4.1 Charge and current

• Electric current is the rate of flow of charged particles
• current =change in charge / change in time
• 1 ampere = 1 coulomb of charge passing a point in 1 second
• electric charge is a physical property of an object, measured in coulombs
• coulomb : the charge that flows past a point in 1 second when there is a current of 1A
• elementary charge

  1.6 x 10^-19 C
• how does an object become charged?

  when they gain or lose electrons or ions
• Quantised charge means it represents multiples of the elementary charge
• Conventional current flows from positive to negative
• Making one end of a wire positive and the other negative will cause electrons to be attracted to the positive end and repelled from the negative end causing a flow of charge forming an electric current
• electrolytes are liquids that conduct electricity
• ions carry charge in electrolytes
• ammeters should be connected in series with the lowest possible resistance
• conservation of charge

  electrical charge can neither be created nor destroyed; the total amount of charge in the universe is constant
• Kirchhoff's First Law 


  for any junction in an electrical circuit the sum of the currents into that point is equal to the sum of the currents out of that point
• $\Sigma_{in}=$$\Sigma_{out}$


  Kirchhoff's First Law
• Number density is the number of free charge carriers per unit of volume
• $m^{-3}$is the unit of?


  number density
• conductors have a very high number density $10^{28}m^{-3}$
• semiconductors have a much lower number density $10^{17}m^{-3}$
• insulators have a very low number density - ideally 0
• electrons move slowly through a wire to the positive terminal as they bump into positive metal ions
• in an unconnected wire, electrons move in random directions and collide with each other resulting in a mean drift velocity of 0
• current (I) = Cross sectional area of the wire (A) X number density (n) X elementary charge (1.6x10^-19) X mean drift velocity (v)
• Mean Drift Velocity


  The average displacement of a charge carrier in a unit of time within an electric field
• Cross sectional area of a wire and mean drift velocity are inversely proportional
• Why is cross sectional area and mean drift velocity inversely proportional?

  number density and the elementary charge are uniform
• Kirchhoff's First Law

  $\Sigma I_{in}=$$\Sigma I_{out}$