Module 4

• Electric current, I


  Rate of flow of charge, flow of charged particles, normally a flow of electrons in metals or a flow of ions in electrolytes
• Electric Current, I


  Charge transferred, Q (coulombs, C) / change in time, t (s) aka amount of charge passing a given point in a circuit per unit time
• Electric charge, Q


  A physical property either positive or negative or as a relative charge
• Charge carrier


  Any particle that has an electric charge
• Relative charge


  Simplified measurement of the electric charge of a particle or object, measured as multiples of the elementary charge
• Elementary charge, e


  Electrical charge equal to 1.6x10-19 which is the charge on a proton
• A proton has a charge +1e and electron has a charge -1e
• Net charge on an object, Q


  Number of electrons added/removed, n x elementary charge, e
• Quantisation


  Availability of some quantities, such as energy or charge, only in certain discrete values
• When a charge is quantised, it means that the charge on an object can only be a multiple of the elementary charge (e)
• Oil drop experiment


  1. Analysed motion of electrically charged oil particles between oppositely charged parallel plates

  2. Oil droplets falling experiences air resistance, gravity and upthrust

  3. The occasional oil drop was held stationary as the downward gravitational force was balanced by the upward attractive electrostatic force whilst others drifted slowly through the electrical field

  4. He found the charge on the droplets was quantised and all the values were multiples of elementary charge

  5. Using his data e was calculated to be -1.59x10-19
• Electric current in metals

  Usually a flow of electrons from the few that are not fixed to their atom (most are fixed because of the way atoms in metals are bonded as they are usually in a lattice)
• To make an electric current, each end of the wire should be positive and negative so electrons move towards the positive end
• Greater the charge


  The greater the current
• Greater current


  Caused by a greater number of electrons moving past a given point each second (e.g. due to a greater cross sectional area) or same number of electrons moving faster through the metal
• Conventional current


  Current from a positive terminal to a negative terminal but electrons flow from negative to positive
• Electrolyte


  Liquid that can carry an electric current created by a flow of ions
• Electrolyte current


  1. If a positive electrode (anion) and a negative electrode (cathode) are placed in a solution, ions are attracted to the electrodes so move towards it, it creates an electric current

  2. When the positive ions reach the cathode they accept an electron and when the negative electron reaches the anode, they donate electrons
• Ammeter


  Measures the current at any point in a circuit and is placed in circuit at the point you want to measure
• Ammeters should have the lowest possible resistance to reduce the effect they have on the current
• Electric charge can neither be created nor destroyed
• Kirchhoff's first law

  1. For any point in an electric circuit, the sum of currents into that point equals to the sum of currents out of that point

  2. This can also be written as: Iin=Iout where Σ denotes "sum of"

  3. This law is based on the conservation of charge (Q=It) so the charge carrier entering a point in a given time must be equal to the total number of charge carriers leaving that same point during the time
• Mean drift velocity, v

  The average velocity of electrons as they move through a wire
• Number density, n


  Number of free charge carriers per unit volume
• Conductors have a high n of the order of 10^28m-3 e.g. copper has 8.5x10^28 and semiconductors (which have a n in between the conductors and insulators) is around 10^17m-3 e.g. germanium with 2x10^18
• As semiconductors have a lower n than metals, in order to carry the same current as them the electrons in semiconductors need to move much faster, increasing the temp of it
• Electrons move really slowly as free electrons repeatedly collide with the positive metal ions as they drift through the wire towards the positive terminal
• Current, I


  Cross sectional area, A x number density, n x elementary charge, e x mean drift velocity of the charge carriers, v aka I=Anev
• The mean drift velocity and the cs-a is inversely proportional so if the radius will decrease by a factor of 4 the drift velocity will increase by a factor of 4
• Circuit symbols


  • Most circuits will contain a cell(s) or a mains power supply
  • Battery: 2 or more cells connected in series, longer terminal represents positive
  • When using a power supply, a plus sign is placed next to the positive terminal
  • Polarity: of charge or orientation of a cell relative to a component
• Potential difference/voltage, V


  Energy transferred (by charge carriers, when they lose energy in a component) from electrical energy to other forms per unit charge
• Volt

  P.D. Across a components when 1J of energy is transferred per unit charge passing through the component
• Voltmeter


  Measures p.d. And they are always connected in parallel across a particular component
• An ideal voltmeter should have infinite resistance but this isn't possible in reality so usually around several million ohms
• Potential difference


  Used to describe when work is done by charge carriers
• Electromotive force


  Used to describe when work is done on the charge carriers aka the energy transferred (work done) from chemical energy or or other forms into electrical energy per unit charge
• Electron gun


  • Device that uses a large accelerating p.d. To produce a narrow beam of electrons
  • These electrons can be used to ionise particles by adding or removing electrons from atoms
  • They can have precisely determined kinetic energies
  • Used in specific instruments eg electron microscopes, mass spectrometers and oscilloscopes
• How electron gun works

  1. They need a source of electrons

  2. Usually a small metal filament is heated by a current so the electrons in the wire gain K.E.

  3. Some gain enough K.E. to escape from the surface of the metal (thermionic emission)

  4. If the heated filament is placed in a vacuum and a high p.d. is applied between it and an anode, the filament acts as a cathode so the freed electrons accelerate towards the anode gaining K.E.

  5. If the anode has a hole then electrons in line with this hole can pass through it creating a beam of electrons with a specific K.E.
• Thermionic emission


  The emission of electrons from the surface of a piece of heated metal
• Resistor


  Electrical component that has a known resistance which resists the flow of charge carriers through it