Topic 5 - Electricity and Magnetism

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10 Terms

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Coulomb's Force Law (unit: N)
The force between two point charges is proportional to the product of the charges and inversely proportional to their separation squared.

F = | q1q2 | / rÂČ
k = 1/4πΔ
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Electric field strength (unit: N/C \= V/m)
The force per unit charge on a positive point charge.

E = F/q
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Electric potential (unit: J/C \= V)
The work done by the electric field per unit charge

V = W/q
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Resistance (unit: Ω)
The ratio of the applied voltage to the resulting current.

R = V/I
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Electromotive Force (emf) (unit: V)
The energy, per unit charge, available to the entire circuit.
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Ohm's 1st law
When voltage is proportional to current (i.e. resistance is constant) we say Ohm's 1st law is satisfied (or we say the device is ohmic). E.g. a metal wire at constant temperature is ohmic.

When satisfied, V = RI
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Internal resistance (unit: Ω)
A constant resistance inside a cell, which leads to energy dissipation in the cell. Modeled as a resistance in series with an ideal emf , hence the terminal voltage of a cell is V = E-rI.

Note: Sometimes the above equation is also referred to as Ohm's 2nd law.
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Current (unit: A)
The amount of charge per unit time passing through a cross
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The copper wires and insulator are both exposed to an electric field. Discuss, with reference to charge carriers, why there is a significant electric current only in copper wires.
* when an electric field is applied to any material «using a cell etc» it acts to accelerate any free electrons 


* electrons are the charge carriers «in copper»
* metals/copper have many free electrons whereas insulators have few/no free electrons/charge carriers 
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Explain, in terms of electrons, what happens to the resistance of the cable as the temperature of the cable increases
* «as temperature increases» there is greater vibration of the metal atoms/lattice/lattice ions OR increased collisions of electrons


* drift velocity decreases «so current decreases» 
* «as V constant so» R increasesÂ