superconductors are needed to create extremely strong, stable magnetic fields.
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