Current (Cambridge International IGCSE Physics)

Spec point — 4.2.5 · - Know that electric current is related to the flow of charge

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

Spec point — 4.2.5 · Describe the use of ammeters (analogue and digital) with different ranges

• 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 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

An analogue ammeter 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

A digital ammeter display

Electrical conduction in metals

Spec point — 4.2.5 · Describe electrical conduction in metals in terms of the movement of free electrons

• 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

Spec point — 4.2.5 · - Define electric current as the charge passing a point per unit time; recall and use the equation I = Q/t (E)

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)

Worked Example When will 8 A of current pass through an electrical circuit?

A. When 8 J of energy is used by 1 C of charge

B. When a charge of 4 C passes in 0.5 s

C. When a charge of 8 C passes in 0.1 s

D. When a charge of 1 C passes in 8 s

[1]

Answer:

• The equation relating current, charge and time is:

$Q=I\times t$

• Rearrange to make current I the subject of the equation:

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

• Consider option B, where Q = 4 C and t = 0.5 s:

$I=\frac{4}{0.5}=8A$

• Therefore, the correct answer is B [1 mark]

A is incorrect as this is the definition of a voltage of 8 V between two points and does not describe current

C is incorrect as $I=\frac{8}{0.1}=80A$

D is incorrect as $I=\frac{1}{8}=0.125A$

Exam Hint Electric currents in everyday circuits tend to be quite small, so it's common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Make sure you can convert these into standard units, e.g. $10\mathrm{mA}=10\times10^{-3}A$.

Conventional current

Spec point — 4.2.5 · State that conventional current is from positive to negative and that the flow of free electrons is from negative to positive

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