Electric Charge – why it is difficult to understand and how to help.

My heart has never been in definitions. It was set against them in Africa 20 years ago, when I was teaching physics in Ghana. The exams, and the students, prioritised the recall of definitions. And I didn’t know them – I just converted the equation into words (I=Q/t Definition: current is the rate of flow of charge).

When my definitions disagreed with the examboard’s definition, I saw doubt, fear and sometimes anger on the faces of my students. So I learnt the exam board’s definitions, sadly, not with good grace.

Recently, I have begun thinking about definitions again. I often see teachers asking students to write their own definitions as either a warm-up or assessments task. But I think this is too hard. If you want students to learn a definition, learn the exam board one.

But definitions are not the key to understanding a concept. Daisy Christodoulou’s new book (Making Good Progress) quotes Thomas Kuhn when talking about definitions. She (and he) make the point that a definition doesn’t lead to understanding: repeated exposure to the concept through discussion, models and texts; solving the discipline’s standard questions about the concept and carring out the standard practicals leads the learner to a rich understanding. Then the definition becomes useful.

Electricity has three concepts which hold the key to understanding the subject: voltage, current and charge. These cannot be taught one-by-one. An effective understanding is built up by repeated exposures to these ideas over years. I will begin with charge, because it is the most fundamental, and the most slippery.

Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.

The definition alone is hopeless.

In physics, the word charge is used in at least three different ways. First, and most important, charge is used as an uncountable noun – physicists often talk about charge as though it were an amount of a substance, like water or plasticine, not separated into separate droplets or chunks. This idea developed because physicists initially understood electricity as a flow of charge, imagining water flowing in a pipe. This use of charge is important, because it allows us to  understand current, voltage, electric fields and the conservation of charge.

A concept map exploring charge as an uncountable noun

The concept of charged particles, such as electrons, protons and ions developed much later, introducing the next usage of the word charge: this time, charge means charged particle. We talk about a flow of charges,meaning a flow of electrons or ions. This idea introduces the confusing idea that current flows in the opposite direction to the flow of electrons in a circuit. The way out of the confusion is to remember that charge and charges are not the same thing.

The third use of the word charge is as a verb: to charge. It can be used correctly to mean put charge onto an object, for example when a balloon is rubbed on a jumper (notice, I used charge as an uncountable noun), or incorrectly, such as charging a battery (when we really mean putting energy into a battery – this is the use of the word charge we all use everyday!)

We also used charged as an adjective (charged particle, charged capacitor, charged balloon and, unfortunately, charged battery).

You can see how the definition of charge is unhelpful. If you need to teach the definition, just make sure they learn the exam version. Otherwise, help your students construct a rich understanding, through the use of a mixture of models, problems to solve and questions to answer and practical work. Be explicit – charge is a challenging concept that many of your students won’t develop on their own or by chance – and very patient.

Please comment on this text, as I intend to use it elsewhere and I would like to get it right!

Thanks,

Ben

@benrogersedu

6 thoughts on “Electric Charge – why it is difficult to understand and how to help.”

• Deep Ghataura says:

I would argue that an even more unhelpful term is “electricity”. We wouldn’t say that biology flows through our veins.

It is unhelpful because it permits the sloppy use of “charge”. It might take much longer to say but we can be precise when we talk about charges and charged capacitors and so on. Greater precision in the hustle and bustle of the classroom is difficult and I don’t begrudge any teacher who accidentally slips now and then.

Like

1. Ben, firstly thanks for a very thoughtful piece. This shows one of the major issues we always face in science; we’re using language and communication skills which evolved when apes wanted to point each other towards the best bananas.

I like the way you’ve explored the three main things scientists mean when they use the word ‘charge’. It’s a shame the word doesn’t change – I wonder if languages other than English are more subtle? I’m thinking of the way that in English advise is the verb and advice is the noun. I suppose we could change the sentence around to say “I’m moving the charge around by friction” rather than “I’m charging up the balloon”, but that avoids rather than solves the issue.

Would using letters be a good way to distinguish between the uncountable noun/property of charge and the quantity charge Q? I think it could be argued that these are qualitative and quantitative uses of the word, perhaps.

Finally, comparing it to the use in ‘everyday’ English. I keep coming back to the idea that science vocab is like legal text or some computer code. It looks like English, but it isn’t. We need to remember to treat our students as language learners, because Science as Additional Language (SAL) is a real thing. It has the added complication that because it looks the same,, we make assumptions about what we think we already know. The concept of ‘false friends’ – words in another language that look familiar, but mean something different – means that it’s actually harder than if the words were totally different. Kids don’t mistake ‘photosynthesis’ for a word they know. But ‘force’, ‘energy’, ‘power’ – they’re similar enough to cause problems. I keep thinking this would be a really interesting article for ScienceInSchools, comparing how different languages deal with these technical words similar to those in everyday use.

Looking forward to your thoughts on pd and current.

Like

• Thank you Ian,

I’ve just looked up the SI units on the German Wikipedia page (https://de.wikipedia.org/wiki/Internationales_Einheitensystem) – It appears thet German has similar, if not identical problems – for example, pressure (in pascals) is Druck (push). It is interesting that current translates to electricity strength – which I can see would cause it’s own problems. To determine whether German uses charge as countable and uncountable would need a better German speaker than I am!

Like