Electric Charge – why it is difficult to understand and how to help.
By Ben Rogers
18 years secondary teacher: 4 years primary. MA in SciEd, . .Chartered Science Teacher. Author of 'Big Ideas of Physics and How to Teach Them.' here Works for Paradigm Trust.
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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 exam board’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 carrying 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.
[my additional note - - -The most significant difference between the charge and the current is that the charge is the physical property of matter, whereas the current is the rate of flow of charges. source ignore police inclusions]
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 [and curriculum] 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 be very patient.
