Electricity

• Positive & Negative Charges

  Objects can be given one of two types of electric charge: Positive or Negative
• When two charged objects are brought close together

  There will be a force between those objects
• Electric charge


  Measured in units called coulombs (C)
• Electrostatic forces share many similarities with magnetic forces, but they are diyerent phenomena
• Demonstrating Electrostatic Charges


  1. Suspend insulating material

  2. Rub one end with cloth to charge it

  3. Hold charged end near second charged material

  4. If first piece rotates away, materials have same charge

  5. If first piece moves towards, materials have opposite charges
• Experimental demonstrations are diyerent from experiments where you take measurements. Your results are your observations.
• Electric Field


  A charged object creates an electric field around itself
• Electric Field Lines


  • Always point away from positive charges and towards negative charges
  • Direction of the force on a positive charge at that point
• Strength of electric field

  Strongest close to charged object, weaker further away
• Objects in an electric field

  • Experience an electrostatic force
  • Force is either attractive or repulsive depending on charge types
• Conductors


  Allow charge carriers to freely move
• Insulators


  Do not allow charge carriers to move
• Conductors


  • Made of positively charged metal ions with delocalised outermost electrons
  • Electrons are free to move, so conduct electricity well
• Insulators


  Have no free charges, so do not allow flow of charge through them easily
• Using a Gold-leaf Electroscope (GLE)


  1. Charge the plate

  2. Touch items to the plate

  3. Leaf falls = good conductor

  4. Leaf remains = poor conductor (good insulator)

  5. Leaf falls slowly = poor conductor
• Metals are very good conductors, non-metals tend to be good insulators
• Current


  The amount of charge passing a point in a circuit every second
• Charge, current and time

  Related by the equation: Q = I x t
• Leaf falls


  Material is a good conductor
• Leaf remains in place


  Object is a poor conductor (good insulator)
• Leaf falls slowly


  Material is a poor conductor
• Overall, metals are very good conductors whilst non-metals tend to be good insulators
• Conductors and Insulators Summary

  • Metals are very good conductors
  • Non-metals tend to be good insulators
• Charge, current and time

  Related by the equation: Q = I * t
• Using the formula triangle

  Rearrange the equation Q = I * t
• Direct Current (DC)


  Charge flows in one direction only, from negative to positive terminal
• Alternating Current (AC)


  Direction of electron flow changes regularly, typically 50 Hz frequency
• If asked to explain the difference between alternating and direct current, sketch the graphs showing the variation of current with time
• Ammeter


  Measures the amount of charge passing through it per unit time, connected in series
• Ammeters


  • Can be digital (electronic readout) or analogue (needle and scale)
  • Analogue ammeters subject to parallax error, should be read perpendicular to scale
  • Digital ammeters can measure very small currents, more accurate but may flicker
• Ammeters should be checked for zero error before use
• Electrical conduction in metals

  Current caused by flow of free (delocalised) electrons
• Conventional current


  Defined as going from positive to negative, even though electrons flow from negative to positive
• Electromotive Force (EMF)


  The electrical work done by a source in moving a unit charge around a complete circuit, measured in volts
• Potential Difference (PD)


  The work done by a unit charge passing through a component, measured in volts
• Voltmeters


  • Connected in parallel to measure potential difference
  • Can be digital or analogue, subject to same considerations as ammeters
• When building a circuit, connect the voltmeter last to avoid troubleshooting
• Resistance


  The opposition to current, measured in ohms
• Ohm's Law


  Current is directly proportional to potential difference as long as temperature remains constant
• Using the formula triangle

  Rearrange the equation R = V/I