Forces and Motion | Electricity and Magnetism

Charging objects

Teaching Guidance for 11-14 Supporting Physics Teaching

Charging and charged objects

Wrong Track: Positive charge flows onto the sphere, then it's charged.

Right Lines: The material is made of atoms, which have negative electrons and positive protons. Atoms are neutral, because there are equal numbers of protons and electrons. However the electrons are easy to remove from the atoms, so its these that are transferred to charge an object. Adding electrons makes the object negatively charged, and subtracting electrons makes the object positively charged. The electrons that one object loses, another object gains.

Working on charging

Thinking about the learning

It's likely that pupils don't have a secure description of the structure of atoms. So if you want to discuss the process of charging or discharging, you may well have to re-activate, or perhaps even develop, this knowledge. Fortunately the model of an atom we need is really rather simple: pupils need to know that atoms are neutral, as a result of an equal number of positively charged particles (protons) and negatively charged particles (electrons). Electrons can be removed from atoms rather easily, but protons are very hard to remove.

To charge a material you can remove some of the electrons, or add some electrons. As you cannot lose electrons, making one thing positively charged necessarily makes another negatively charged. To make something neutral again, you restore the balance between the electrons and the protons, a process known as discharging.

In this way any material can be charged or uncharged, as a result of shifting electrons to or from it. This is very different from persuading charged particles to flow through it, which is what you do in electrical circuits. Materials which can be used in electrical circuits are conductors: those which do not permit charged particles to flow are called insulators. This categorisation is to do with charged particles moving through the material, and not with shifting charged particles to or from an object.

Thinking about the teaching

Many pupils will firm encouragement to distinguish between the properties of a material (whether it's an electrical insulator or electrical conductor) and its charged state (whether is positively charged, negatively charged, or neutral). In particular there is a tendency to conflate the state of being charged with the property of being a conductor.

You'll need to emphasise that to charge something you need to shift electrons to or from it, so you'll need to contact it. This makes it easier to understand that isolated means that it cannot become charged as other charged objects come close to it. Contact is needed.

Pupils also tend to think of charge as a something that exists quite separately from electrons or protons, and you'll need to be careful not to reinforce this by talking as if this were the case.

Teacher Tip: Add electrons to something to make it negatively charged, don't add negative charge to an object.

Limit Less Campaign

Support our manifesto for change

The IOP wants to support young people to fulfil their potential by doing physics. Please sign the manifesto today so that we can show our politicians there is widespread support for improving equity and inclusion across the education sector.

Sign today