Forces and Motion | Electricity and Magnetism

Electrical forces

Teaching Guidance for 11-14 Supporting Physics Teaching

Forces on objects with small and large charge

Wrong Track: The big charge will push hard on the little charge, sharing some it's charge with the little one.

Right Lines: The are two forces, one acting on each charged object. The forces are equal in size, but opposite in direction. Both forces are examples of action at a distance, so there does not need to be any transfer or contact to enable the forces.

Working with electrical forces

Thinking about the learning

The idea of action at a distance is hard, and sufficiently counter-intuitive that physicists arguably invented the idea of the field to bridge the gap. But this bridging does not involve the transfer of any properties, such as charge or mass, from one object to another. It's simply a way of re-describing the interaction, different from the approach of isolating objects from their environment and figuring out the forces acting on them as a result of interactions with the environment. No charged particles (electrons or otherwise) move between interacting charged objects. The field demarcates a volume of influence, within which you can expect other charged objects to have an electrical force acting on them. Emphasising the essential nature of the electrical force as action without contact helps to make this clear.

Forces acting on a pair of objects, one with a large charge, and the other with only a small charge, are often seen to be unequal in magnitude. As they're representations of the same interaction, they are necessarily the same (later you might want to connect this principle with Newton's third law – you can find out more in the SPT: Force and motion topic).

Thinking about the teaching

This is another place where the idea of compensation is often ignored: pupils tend to focus on either the charge of, or the distance to, the object in deciding whether to expect the object to exert a large or small electrical force. It's good to be aware of this widespread tendency, and to explicitly model considering both, even if a precise analytic discussion is beyond expectations at this age. The same errors in reasoning turn up elsewhere, for example in the SPT: Machines topic, and later in the SPT: Electricity and energy topic. It's a pervasive mistake.

Pupils are likely to seek to have some physical contact-like interaction between charged objects, whether to reduce their discomfort with action at a distance or for other reasons. This often takes the form of suggesting that charge is transferred from one charged object to the other, when one exerts a force on another.

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