Ch.8 Charge and Current

Ch.8 Charge and Current

1. Current and Charge

   • Electric current is defined as the rate of flow of charge
     • I = Q/t
     • Unit = amperes (A)
   • Electric charge is a physical property - a measure of "chargedness"
     • Objects with charge interact and exert forces on each other
       • Unit = coulombs (C)
   • Any particle with electric charge is a charge carrier
     • Relative charges (+1 or -2) are measured against 'e' - the elementary charge (1.6x10^-19)C
   • Charge of most objects is due to loss or gain of electrons (+-e)
     • Therefore net charge (Q) = +-ne (where n = number of electrons added or removed)
       • Charge on an object is described as being 'Quantised'

2. Moving Charges

   • In metals the charge carriers are electrons, but in liquids they tend to be ions
   • In metals, an electric current is usually a flow of electrons
     • Most electrons in metal atoms remain fixed to their atom but a small number from each atom are free to more - these are called 'free electrons'
     • One way to make electrons move is to make one end of a wire positive and the other negative
       • This will cause the electrons (with a negative charge) to be attracted towards the positive end, and move through the wire as an electric current
         • A greater rate of flow of charge (greater current) may be due to a greater number of electrons moving past a given point each second - e.g. a wire with a greater cross-sectional area - or the same number of electrons moving faster through the metal
   • Conventional current was defined long before the discovery of the electron, as a current from aa positive terminal towards a negative one
     • This means that conventional current (positive to negative) flows the opposite way to direction of motion of electrons (negative to positive)
       • The direction of all electric currents is still treated as from positive to negative, regardless of direction of flow of electrons
   • Liquids that carry an electric current are called electrolytes
     • In these cases the electric current is not a flow of electrons but a flow of ions
       • All electrolytes are either in molten ionic compounds or, more commonly, ionic solutions
         • If salt is dissolved in water, the salt separates into positively charged sodium ions (cations - Na+) and negatively charged chlorine ions (anions - Cl-)
           • If a positive electrode (anode) and a negative electrode (cathode) are placed in the solution, cations move towards the cathode and anions move towards the anode (opposite charges attract)
             • This movement of ions is a flow of charge, an electric current
   • An ammeter is used to measure the electric current at any point in a circuit, placed in series
     • Ammeters should have the lowest resistance as possible as to reduce the effect on the current it is measuring

3. Kirchoff's First Law

   • For any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point
     • In other words, charge cannot be created nor destroyed - total electric charge in the universe is constant

4. Mean Drift Velocity

   • Number density (n) is the number of free electrons per cubic metre of material
     • Greater number density = better electrical concuctor
   • Materials can be classified according to number density
     • Insulators = Low number density
       • Conductors = Large number density
         • Semiconductors = Inbetween
           • Semiconductors have a much lower 'n' than metals, therefore electrons must move much faster in order to carry the same current - causing them to heat up
   • Most charge carriers move slowly, however free electrons repeatedly collide with the positive metal ions as they drift through the wire
     • This means that the free electrons already throughout the wire, when a switch is turned on, instantly start drifting, which is why when a light switch is turned on, the light turns on almost immediately
     • Mean drift velocity (v) can be found using the equation I = Anev where I = current, A = cross-sectional area, e = elementary charge and n = number density
   • If the cross-sectional area of the wire changes, in order for the current to remain constant, the mean drift velocity must change
     • Decrease in cross-sectional area results in increased mean drift velocity (to keep current constant)

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