Sensing

• Current


  The rate of flow of charge in a circuit
• How to measure current at a point in a circuit

  Attach an ammeter in series with this point
• Current variations in series circuits

  • The current is the same at all points in the circuit
• Current variations in parallel circuits

  • The current is split between the branches of the circuit
• Kirchhoff's first law


  The total current entering a junction must be equal to the total current leaving it
• Work done when a charge moves across a potential difference

  Work Done = Charge x Voltage
• Power


  The rate of transfer of energy
• Equations to calculate power

  • P = Energy Transferred / Time
  • P = Current x Voltage
• The unit of power is Watt, W
• Joule heating


  When a resistance opposes the flow of charge, causing energy to be wasted as heat
• Resistance


  The opposite of conductance
• Conductance


  A measure of how easily electrons can flow through a given material
• Equations for conductance (G)


  • G = 1 / Resistance
  • G = Current / Voltage
• Total resistance of resistors in series

  R = R1 + R2 + ...
• Total resistance of resistors in parallel


  1/R = 1/R₁ + 1/R₂ + ...
• Total conductance of components in series

  1/G = 1/G1 + 1/G2 + ...
• Total conductance of components in parallel

  G = G1 + G2 + ...
• Resistance increases with temperature because as temperature increases, the ions in a metal gain more kinetic energy, meaning they vibrate more, and consequently, electrons collide more frequently with the ions, making it harder for the current to pass through
• What makes conductors good at conducting electricity

  • They have a large number of mobile charge carriers (free electrons) that can carry a current
• How a semiconductor works


  The number of mobile charge carriers increases with a factor such as light or heat, meaning their ability to conduct electricity also increases
• Examples of semiconductors


  • Light Dependant Resistors (LDRs)
  • Thermistors
• Electromotive force (EMF) of a power supply


  EMF is the energy provided by the source per unit charge
• Kirchhoff's second law


  The sum of all the voltages across a circuit must equal the EMF of the circuit
• Internal resistance


  The resistance inside a power supply, due to the materials that it consists of