Sunday, 2 June 2013

Van Der Graaf Demonstrations - Induced Charge

It's demonstrations like this that really show the limits of Inductive-only learning in Science. Don't get me wrong, inductive teaching is a powerful tool under certain circumstances, but how the hell do you design an activity to inductively derive all the complexities of induced charge? Even if you can come up with one, it won't have as much impact or be as quick to do as the mixed-method demonstration discussed below.

You should have already done the Hair Club demonstration mentioned here, and hopefully the Einstein Hair demonstration as well. You need to have really emphasized the roll that like-charge repulsion has around the Van Der Graaf machine.

  1. Write the two rules of charge interaction on the board. Like charges repel, unlike charges attract.
  2. Place the discharge sphere in it's holder at the back of the Van Der Graaf machine and charge up the main sphere.
  3. You need a ping-pong ball or Styrofoam ball (1-2cm diameter) attached to fishing line (or any fine thread) and the other end of the line tied to a stick.
  4. Bring the Styrofoam ball near the Van Der Graaf machine without touching. Ask the students to explain what is happening. If you've got a class that is good at participating, they will state that the Styrofoam is attracted to the sphere due to unlike charges attracting. This is where the hard teaching part starts. 
  5. Bring the Styrofoam ball close enough to the Van Der Graaf machine so that it touches. When it jumps away, ask the students to explain why. (The ball is now the same charge as the main sphere, and like charges repel.)
  6. Remind the students that the Styrofoam ball doesn't have a charge. Bring it away from the machine, hold it in your hand, describe how it's now discharged and neutral. Bring it back to the machine and demonstrate that the ball swings towards the main sphere. 
  7. At this point, most students will adjust the rules to say "Unlike charges attract, as do unlike and neutral." You need to go to theory now to prevent this alternative conception from sticking.


Reinforce that individual neutral particles don't interact with charged objects - cite the fact that the neutron in the center of an atom doesn't have any effect on the outer electrons.

Induced charge is the hardest concept in the series of Van Der Graaf demonstrations, but if you want to go on to explain how lightning forms, the majority of your students will need to understand it. And if the students have been watching you throw miniature lightning bolts around, they will want to understand lightning. Don't waste a teaching moment and put the following questions on the board.
  • The Van Der Graaf's sphere is negatively charged (change everything around if you've got the other type of Van Der Graaf machine). What will happen to any electron (including those in the Styrofoam ball) as it is brought close to the main sphere?
  • Which side of the sphere will the electrons end up on?
  • What happens to the Van Der Graaf's electrostatic field as you move away from it?
  • What type of charge is left on the closer side of the Styrofoam ball?
You now need to draw the following diagram:
And explain that when the Styrofoam ball is brought near the Van Der Graaf sphere, the electrons are pushed to the other side of the Styrofoam sphere. Once this induced charge is established, the positive charges left behind are closer to the Van Der Graaf, and thus more strongly attracted to the sphere, than the electrons are pushed away. The end result is that the neutral object is attracted to the sphere, but only because it consists of both positive and negative charges.

If your splitting the demonstrations across two lessons, you can send the students onto the following web page to satisfy their curiosity. Or you can continue with the demonstrations and come back to lightning the next lesson (or maybe an assignment).

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