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12 - Electricity (12.3 Resistance (Measurement of resistance (A resistor…
12 - Electricity
12.1 Current and charge
Electrical conduction
The electric current is the rte of flow of charge in the wire or component due to the passage of charged particles
These charged particles are known as charge carriers. In metals these charge carriers are conduction electrons while in an electrical current that is passed through a salt solution, the charge is carried by ions(charged atoms or molecules)
The convention for the direction of current in a circuit is from positive to negative. Electrons(negative) flow one way and as the opposite is considered positive, the current goes the opposite way compared to what way the electrons are flowing
The unit of current is ampere(A), which is defined in terms of the magnetic force between two parallel wires when they carry the same current. The symbol for current is I
The unit of charge is the coulomb(C), equal to the charge flow in one second when the current is one ampere. The symbol for charge is Q
△Q=I△t, charge flow =current x time
Charge carriers
Materials can be classified in electrical terms as conductors, insulators or semiconductors
In a insulator, each electron is attached to an atom and cannot move away meaning no current can pass through it as no electrons can move
In a metallic conductor, most electrons are attached to atoms but some are delocalised so when voltage is applied the delocalised electrons are attracted towards towards the positive terminal of the metal
In a semiconductor, the number of charge carriers increases with an increase of temperature while resistance decreased. Pure semiconductors are referred to as an intrinsic semiconductor as conduction is due to electrons that break free from the atoms
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12.3 Resistance
Definitions and laws
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Resistance of components in a circuit is a measure of difficulty of making current pass through component. Resistance is caused by repeated collisions of charge carriers and fixed positive ions of material
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Resistivity = resistance x area of cross section/ length of conductor, p=RA/L(unit = ohm meter, Ωm)
Superconductivity
Superconductor is a wire or a device made of material that has zero resistivity at and below a critical temperature that depends on the material
When resistance is zero, there is no pd across it has current passes through it(no heating effect)
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Any material with critical temp above 77K(-196°C), boiling point of nitrogen, is refereed to as a high-temperature superconductor
Superconductors are used to make high-power electromagnets that generate very strong magnetic fields in devices such as MRI scanners and particle accelerators
Strong magnetic fields also used in development of new applications such as lightweight electric motors and power cables that transfer electrical energy without energy dissipation
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