Chapter 8: Charge and Current

Chapter 8: Charge and Current

1. moving charges

   • Flow of charge
     • Current is the flow of charge, movement of charge carriers. in metals they are usually electrons but in liquids, the charge carriers are usually ions.
       • liquids that can carry an electric current are called electrolytes.
         • all electrolytes are either molten ionic compounds or ionic solutions.
     • the greater the rate of charge flow the greater the electric current in the wire.

2. Electric Charge

   • Charge of an electron : 1.6 * 10^-19 Coulombs
   • Charge (Q)measured in Coulombs(C)
     • A coulomb is defined as the electric charge flowing past a point per second when there is a current of 1 Amp
   • Electric charge is a physical property .
     • Net charge
       • Example: An object with a charge of 1.92 * 10^-18 has a charge of +12e. 12 * 1.6* 10 ^ -19
       • Q= +-ne
         • e is the elementary charge e.g 1.6 *10 ^-19
         • n is number of electrons
         • Q is net charge in Coulombs
       • charge Is quantised(can only have certain values e.g -2 , +1)
   • like charges repel. Opposite charges attract

3. Current:The rate of flow of charge

   • conventional current is opposite to electron flow and is from positive to negative
   • I = Q/T
     • I is amperes(base unit)
     • Example: Current in a heater. A charge of 0.26 MC passes through a heater in 6 hours. Calculate the average current in the heater at the time giving your answer to an appropriate number of significant figures. Step 1: Identify equation needed I= Q/T. Step 2: Substitute known values in SI into the equation and calculate the answer. I = (0.26* 10^6) / (6.0 *3600). Express your answer to the correct number of significant figures.
     • Time (s)
   • Causes of a larger current
     • a greater number of electrons moving past a given point each second. E.G a wire with a greater cross-sectional area
     • the same number of electrons moving faster through a metal
   • measuring electric current
     • ammeter is used to measure the electric current at any point in a circuit.
       • ammeters are placed in series so they have the lowest possible resistance in order to reduce the effect they would have on the current.
         • an ammeter with a high resistance would decrease the current it should be measuring
         • the perfect ammeter has 0 resistance so has no effect on the current it is measuring

4. Modelling electric current in metals.

   • The structure of metal
     • a regular crystal structure or lattice of positive ions, surrounded by a number of free electrons(sometimes called conduction or delocalised electrons)
       • positive ions
         • positive ions are not free to move
         • the positive ions vibrate around fixed points. they vibrate more vigorously as the temperature of the metal increases
       • free electrons

5. kirchoffs first law (conservation of charge)

   • conservation of charge states that electric charge can neither be created or destroyed. so the total amount of charge in the universe in constant
   • At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node
     • therefore conservation of charge- what flows in must flow out.

6. I =Anev

   • v Drift velocity
     • speed at which charge carriers move (usually quite slowly)
       • lights turn on fast due to the fact that all the free electrons start moving almost at once
   • e elementary charge
   • n number density
     • The number density is the number of free electrons per cubic metre of a material.
     • the greater the number density. The greater the number of free electrons per m ^3 therefore the better the electrical conductor
       • Insulators
         • e.g wax , rubber
         • have a very low number density and dont conduct electricity
       • semiconductor
         • silicon, germanium
         • have a number density between insulators and conductors
           • semi conductors have much lower number density than metals, so in order to carry the same current the electrons in semiconductors need to move faster. this increases the temperature of a semiconductor. e.g computer cpu
         • transistor
       • conductor
         • e.g lead, copper, zinc
         • have a high number density and are good electrical conductors
   • A cross-sectional area of the conductor e.g wire
     • The effect of changing cross-sectional area
       • if the cross sectional area of the wire changes so must the drift velocity
         • the narrower the wire the greater the drift velocity as the current has to stay constant
           • so the mean drift velocity is inversely proportional to the cross sectional area of the wire v = 1/A

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