Current (Cambridge (CIE) IGCSE Physics)

Current

• Electric current is defined as

The rate of flow of electric charge

• Current flows

  • when a circuit is formed e.g. when a wire connects the two oppositely charged terminals of a cell

  • from the positive terminal to the negative terminal of a cell

Charge flows from the positive terminal to the negative terminal

Measuring current

• Current can be measured using an ammeter

• Ammeters must be connected in series with the component being measured

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

• Ammeters can be

  • 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

Electrical conduction in metals

• The wires in an electric circuit are made of metal because it is a good conductor of electric current

• In the wires, the current is a flow of negatively charged electrons

In metal wires, the current is a flow of negatively charged electrons. When a potential difference is applied, electrons flow through the lattice of metal ions

Calculating current

Extended tier only

• Electric current can be defined more precisely as:

The charge passing a point in a circuit per unit time

• Current is measured in units of amperes or amps (A)
  

  • 1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C/s

• This means the size of an electric current is the amount of charge passing through a component each second

• Current, charge and time are related by the equation:

• Where:

  • Q = charge, measured in coulombs (C)

  • I = current, measured in amps (A)

  • t = time, measured in seconds (s)

• The current, charge and time equation can be rearranged with the help of the following formula triangle:

Current charge time formula triangle

Formula triangle for the charge, current and time equation

Conventional current

Extended tier only

• Current is the flow of positive charge i.e. from the positive terminal to the negative terminal of a cell

  • This is known as conventional current

• This is in the opposite direction to electron flow

  • Electrons are negatively charged, so they flow from the negative terminal to the positive terminal of a cell

Conventional current and electron flow

By definition, conventional current always goes from positive to negative. This is in the opposite direction to the flow of electrons