Electricity Basics – AS Electronics Revision

Current and Voltage / Potential Difference / EMF

  • A battery (or other source of EMF) can make electrons flow if there is an unbroken circuit of conductors between its terminals
  • (Conductors are things that can conduct electricity – they tend to be things with lots of free electrons, such as metals. That’s physics knowledge, though, not electronics)
  • An electric current flows through a conductor when all the free electrons in it move the same way at once
  • The ‘force’ that drives the electrons through the conductor is called EMF (Electro-Motive Force)
  • EMF is often just called voltage (or potential difference)
  • The symbol, V, for voltage is the same letter as the unit, V (volts), which can be confusing, since other units and symbols normally use separate letters
  • Current, by the way, is measured in Amps (unit represented by the letter A), and the symbol for current is I

Which Way?

English: conventional notation of electric cur...

English: conventional notation of electric current and the flow of electrons (Photo credit: Wikipedia)

  • People originally thought that current flowed from positive to negative
  • It was later discovered that electrons actually go the other way
  • Instead of rewriting all the books, scientists decided they’d pretend current still went the same way
  • The idea that current flows from positive to negative is called conventional current
  • A positive current flowing from positive to negative is the same as a negative current flowing from negative to positive… which is why we say electrons have negative charge
  • (It worked out quite nicely, because that meant the charge on protons is called positive. I guess we may one day encounter some aliens who got it the other way around, calling protons negative and electrons positive…)
  • If you see a line on a circuit diagram, indicating the direction of current flow, it means conventional current flows that way


  • Electrons flowing through a circuit encounter resistance
  • (i.e. they crash into atoms)
  • This means some energy is lost (as heat, usually)
  • …so some voltage is dropped across the resistor
  • That’s why a resistance causes a potential difference to develop across it
  • The symbol for resistance is R
  • It’s measured in Ohms (the unit is represented by that Greek letter Omega)

Series and Parallel Circuits

  • Components connected in series only meet at one point
  • Series circuits have all their components ‘in a row’
  • There’s only one path the current can take
  • Parallel circuits have more than one path for current to take
  • Components connected in parallel meet at two points
  • You may remember from GCSE Physics that everything in a series circuit stops working if one component fails, whereas parallel paths can fail separately and not affect the others. AS Electronics isn’t really about components failing, though.

About Matt

I like writing, filmmaking, programming and gaming, and prefer creating media to consuming it. On the topic of consumption, I'm also a big fan of eating.
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