Using the simulation in class / Teaching the meaning of potential difference and how to measure it using a voltmeter

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HEADLINE LEARNING POINTS

  • Potential difference is measured using a voltmeter

  • The voltmeter measures the difference in potential between two points…

  • …so it’s connected in parallel with a component

  • We talk about the potential difference across a component

  • The potential difference across the battery is the same as the potential difference across the bulb

UNDERSTANDING

  • Voltmeters are used to ‘sample’ the voltage at two points and then output the difference between them

  • When you use an ammeter you have to break the circuit to put the ammeter in the way of the current, so it becomes part of the circuit - that’s why ammeters have a very low resistance

  • When you use a voltmeter you don’t break the circuit - you divert a tiny sample of current through the voltmeter, which you want to make as small as possible - that’s why voltmeters have a very high resistance

  • Ideal batteries don’t change the potential difference across their terminals regardless of the load resistance

  • This means changing the resistance of the bulb only changes the current, not the pattern of potentials round the circuit

USING THE SIMULATION

  • Ask students to look at the amount of energy per charge in different parts of the circuit and suggest what the potential is in those parts

  • Make sure you ask about the whole positive side of the circuit, the whole negative side, and through the bulb filament

  • Pick two points and ask students to suggest the potential difference between those two points - then check with the voltmeter

  • Show why the potential difference across the battery is always the same as the potential difference across the bulb

  • Change the battery voltage and show the reading of the voltmeter changing

  • Change the resistance of the bulb and show that the reading on the voltmeter stays the same - but the speed of the charges show that the current is changing

SUBTLETIES

  • Real batteries are designed to provide a constant voltage within a set working range of currents - in other words if they don’t have to work that hard

  • Physically big batteries can typically provide higher currents without their voltage dropping that much - which is why electric car batteries are very big

  • If you need your battery to be small - maybe to fit inside a watch - then it can only provide a very small current and still keep the voltage constant - which is why small batteries are only used for very low-power devices

  • In our circuit simulation the battery is ideal - it keeps the potential difference across its terminals constant, regardless of how much current it’s providing

MISCONCEPTIONS

  • You sometimes hear people imply that voltage is a flow of some kind - when they say something like ‘he got 50,000 volts through him’

  • The rope loop analogy causes all sorts of problems with potential difference

  • There is a sort of mathematical analogue between the frictional force of the person gripping the rope and the potential difference - it’s just not how circuits behave

  • The problem is that if you grip the rope harder then you have to pull it harder - this suggests that the p.d. across the battery changes to match the p.d. across the load, which is exactly the wrong way round

  • Another problem is that dynamic frictional force doesn’t change much with speed - so if the rope slips faster through the load hand the frictional force doesn’t change much, but with a resistor a higher current through a resistor means there has to be a bigger p.d. across it - that’s the whole point with V = IR.