Using the simulation in class / Teaching the relationship between voltage, current and resistance

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HEADLINE LEARNING POINTS

  • Increasing the battery voltage increases the current

  • Increasing the bulb resistance decreases the current

UNDERSTANDING

  • When you change the battery voltage, the new potentials are set up everywhere round the circuit at (very, very nearly) exactly the same time

  • So the current changes everywhere at (very, very nearly) exactly the same time

  • Increasing the battery voltage makes the bulb get brighter because both the current through the bulb and the potential difference across the bulb increase

  • You can see this in the animation because each charge arrives at the bulb with more energy, and the charges with energy arrive faster

USING THE SIMULATION

  • Change the battery voltage and observe what happens to the speed of the charges - the current

  • Big voltage gives big current

  • Change the bulb resistance and observe what happens to the current

  • Big resistance gives small current

  • Ask students:

    • What are the two reasons the bulb gets brighter when you increase the battery voltage?

    • What happens to the potentials round the circuit when you change the bulb resistance?

    • What happens to the current when you increase the bulb resistance?

    • Which are brighter, high resistance bulbs or low resistance bulbs?

SUBTLETIES

  • You can’t easily intentionally change the resistance of a bulb - all you can do is take one bulb out and replace it with a different one

  • The relationship between voltage, current and resistance is often misnamed Ohm’s Law or V = IR

  • Ohm’s Law just says that for a very small number of components - mostly wires that aren’t too warm - current is directly proportional to p.d.

  • Real filament bulbs are nowhere near Ohmic - their resistance rises sharply with temperature - except when the voltage is very low and the filament isn’t very warm

  • The bulb in the simulation is Ohmic to make the numbers easier

  • At this early stage it’s probably best not to mention Ohm’s Law or what resistance is - the safest analogue for increasing resistance is a marble falling through syrup rather than water, since charges in a circuit are moving at a terminal velocity

MISCONCEPTIONS

  • The idea of ‘resistance’ is really horrible and causes all sorts of problems

  • Physical systems where you feel a ‘resistance’ to motion (e.g. the rope loop analogy) are not analogous to how circuits work - but they appear to be - even to clever people who should know better

  • Resistors do not ‘resist the flow of current’ because there’s no fixed current for them to resist

  • Students think batteries have to ‘try harder’ to ‘push’ current through the ‘resistance’.

  • This is part of the ‘constant current misconception’

  • Batteries are constant voltage providers, but students often implicitly believe they are (or try to be) constant current providers

  • If you increase the resistance then the current is smaller

  • This means the battery is working less hard

  • It doesn’t struggle to try and maintain a constant current - it just gives up a bit