Electrical Circuit
Electricity and Magnetism

Forcing charged particles round bends

Teaching Guidance for 11-14 Supporting Physics Teaching

Forces on charged particles drifting around electrical loops

Wrong Track: The concentration of charged particles on the battery repels and attracts the charges around the circuit.

Right Lines: It's the force where the drifting charged particles are that changes the motion of the particles. So you need to see how the charged particles are arranged around the circuit by the battery, and how this arrangement of charged particles affects the moving charged particles.

Dealing with circuit loops – issues that may be triggered

Thinking about the learning

Here it's possible to mislead by acting and talking as if the battery acts direct on the charged particles at each point in the circuit, or even that the concentrations of charged particles on the battery act directly on the moving charged particles at any point. You need to be clear that the charged particles are acted on by forces at their location. These local forces do exist as a result of the repulsion between the flowing charged particles and other charged particles. But by far the most significant charged particle concentrations for the individual moving charged particles are those close to that moving charged particle. These distributions of charged particles are set up by the concentration of charged particles in the battery. So we think that this is another place where it's important to emphasise systematic thinking. It's all about the whole electrical loop.

Imagine the kind of difficulties you might get in to if you try and explain using the repulsion and attraction between the charged particles on the battery terminals and the drifting charged particles, using what you know about the forces between charged objetcs (from the SPT: Forces topic). A circuit laid out on the benchtop will work perfectly well with the wires snaking back and forth. At some points the force acting on the drifting charged particles from the concentration of charged particles on the battery will seems to be driving the charged particles in just the wrong direction – perhaps even out of the wire.

Thinking about the teaching

Here again we'd suggest not always starting at the battery when discussing the flow of charged particles around any selected loop. The important thing is to emphasise the complete loop, acting as a unit, or system, and to avoid leading children into using patterns of simple sequential reasoning.

You'll have to pay attention to the distribution of charged particles around the loop when there is a current in the loop in order to generate a good account of the forces on the drifting charged particles.

A rather convincing explanation is presented in the Physics Narrative, but that is for teachers, and probably not for children at this age.

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