Thursday, 4 October 2012

Flipping Bad Physics: David Blaine

The newspapers are ringing with the latest exploits of "magician" David Blaine. To a physicist, hopefully those who recently completed my first year course on electromagnetism, it is obvious that this story is little more than "man sits in a fancy Faraday cage for a while".

But that's not the point of the post. It's the text accompanying the image. And specifically the phrase "a million volts of fiery electric current"! ARGH!!!

I know Joe Public does not care, but such a sentence causes the inner guts of those with a passing knowledge of physics to twist in an awful pain. Put simply, voltage (measured in volts) is not the same as current (measured in amps).

If you are being electrocuted, the difference may not bother you, but the fact that smoke is coming out of your ears is not an excuse for physics illiteracy.

Right. This is going to be a short post, as I have lots to do, but let's use a gravitational analogy.

Let's imagine you are on the top of a very tall building. It is a long, long way down. You have a certain amount of gravitational potential energy which is proportional to your height off the ground. Simply put, this is your gravitational "voltage".

You step off the building. Let's ignore the acceleration time, but assume you instantaneously fall at the terminal velocity in air. This is almost 200 km/h, and will probably result in a messy end. The speed at which you fall is the gravitational "current" (in reality, you want a line of people jumping off to get a real current, but let's not have that disturbing picture).

Let's consider changing the experiment slightly. We keep the height the same, so the gravitational "voltage" stays fixed, but now you jump into a tube on the side of the building filled with treacle. Again, you fall at the terminal velocity, but this is much lower.

You gravitational "current" has gone down because the "resistance" to your motion has gone up.

Is it really that difficult? If you have a journalist friend, get them to watch this all explaining video
The penguins going up the steps are the electrons inside a battery. The height they go to is the voltage. The rate at which they slide down the path is the current. Simple.

5 comments:

  1. A very good analogy - especially the penguins - and a great way to explain electricity.

    Technically it might be more appropriate to compare the current (amps) to the total number of penguins (electrons) passing around the circuit at any point in time.

    The rate at which they pass around (electrons per second) is analogous to the electric charge (amps per second or coulombs).

    I get concerned when I see anyone playing with electricity the way this man appears to be doing. It puts silly ideas into some people's heads. The end result can be electrocution or injury if something goes wrong - especially if there is no treacle and the number of penguins is high!

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    Replies
    1. The rate at which they pass around (electrons per second) is analogous to the electric charge (amps per second or coulombs).

      charge --> current

      Of course, an amp is a coulomb per second, not vice versa.

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  2. Yes, I should have typed ampere-seconds not amps/second

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  3. I agree my analogy is not perfect, but I am a firm believer than the penguin roller-coaster is a better description of electricity than that presented by many science journalists :)

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