Electrical Current (Cambridge (CIE) O Level Physics): Revision Note

Current

• 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:

  • Q = charge (measured in coulombs, C)

  • I = current (measured in amps, A)

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

Equation Triangle for Current Equation

Use the formula triangle to help you rearrange the equation until you feel comfortable doing it on your own 

Measuring Current

• 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

Simple Circuit Diagram

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

Electrical Conduction in Metals

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

Conduction in Metals

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

Conventional Current

• 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

Conventional Current Versus Electron Current

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

Direct & Alternating Current

• An alternating current (a.c.) is one that continuously changes its direction, going back and forth around a circuit

• A direct current (d.c.) is one that is steady, constantly going the same way around a circuit, from positive to negative

A.C. and D.C. Current Graphs

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

• Electric cells (batteries) produce direct current (d.c.)

• Mains electricity is an alternating current