DC and AC Electricity Pt1: Electron Movement

One of the most famous technological “wars” was the battle that raged almost 150 years ago between Thomas Edison’s DC electricity and Nikola Tesla’s AC electricity. For our purposes, we will put aside the stories of the two men (Edison was an inventor and businessman whereas Tesla was a scientist and idealist) and focus on the differences between DC and AC electricity.

Let’s start with DC, which stands for direct current. This is the most common form of electricity that is used in portable devices and digital electronics; it is used in everything from torches and smartphones to drones and television sets. It is also the form of electricity that is most often used when carrying out experiments in the lab, whether you are investigating the resistance of a lamp or performing electrolysis.

In a direct current circuit, electrons move in a constant direction around the circuit, travelling from the negative side of the power supply to the positive side. Note that this is the opposite direction to conventional current, which travels from the positive terminal to the negative terminal. This is due to the fact that electrons were unknown when electricity was first being explored and when electrons turned out to have a negative charge, they had to move in the opposite direction to the positive current that we know so well but which is actually just an imaginary relic of a bygone time!

In an AC circuit, electrons are constantly changing direction, moving forwards and backwards. We call this an alternating current. Forwards movement has a positive value and backwards movement has a negative value. You should recognise this direction-sensitive behaviour as a vector characteristic: this tells us that current is a vector quantity because it has not only magnitude but also direction.

You might wonder how a forward-and-backward movement can pass energy around a circuit. This can be explained by realising that our simple model of electrical energy as something that is carried, like water in a bucket, when electrons travel from one battery terminal to the other, isn’t quite true. In fact, the energy is passed from one electron to another and moves around the circuit almost instantly whereas the electrons themselves move very slowly (measured by their drift velocity – but that is beyond what you need to know for your GCSE course).

Given that energy is passed, not carried, it is much easier to imagine electrons acting in relay around the circuit – and in this case it doesn’t matter whether the electrons move in the same direction as the energy or not.

To link this to previous learning, think back to the two different types of waves… In longitudinal waves, the particles vibrate backwards and forwards, moving parallel to the direction of the energy transfer. In the case of sound, the rate at which they vibrate back and forth determines the note that we hear. Conversely, in transverse waves the particles vibrate up and down as the wave energy moves forwards: think of a Mexican wave where the people raise and lower their arms in turn to create a wave that moves through the crowd at right angles (perpendicular) to the direction of the body movement.

If you are happy with the idea of the two types of waves then you should be able to convince yourself that electrical energy can also be conveyed in two different ways; either by electrons that are constantly moving in one direction (DC electricity) or by electrons that are being forced to move back and forth (AC electricity).

In the next post, we will discover further links between waves and electricity when we realise that AC electricity can be measured using the same characteristics that we use to measure waves.

Before you get to that, you must complete a self-assessment test to check that you understand the facts that have been explained here. The test is on liveworksheets.com: click here to access it. Remember to send me your results by selecting Email results to your teacher when you finish.