Charge and current

• Current:

  • The rate of charge flow.
  • Measured in amperes(A)/Amps, using an ammeter in series.
  • $I=$$\frac{Q}{T}$
  • The ammeter should have a very very low resistance, which allows the current to pass through without being affected.

  
• Charge:

  • Can be positive, negative or neutral.
  • Measured in coulombs(C), using a coulomb-meter
  • The coulomb, in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. 1 C = 1 As
  • $Q=$$IT$
  • Charge is quantised, it comes in definite/finite quantities.
  • Elementary charge is $1.6\times10^{-19}$C.
  • The up quark has a charge of $+$$\frac{2}{3}e$
  • The down quark has a charge of $-\frac{1}{3}e$
  • The strange quark has a charge of $-\frac{1}{3}e$
• Kirchhoff's 1st law:

  • The sum of the currents entering a junction is equal to the sum of the currents leaving the junction.
  • In a simple circuit with a single loop, the current is the same throughout the loop.
  • In a parallel circuit, the total current entering a junction is equal to the sum of the currents in each branch.

  
• Kirchhoff's 2nd law:

  • The sum of the Emf's in a closed circuit is equal to the sum of the potential difference.
  • Total energy gained equals total energy lost.
• Potential difference:

  • A measurement of the energy transferred per coulomb of charge, as the charge passes through a component.
  • Measured in Volts(V), using a voltmeter in parallel.
  • An ideal voltmeter should have infinite resistance, otherwise it will draw extra current from the cell
  • $V=$$IR$
  • $V=$$\frac{E}{Q}$ or $V=$$\frac{W}{Q}$
  • A potential difference of 1.5v means each coulomb of charge transferers 1.5j of energy to a component.
• Filament lamp:

  • As potential difference increases, resistance does also. This is because the bulb get hotter so electrons have more kinetic energy, therefore the amplitude and frequency of collisions increases.

  
• Diodes:

  • Only allows current to flow in one direction,
  • In reverse they have incredibly high resistance.