# PHYSICS- ELECTROSTATICS, RESISTANCE, RESISTIVITY AND BASIC ELECTRIC CIRCUITS

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• Created by: Emily
• Created on: 01-11-12 11:26
When an object has become charged..
it has lost or gained electrons, giving it a charge of either + (lost) or - (gained). Electrons are neither created nor destroyed therefore electrons must be TRANSFERRED from one material to the other.
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Explain how friction can transfer electrons
Imagine an uncharged polythene rod, by rubbing it with a piece of cloth the polythene gains electrons, and becomes negatively charged (the duster must lose electrons and becomes positively charged). As polythene is a insulator the charge is static.
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Conductors can not be charged unless...
Insulated from Earth. Because; any charge transferred to the conductor will flow through it and down to the earth.
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Charging by Induction
When a negative rod is brought close to 2 metal spheres electrons (-) are repelled from the closest sphere, leaving the side of the sphere charged + . With the negative rod by the sphere, they are separated, then the rod removed - see diagram.
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Electrostatic Uses
crop spraying, photocopying, ink-jet printers, lightning conductor,
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What is charge measured in?
COULOMBS. Note: 1C is a lot of charge, so mC (mili coulombs - 1/1000 C) is often used , or uC, (micro coulomb - 1/100,000 C). Also; charge is given the symbol q or Q in formulas.
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What is used to measure charge?
A coulombmeter. The object that is measured is scraped across the metal cap of the device, placing some charge on it. The coulombmeter then measures the charge placed on its cap.
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What is the charge on an Electron?
e = -1.6 x 10 ^ -19 C
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Formula to work out charge transfer, or number of electrons:
number of electrons = the charge transfer (in C)/ The charge of an electron. NOTE: Should be a large number!
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What is an Electric Current?
In a metal, an electric current is a flow of electrons, so a flow of charge.
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Formula for Current?
Current (I)= Charge Transferred (Q)/ Time Taken (t). NOTE: current is measured in Ampere (amp)
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Describe a Crystal Lattice Structure.
Where atoms are lined up in a fixed, orderly pattern, and not all electrons are used in bonding so some are free to move between the atoms. These FREE CHARGE CARRIERS move randomly.
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What is Resistance?
Resistance is caused by collissions between atoms and electrons. Energy is used up in these collisions and is given off as heat. more resistance = more collisions = more heat.
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Describe What is happening in this Diagram:
If an electric field is set up across a wire, The electrons are attracted to the + end of the wire, moving randomly but with a drift. As they move they collide with atoms of the lattice structure (resistance) which restricts their movement.
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Formula for Density, Mass and Volume:
Density (g/cm^3)= Mass (g)/ Volume(cm^3)
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What has an affect on Resistance?
The cross sectional area, the length and the properties of the material. (this property is called RESISTIVITY - with the symbol 'p' (greek letter - roe)
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formula for Resistance:
Resistance = Resistivity x Length / Area NOTE: measured in the units 'ohm meter' _n_ m. For conductors is should be a very small value (x10 ^ -8). Can plot Resistance (y) and length over a range of lengths for accuracy.
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Variation of Resistance with Temperature.
at a higher temperature (more energy), atoms vibrate more (more kinetic energy) = they occupy more space = more chance of collisions.
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experiment to investigate the effect of temperature on resistance:
Diagram including: ohm meter, liquid paraffin, water, temperature sensor, metal coil, beaker, test tube, circuit.
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Typical graph of resistance and temperature (including superconductor)
* positive correlation, *at 0'C the resistance is not 0 ohm, *liner graph (only applied to metals)
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OHMS LAW.
The current flowing in a conductor is directly proportional to the potential difference across it, provided there is no change in temperature.
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## Other cards in this set

### Card 2

#### Front

Explain how friction can transfer electrons

#### Back

Imagine an uncharged polythene rod, by rubbing it with a piece of cloth the polythene gains electrons, and becomes negatively charged (the duster must lose electrons and becomes positively charged). As polythene is a insulator the charge is static.

### Card 3

#### Front

Conductors can not be charged unless...

#### Back ### Card 4

#### Front

Charging by Induction

#### Back ### Card 5

#### Front

Electrostatic Uses

#### Back 