Using the simulation in class / Teaching the meaning of current in amperes by counting charges passing point in a given time

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HEADLINE LEARNING POINTS

  • Current in amperes is the number of coulombs of charge passing a point on the circuit each second

  • Current is measured by connecting an ammeter in series

  • charge = current x time

  • current = charge / time

UNDERSTANDING

  • Current is connected to how fast charges move - fast charges means big current

  • At a very simple level, an ammeter is a bit like a speedometer for charges

  • The unit of current - the ampere - is defined as a current that moves 1 coulomb of charge past a point in 1 second

  • So current in amperes depends on both how close together the charges are and how thick the wire is, as well as how fast the charges are moving

  • Current is measured at a given point in a circuit - it says stand on this point and count how many charges go past in a second

  • You always have to break the circuit to insert an ammeter - this means you put the ammeter in the way of the current so it can measure it

  • Ammeters have to have a very low resistance so they don’t change the current they’re trying to measure

USING THE SIMULATION

  • It’s best to uncheck the Show energy checkbox, so you focus only on the black dots

  • You can drag the ammeter into place and show how its reading changes as you change the battery voltage thus making the speed of the charges change too

  • You can also move the ammeter to show that the current is the same everywhere round the circuit

  • Start with charge = current x time

  • Adjust the battery voltage/ bulb resistance so the ammeter shows the current is say 2 A

  • Ask the students how much charge will flow past a point in say 5 seconds, then use the stopwatch to check by counting charges

  • Then move on to current = charge / time

  • Put the ammeter back

  • Set the current (secretly) to some known amount e.g. 10 V and 5 ohms gives 2 A

  • Count the charges flowing past a point in 10 seconds and use that to calculate current in amperes

  • Check using the ammeter

SUBTLETIES

  • Ammeters don’t count or time charges per se

  • Analogue ammeters use the current’s magnetic effects to rotate a coil against the effects of a spring - big current means big rotating force means big reading

  • Digital ammeters typically work by measuring the voltage across a known resistance inside the meter then implicitly outputting I = V/R

MISCONCEPTIONS

  • It’s important to realise that charge = current x time is only meaningful if you specify a point past which the charge is flowing

  • You someimes see questions asking the amount of charge flowing ‘in the circuit’ which simply doesn’t make sense, since that would depend on the size of the circuit, and wouldn’t depend on the current