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Chapter 12 Electricity (12.3 (Measurement of resistance (Resistor-…
Chapter 12 Electricity
12.1
Electrical conduction
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Metal- Charge carriers are conduction electrons, they move about inside the metal colliding with the metal and fixed +ve ions
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Charge Carriers
Insulator- Each electron is attached to an atom and can't move away from the atom. When PD applied no current as no electrons can move through the insulator
Metallic Conductor- Some electrons are delocalised (charge carriers)- when PD applied the conduction electrons are attracted to the positive terminal of the metal
Semiconductor- Number of charge carriers increase with temperature- resistance decreases as temp raised
Pure semiconductor- Intrinsic semiconductor because conduction is due to electrons that break free from the atoms of the semicoductor
12.2
Energy and PD
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Each electron passing through a circuit component does work to pass through the component- transfers energy
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Energy transfer
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Device with resistance- work done is transferred as thermal energy- because charge carriers repeatedly collide with atoms in the device, transferring energy causing them to vibrate and heat up
Electric motor (constant speed)- Work done is equal and to energy transferred to the load and surroundings by the motor so KE is constant. Charge carriers = electrons that need to be forced through the wires of motor coil against opposing force of electrons from magnetic field of motor
Loudspeaker- Work done transferred as sound energy. Electrons must be forced through wires of the vibrating loudspeaker coil against the force from the loudspeaker magnet
12.3
Definitions and laws
Resistance- Measure of the difficulty of making current pass through the component Units (Ohm Ω) 1 Ohm- 1 volt per ampere
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Resistivity
For a conductor length L and a uniform cross sectional area A- its resistance R is proportional to L and inversely proportional to A
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Superconductivity
Superconductor- wire or device made of a material with 0 resistivity at and below a critical temperature
Below the temp, when a current passes through it there is no pd across it as resistance is 0- current has no hating affect
Uses
High-power electromagnets- generate strong magnetic fields for MRI scanners and particle accelerators
12.4
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Diode- current very small when diode is reverse biased, measure with it forward biased and then reverse biased and plot a graph
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