not just described, Explained

Home

Contact us

Misconceptions quiz

Subscriptions FAQ


Physics

Radioactivity

Electricity

Physics subscription prices

subscribe log in


PSHE

Eating

Work and Money

PSHE subscription prices

subscribe log in

Criticisms of the Furry Elephant electricity animation

What is the Furry Elephant electricity animation?

Let's first of all that the electricity is not meant to show what's 'really happening' in simple electric circuits.  Bearing this in mind, here are some of the common objections to electric current in this way.

Try Why Do Astronauts Float by Julian Hamm

The charges go the wrong way

There are many reasons why we show convential current and conventional charge but the main reason is that it's the only way to show energy flows consistently.  A general rule in physics is that systems tend to reduce their energy if you release them.  Drop a ball, let go of a coiled spring, run a battery down; the energy of the system decreases.

In an electric circuit charges move from high energy to low energy.  That's why useful energy is liberated.  If they moved in the opposite direction you'd have to put energy into the system somehow.  It would be odd to define the negative terminal of a battery as being at higher energy than the positive but this is how you'd have to do it if you want to use electron flow.

There are no really clean answers to this one.  History has dealt us this rather nasty hand by defining energy in terms of the flow of positive charge and we just have to play it the best we can.  I appreciate this isn't to everyone's liking.  Sorry, but there you have it.

Individual charges don't really carry energy

This is a perfectly valid criticism.  A university-level treatment suggests that electric charges on the surface of the wires cause energy to move through empty space outside the wires directly from battery to bulb via something called the Poynting field.

Good analogies that preserve this field-type quality are the band-saw cutting wood, the bicycle wheel and brake, and the rope-loop.  However you could argue that these analogies miss the point as well because their 'fields' are set up IN the 'wires' rather than around them.  Another problem is that these field analogies aren't very good at explaining the 'per charge' part of voltage and current.

Voltage is to do with the energy transferred per charge.  Current is to do with the number of charges passing a point each second.  The advantage of treating energy as a property of individual charges is that you can maintain this 'per charge-iness' and so it's much easier to understand voltage, current and, by extension, power.

So it's true that individual charges don't carry energy.  But given that you're not going to teach the Poynting field, you might as well choose an analogy that's as useful as possible.

The charges should still have some energy when they get back to the battery

I'm never quite sure whether this is about resistance, or about the fact that the charges are still moving and so should have some energy, or something else.

It's true that the wires always have some resistance and so energy is transferred all round the circuit, not just in the bulb.  But the idea of neglecting the resistance of the wires is perfectly standard practice.

Even if you include the resistance of the wires, the charges must have transferred all their electrical energy by the time they get back to the negative terminal of the battery.  This follows on directly from the idea of potential difference.  By saying the positive terminal of the battery is 6 V higher than the negative terminal, you're implicitly defining the negative terminal as being at 0 V, regardless of the set-up of the circuit.

Think of a battery being like a ladder.  The height of the ladder is like the battery voltage.  We can just define the foot of the ladder to be at zero energy regardless of whether we have it on the 7th floor or in the basement.

Another possible interpretation of this objection is that the charges must have some energy when they get back to the battery otherwise they'd be stopped.  I suspect that this comes from a faulty understanding of what might constitute 'electrical energy'.

One thing that electrical energy isn't is the kinetic energy of the charges.  Though we show the extremely idealised case of wires of constant thickness everywhere, this is never the case in a real circuit.  Not only does the thickness of the wires change but often the material they're made from and their temperature.

All of these cause the average speed of the charges to be widely different in different parts of the circuit.  The key point to note is that where the resistance is higher, for example where the wire is narrower, the charges actually tend to move quicker so that the same number of charges can get through each second.

This seems rather counter-intuitive but makes sense if you think that energy might reasonably be transferred where the electrons make more interactions with the positive ions each second.  In other words where the electrons tend to be going faster.

In an ideal circuit consisting of a uniform length of wire the speed of the charges would be constant along the entire length.

Modelling charges with springs between them is unhelpful

I've tried to explain electrical energy by saying that the battery tends to squash the charges together on the positive side of the circuit and stretch them out on the negative side.  This fits in with the common idea of charge separation.  I've extended this idea by saying that this is a bit like having springs between the charges that are stretched or compressed and that this is where the energy is stored.

At a fundamental level energy is carried by the Poynting field outside the wires directly from the battery to the bulb.

The springs-between-charges idea is meant capture some of the of the Poynting field and fits in with the band-saw, bicycle wheel and rope-loop analogies.

Except there is a problem; and that is that band-saws are designed not to store energy in the saw itself.  In other words you want the band-saw to be as stiff as possible so that you're not wasting energy stretching the saw.  In an ideal band saw, no energy is stored in the blade-loop but lots of energy is still transferred from the motor to the wood being cut.

The band-saw analogy and others like it aren't good at explaining voltage in open circuits.  For example, if a circuit is incomplete then you can still say 'this bit of the circuit is at 6 V and this bit is at 0 V' (or +3 V and -3 V).  What is it about the arrangement of charges that makes the voltage different?  The advantage of the spring analogy is it provides some sort of explanation as to how a voltage can still be there even if there is no current because the springs are still stretched or compressed even though they're not moving.

back to Lesson 2: Inside Circuits