Electricity

• Electromotive Force (e.m.f.)


  The potential difference of the power source in a circuit
• Electromotive Force (e.m.f.)


  The electrical work done by a source in moving a unit charge around a complete circuit
• Electromotive Force (e.m.f.)


  Measured in volts (V)
• Potential Difference


  The work done by a unit charge passing through a component
• Potential Difference


  Measured in volts (V)
• Measuring Potential Difference


  1. Using a voltmeter (digital or analogue)

  2. Connected in parallel with the component being tested

  3. Measures the difference in electrical potential between two points
• Analogue Voltmeters


  • Subject to parallax error
  • Typical ranges 0.1-1.0 V and 0-5.0 V
  • Need to check for zero errors before using
• Digital Voltmeters


  • Can measure very small potential differences (mV or μV)
  • More accurate than analogue
  • May 'flicker' between values, need to make a judgement
• Voltmeters are connected in parallel to the component being tested
• When building a circuit, save the voltmeter until last, connect it between the two points you want to measure
• Potential divider


  A circuit which produces an output voltage as a fraction of its input voltage
• Potential dividers


  • Have two main purposes: To provide a variable potential difference, To enable a specific potential difference to be chosen
  • Split the potential difference of a power source between two or more components
• Potential divider equation


  V_out = (R_2 / (R_1 + R_2)) * V_in
• The potential difference across each resistor depends upon its resistance
• The resistor with the largest resistance will have a greater potential difference than the other one
• If the resistance of one of the resistors is increased

  It will get a greater share of the potential difference, whilst the other resistor will get a smaller share
• Potentiometer


  A single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it
• Potentiometer


  A kind of variable resistor
• Potentiometer


  • The sliding contact has the effect of separating the potentiometer into two parts – an upper part and a lower part – both of which have different resistances
  • Moving the slider changes the resistances (and hence potential differences) of the upper and lower parts of the potentiometer
• If the slider is moved upwards

  The resistance of the lower part will increase and so the potential difference across it will also increase
• Transformer


  A device used to change the value of an alternating potential difference or current
• Operation of a transformer

  1. Alternating current supplied to primary coil

  2. Current continually changing direction

  3. Changing magnetic field around primary coil

  4. Iron core easily magnetised

  5. Changing magnetic field passes through secondary coil

  6. Changing field cuts through secondary coil

  7. Induces alternating potential difference in secondary coil

  8. Alternating potential difference has same frequency as alternating current in primary coil

  9. Alternating current flows in secondary coil if it is part of a complete circuit
• Step-up transformer


  Increases the potential difference of a power source
• Step-up transformer


  • Has more turns on the secondary coil than on the primary coil (Ns > Np)
• Step-down transformer


  Decreases the potential difference of a power source
• Step-down transformer


  • Has fewer turns on the secondary coil than on the primary coil (Ns < Np)