The Speed of Electrons in Wires

When you flick a light switch the light comes on straightaway. A common misconception is that this is because the electrons leave the power supply and travel very quickly through empty wires and then back to the power supply again.

In fact the electrons are already there everywhere in the circuit and they all start moving very slowly at almost exactly the same time. Just like a wheel, there's no part that begins first.

If you look a little more deeply there are actually three very different speeds, which are very difficult to imagine all at the same time.

1. (VERY FAST) The random thermal jiggling of the electrons. The electrons whizz around colliding billions of times a second with the surrounding atoms (or, more accurately, ions). This depends on the metal and the temperature but is typically around 1% of the speed of light.

2. (VERY VERY FAST) The speed at which the electrons find out that they should start moving. Think of the rear carriage of a train starting to move at almost exactly the same time that the locomotive begins to pull. This is called the signal speed and is typically a bit less than the speed of light.

3. (VERY VERY SLOW) The speed that the electrons actually make progress along the wire. This is called the drift speed and is typically around half a metre an hour - slower than a snail. This is because the electric field in the wire (think push from the battery) doesn't have much space to accelerate the electrons before their high thermal speed causes them to be scattered off another atom.

In this simulation I try to show all three speeds at the same time.

Half-life and Health Risks

Imagine radioactive material is accidentally released into the environment. Which is the most hazardous half-life for it to have? A few hours? A few years? Millions of years?

To answer this question we need to think about why radioactivity decreases with time. The simple answer is that every time a nucleus decays and releases a particle (like an alpha or beta) then there's one fewer undecayed nucleus left. For a given isotope every nucleus has the same chance of decay each second. It doesn't matter how long it's already been around for or what its neighbours are doing.

If the chance of decay is high then lots of nuclei decay each second (so lots of radiation is given off) but you quickly end up running out of undecayed nuclei. This means the half-life is short.

If the chance of decay is low then very few nuclei decay each second (so very little radiation is given off) and there are still lots of undecayed nuclei left a long time later. This means the half-life is long.

The key point is that isotopes with a very long half-life are only very weakly radioactive and isotopes that are very radioactive don't stay radioactive for long.

It turns out that the most problematic half-life for the environment is a few decades or so. Isotopes such as strontium-90 (with a half-life of about 30 years) are pretty radioactive and stick around for a time comparable to a human life-time, which gives plenty of opportunity for them to cause genetic damage.

In this activity you go forward in time to see the effect on radioactivity of different samples.

General comments about Radioactivity and Atomic Physics Explained

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The Constant Current Misconception

If you have a simple battery and bulb circuit and you add another bulb in parallel, would you say the current now 'splits' at the junction?

Or imagine starting with the same simple circuit and adding a bulb in series. Could you explain the fact that the bulbs are both dimmer by saying that the battery’s energy is now shared between two bulbs?


If you think either of these explanations seem pretty reasonable then you may hold the constant current misconception.

The constant current misconception is the implicit belief that batteries are constant current providers.

In the parallel example there isn't a 'the current' to split. When you add the extra bulb in parallel then the current drawn from the battery doubles, it doesn't just split differently.

In the series example the assumption is extended to imply that batteries provide energy at a constant rate. They don't. When the extra bulb is added in series then the battery provides energy at half the rate. It doesn't provide energy at the same rate and then share it out differently.

Batteries are constant voltage providers (as long as you don't make them work too hard) and the current they provide depends on the circuit they are connected in. Any change to the circuit will always change the current.

General comments about Electricity Explained

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An Introduction to the Philosophy of Science

Imre Lakatos once said that most scientists have no more idea of the nature of science than a fish has of hydrodynamics. A little harsh, perhaps, but I'm inclined to be deeply cynical of those who blithely talk about science as if there existed this human acitivity that churned out universal truths simply by following a recipe.

I've tried to summarise some of the main ideas in the philosophy of science and I have to say I'm becoming more of a fan of Paul Feyerabend (having just finished re-reading Against Method for the third time) as I get older.

Teaching How Science Works

The emphasis on embedding How Science Works into syllabuses is welcome but my guess is that many teachers, even those who have been research scientists, can find it hard to deliver.

This isn't just because it's difficult to find interesting activities to get across the idea of, say, peer review but more because science is often taught as an end-result rather than a process. And here I'm not talking about 'investigations'. I'm talking about impetus theory and flogiston.

I've taken a fairly uncontentious approach to introducing How Science Works but I'm still deeply suspicious of the idea that there exists some 'scientific method' that leads to 'the truth'.