Current (Cambridge (CIE) IGCSE Physics): Revision Note

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

Measuring current

• Current can be measured using an ammeter

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

• 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 it is not at zero, this value will need to subtracted from all 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 (opens in a new tab) of electric current

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

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:

$I=\frac{Q}{t}$

• Where:

  • $I$ = current, measured in amps (A)

  • $Q$ = charge, measured in coulombs (C)

  • $t$ = time, measured in seconds (s)

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