Aims of this Episode:

  • Define the basic key terms: Charge, Current, Potential Difference and Resistance;
  • Describe the differences between Series and Parallel circuits;
  • Describe the differences between Direct and Alternating Current.


Charge is simply the thing that carries energy around a circuit. In a metal wire the charge is electrons (free electrons or valence electrons). These are put into groups called coulombs. One coulomb contains 16,000,000,000,000,000,000 electrons! Think “Herd of Sheep = Coulomb of Electrons”.


“The rate of flow of charge.”

This is a measure of how many coulombs pass a particular point in a circuit in one second. A current of 5 Amps means that five coulombs pass a point in one second.

Potential Difference

“Energy transferred per unit charge (coulomb).”

As the coulombs of electrons pass through a bulb for example, they pass some energy onto the bulb. Therefore there will be a difference between the amount of energy per coulomb before the bulb and the amount of energy per coulomb after the bulb – this is the potential difference.


Resistance is a measure of how difficult it is for coulombs of electrons to pass through the component (bulb or resistor for example). If the component has a higher resistance then the coulombs will either use more energy (higher potential difference) or move slower (lower current).

Series & Parallel


In a series circuit the components are arranged one after the other (much like the episodes in a TV series come one after another). The current is the same throughout a series circuit, whilst the potential difference is shared between all of the components. This makes sense, since the coulombs will move at the same rate, but will have to share their energy between all of the components because they have to pass through them all before returning to the battery.

This is how you calculate the total resistance of a series circuit:


In this type of circuit the components are arranged in loops, so there are multiple branches to each circuit. Because of these branches, the current splits and is shared out between each branch. However, each coulomb no longer has to share its energy with all of the components, so the potential difference is the same in each branch.

The total resistance is calculated using this equation:

Alternating & Direct Current

Direct Current

The coulombs in a circuit using direct current all flow in the same direction (from positive to negative), this is sometimes called conventional current. Your phone battery uses direct current.

Alternating Current

This type of current is found in mains electricity (the plugs in your wall at home). The coulombs move backwards and forwards (weird, I know), passing their energy a bit like a human chain passing buckets of water to put out a fire. In the UK, the mains electricity has a potential difference of 230V and changes direction 50 times per second, so it is said to have a frequency of 50Hz.

S@S 013: Electricity
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