Current (CIE IGCSE Physics)

• The current is the amount of charge passing a point in a circuit every second

(It is helpful to think of current as the charge per second)

• Charge, current and time are related by the following equation:

• Where the symbols:
  • Q stands for charge (measured in coulombs, C)
  • I stands for current (measured in amps, A)

• You can rearrange this equation with the help of the formula triangle:

Use the formula triangle to help you rearrange the equation

• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors

Charge can flow between two conductors

• This flow of charge is called an electric current
• The greater the flow of charge, the greater the electric current

Direct Current and Alternating Current

• Current can be direct current (dc) or alternating current (ac)
• In terms of calculations they can be treated in the same way

Two graphs showing the variation of current with time for alternating current and direct current

Direct Current (dc)

• Direct current is 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

Alternating Current (ac)

• Alternating current typically comes from mains electricity and generators
• It is needed for use in transformers in the National Grid (covered later in this topic)
• The direction of electron flow changes direction regularly
• A typical frequency for the reversal of ac current in mains electricity is 50 Hz

• Current is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through
  • Ammeters measure the amount of charge passing through them per unit time, so the ammeter has to be in series so that all the charge flows through it

An ammeter can be used to measure the current around a circuit

Digital or Analogue?

• 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 for analogue ammeters
  • 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. They should be checked for zero errors before using
• They are also subject to parallax error
  • Always read the meter from a position directly perpendicular to the scale

Digital Ammeters

• Digital ammeters can measure very small currents, in mA or µA
• Digital displays show the measured values as digits and are more accurate than analogue displays
• They’re easy to use because they give a specific value and are capable of displaying more precise values
  • However digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down
• Digital ammeters 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

Digital meter

• In a metal, current is caused by a flow of electrons

In metals, the current is caused by a flow of free (delocalised) electrons

EXTENDED

• Electrons are negatively charged
• This means that the electrons flow from negative to positive
• Conventional current, however, is still defined as going from positive to negative

By definition, conventional current always goes from positive to negative (even though electrons go the other way)