CHARGE AND CURRENT

• Rate of flow of charge

• Flow of electrons in metals/ ions in electrolytes

I = Q / t

• I = electric current

• Q = charge transferred

• t = time

• I C of charge passing a given point per 1 s

• 1 Cs^-1

• Electric charge passing a given point in 1 s when there is an electric current of 1 A

• 1 As

Electric charge measured as a multiples of the elementary charge

• 1.6 x 10^-19 C

• Equivalent to the charge on a proton

• Proton = +1e
• Electron = -1e
• Neutron = 0e

Electrons added = negative charge

Electrons removed = positive charge

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Q = ±ne

• Q = net charge

• n = number of electrons added or removed

• elementary charge, 1.6 x 10^-19 C

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The charge on an object is quantised (an integer multiple of e) 

Lattice of positive ions surrounded by free electrons

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Positive ions vibrate around fixed points

Vibrate more vigorously as the temperature increases

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Free electrons attracted to the positive end of the wire

Move through the wire as an electric current

• Greater number of free electrons passing a given point per second

• The same number of free electrons moving faster through the wire

• Direction in which positive charges would travel, from positive to negative

• Opposite to the direction in which electrons would travel, from negative to positive

• Liquid containing ions that are free to move and so to conduct electricity

• Ionic solutions or molten ionic compounds

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• Positive ions (cations) are attracted to the negative electrode (cathode)

• Negative ions (anions) are attracted to the positive electrode (electrode)

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• Movement of ions = flow of charge = electric current

• Measures current

• Placed in series at the point where you want to measure the current

• Ideal ammeter has zero resistance and so no effect on the current it measures

• Charge can neither be created nor destroyed

• In any interaction the total charge before and after must be the same

ΣI_in = ΣI_out

Sum of currents into a point = sum of currents out of that point

Number of charge carriers entering = number of charge carriers leaving

Based on the law of conservation of charge

Number of free electrons per cubic metre

Higher number density = better electrical conductor

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Conductors

• High number density

Semiconductors

• Lower number density than conductors

• Electrons must move faster to carry the same current

• Increases the temperature

Insulators

• Low number density

I = Anev

• I = electric current

• A = cross-sectional area

• n = number density

• e = elementary charge, 1.6 x 10^-19 C

• v = mean drift velocity