A Shocking Lesson

Static electricity is an electrifying experience! Sometimes, when you come in contact with a piece of metal, you can receive an electric shock, particularly if you are wearing rubber shoes (like sneakers or runners). Why?

Well, the reason is that you are storing an unbalanced charge. If you remember from our previous studies of the atom, most atoms have no net charge; they have the same number of protons as electrons, and thus have an overall charge of zero. How do you get an excess of charge? Well, that means you have more or less electrons than protons (you can’t gain more protons without changing your nuclear structure, so you must gain or lose electrons.)  and thus you are charge imbalanced. As soon as you touch (or even come close enough that a spark can jump to) another object, the charge will move so that it is more evenly balanced across the two objects. This is called static discharge and can be quite painful! In fact, the average human body can store approximately 3.5 kV of static charge!

There is more fun yet – we can force an object to become charged by connecting it to a DC power source or by creating friction between it and another object. As long as the object is insulated from ground, it will build up an imbalanced charge. We can experiment directly with this effect by using a device called a Van Der Graaf Generator. The first three videos show this device and some of the physics behind it’s operations. The last two use a Tesla Coil, which is a device which uses directly applied high voltage to create massive charge imbalances. Tesla coils are dangerous (and fun!)

What you need to know about charge is quite simple:

  1. What is it and how is it and how is it measured?
  2. What are the effects of charge imbalance?
  3. What is an Electric Field?

1. Charge is a fundamental constant of the universe; like distance and time, it is not a derived unit  (speed is an example of a derived unit – it is the rate of change of distance with respect to time). Charge has the symbol (Q or q) and is measured in Coulombs (C) (Named for Charles-Augustin Coulomb, a French Physicist who investigated the phenomenon in the 18th century). 1 Coulomb is a *very large* unit of charge. An electron has a charge of -0.000000000000000000016 Coulombs (or -1.6 * 10^(-19) C).

2. Charge imbalance occurs when an object has an excess of either positive charge (electron deficit) or negative charge (electron surplus). When there is an imbalance of charge, a force will exist between one charged object and any other object. We call this an Electrostatic Force and the magnitude of this force is determined by Coulomb’s Law:

The force between two charges is equal to a constant multiplied by the magnitude of each charge, divided by the square of radial distance between them. This is an inverse square relationship – something which will see again in the future! The direction of the force is determined by the sign of the charges – two like charges will repel, two unlike charges will attract.

3. Electric fields are an imaginary construct. They are a way of representing the electrostatic force (on a positive test charge) at any point in the space around a charged object. There are two critical points of drawing a field diagram

  • Arrows must be drawn that show the direct of force on a positive charge
  • The relative density of lines represents the strength of the force.

Here are some notes that explore this more fully on physicsclassroom. Here is a student-written application (Mr. Coulomb Takes Charge) that is *excellent* – download all files and select intro.exe to make it work. Finally, here are two applets that will allow you explore field diagrams (1, 2)

See you in class!

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16 Comments on “A Shocking Lesson”

  1. Patrick Says:

    hey sir, thanks for the post, makes things easier to understand.

    i was wondering though, as humans are made up of mostly water, would we be able to store enough static electricity in our body, to cause a reaction with the electrons in the water molecules, causing them become ions themselves?

  2. CyberChalky Says:

    Hi Patrick,

    When we say “humans are mostly water”, we mean that a significant part of the volume and mass of a human have water in them. Water, being a compound doesn’t ionise – if subject to enough charge difference, it can separate into the component parts of water, namely hydrogen and oxygen.

    If an individual were somehow charged high enough for this to happen, I doubt you would be concerned about ionisation – more the fact the you are evaporating!

    • Kane L Says:

      Yummy! could a human, being made up of mostly water, be used as a capacitor in a circuit?

      • CyberChalky Says:

        Any nonconductor can be a capacitor; a human is relatively non-conductive, so yes, a human can be a capacitor.

        Two things, as a capacitor, a human has a very quick discharge time and so is not particularly good for circuits – this happens when an unshielded human touches delicate electric components (such as RAM) – they “fry” them the static charge a human body holds is sufficient to damage the delicate electronic components.

