Current

• Current - the amount of charge passing a point in a circuit every second

  • charge per second
• I = Q/t

  • Q - charge (measured coulombs, C)
  • I - current (measured in amperes, A)
• Electric current - the flow of charge between two oppositely charged conductors that are connected together (by a length of wire)

  • The greater the flow of charge, the greater the electric current
• Current formula triangle

  A) charge

  B) current

  C) time

  
• Electric current diagram

  A) conductor

  B) flow of charge

  
• Current can be direct current (dc) or alternating current (ac)

  • In terms of calculations they can be treated in the same way
• DC vs AC

  A) direct

  B) dc

  C) alternating

  D) ac

  
• Direct current (D.C) - produced when using dry cells and batteries (and sometimes generators, although these are usually ac)

  • The electrons flow in one direction only, from the negative terminal to the positive terminal
• Dry cells - a type of electric battery
• Alternating current - typically comes from mains electricity and generators

  • The direction of electron flow changes direction regularly
  • A typical frequency for the reversal of AC current in mains electricity is 50 Hz
• If asked to explain the difference between alternating and direct current, sketch the graphs
• Current is measured using an ammeter
• • Ammeters - measure the amount of charge passing through them per unit time
  • Ammeters should always be connected in series with the part of the circuit you wish to measure the current through as measures the amount of charge passing through it

  
• Ammeters can be either

  • Digital (with an electronic read out)
  • Analogue (with a needle and scale)
• Analogue ammeters - typical ranges are 0.1-1.0 A and 1.0-5.0 A

  • Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements
  • Always read the meter from a position directly perpendicular to the scale
• Zero errors - when a measuring system gives a false reading when the true value fo the measured quantity is 0
• Parallax error - an error in a reading caused by not reading at eye level
• Digital ammeters - measure very small currents, in mA or µA

  • Shows the measured values as digits and are more accurate than analogue displays
  • Easy to use because they give a specific value and are capable of displaying more precise values
• Digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down

  • should be checked for zero error
  • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results
• • In a metal, current is caused by a flow of delocalised electrons

  A) electrons

  B) metal ion

  C) electron

  
• Electrons are negatively charged

  • This means that the electrons flow from negative to positive but conventional currents are still defined as going from positive to negative
• Actual electron flow vs conventional current

  A) electron flow

  B) conventional current

  
• Define electrical current


  charge passing a point per unit time.