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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