        The second thing is that the discharge could also be harmful to the human involved…

  3. Patrick Says:

    on physics classroom reading the part in lesson 3 that explains coulombs law it said something about coulombs constant, which intrigued me, it said “The value of this constant is dependent upon the medium that the charged objects are immersed in”, does that mean that the constant is subject to change, depending on the environment….and if so, is there a way to work this out?

    P.S i get a feeling that the answer will be beyond my scope, but i’m curious so tell me anyways 😀

  4. CyberChalky Says:

    Hi Patrick,

    Coulomb’s constant is linked to the speed of light, c. As long as the c is constant, k will also be constant.

    Unfortunately, c is dependent on the medium, and the permittivity and permeability of space (these are constant, defined values of our universe. This is not to say that there are not alternative universes that have different values for these “constants” and thus different values for c, and therefore k. This is called “multiverse theory” – you *will* never have to deal with this in VCE physics.)

    • Patrick Says:

      so if we knew what medium the electrostatic force was occurring in, worked out its permeability ratio, and permittivity ratio, we would be able to see what effect it would have on Coulombs constant? and if coulombs constant is linked to the speed of light…and the constant itself has factors which affect it, does that mean, that the speed of light itself has factors in which can decrease it? i say decrease because i believe nothing can go faster then the speed of light.

      • CyberChalky Says:

        Absolutely; if you define the permittivity (ε0) and the permeability (μ0) you can calculate the constant for that medium.

        Yes, it is theoretically possible to decrease the speed of light if the permittivity (ε0) and the permeability (μ0) are increased. The speed of light is determined by the inverse square root of the product of these constants, i.e. 1/√ε0μ0

        If however, you decrease the speed of light, it (should) remain the highest possible speed. However, as we are talking about a theoretical universe in which the fundamental constants have been changed, their may be other unpredictable effects. Thus we can’t predict what such a world would be like. However, there have been some amusing attempts to do so, including Gamow’s famous “Mr. Tompkins” series:

  5. Patrick Says:

    one more to end the night, while looking for the definition of current, i saw a thing called “dark current” is this another form of and electrical current, or something else? if you could, explain in lamest terms, or as simple as possible, please?

    • CyberChalky Says:

      Dark Current can mean many things.
      The simplest (and most boring) one is a current which flows through devices like solar cells when they are completely isolated from any possible light.

      More fun ones talk about the movement of dark energy around the universe – this is also called the “dark current” – and it is very strange. This is *not part of the VCE*. In fact, it is not part of undergraduate (bachelor) university physics…

  6. Patrick Says:

    i was reading the link to the new world of Mr.Tompkins, and a remark came up in the chapter entitled ” quantum snooker” the student sat there and contemplated, if in theory according to the subject of quantum physics, that he might be able to turn his male teacher into a FEMALE teacher. By the way, this is hilarious, and one of the reasons to LOVE physics. Also explosions. 🙂

    But isn’t quantum physics, just teaching quantum mechanics? and isn’t quantum mechanics, just teaching student’s why electrons move in waves inside the atom?

    • CyberChalky Says:

      There’s not much “just” about Quantum Physics – the dual nature of electrons (as first initiated by De Broglie) isn’t really QP, just one of th necessary precursor ideas. The next idea is the uncertainty principle (as stated by Heisenberg) which says that it is impossible to simultaneous physically localise a sub nuclear particle and determine its energy status (i.e. speed). From that point, it actually starts getting weird, for example string theory – the concept that everything has no physical reality and is merely harmonics on vibrating twisted planes of space, or the concept of tunnelling that says that objects can occupy the same physical space concurrnently…

  7. […] of these is explained in some detail in another post I have on this blog – which I expect that all of you will read. (CLICK IT NOW – you […]

  8. […] On Electricity: Fields, Resistance, Static charges […]

  9. […] have two posts about this topic already (1, 2), and I suggest that you read them – and all the comments and links! – after you finish […]

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