This charging by induction process occurs in two steps. The first step is referred to as the polarization step. When the pie tin is held near the positively-charged board, there is a separation of the positive charges from the negative charges within the pie tin. The second step is the charging step. When the pie tin is touched by the finger, it becomes charged. You must be able to explain each step in terms of the movement of particles within the objects. Details about each step are described below.
Polarization Step
During the polarization step, the presence of the positively-charged acrylic board forces (or induces) a movement of electrons within the pie tin. Knowing which way electrons move within the pie tin is based on the two fundamental rules of charge interactions:
- Oppositely-charged objects attract.
- Like-charged objects repel.
Since the electrons and the positively-charged acrylic plate are oppositely-charged, the electrons in the pie tin will move towards the acrylic plate.
What about the protons? Don't they move? No! Protons are located in the nucleus of atoms. They are tightly bound in the nucleus. The nucleus would have to be split open in order for the protons to escape the nucleus. Atom-splitting just doesn't happen when you bring a neutral object near a charged object.
Why don't electrons move from the pie tin onto the acrylic board? Electrons are free to move within conducting materials. The pie tin is made of aluminum - a conductor. This allows the electrons to move from one location on the tin to another location. But air separates the pie tin from the acrylic board. Air is not a good conductor; it prevents the electrons from moving through it. The result is that the electron movement ceases when the electrons reach the edge of the pie tin.
The polarization step polarizes the charges in the pie tin. The mass migration of electrons from the top of the pie tin to the bottom of the pie tin leaves the top part with an overall positive charge and the bottom part with an overall negative charge. Yet because there has been no movement of electrons onto or off of the pie tin, it is still neutral. The charging doesn't occur until the next step.
Charging Step
When the pie tin is touched by the finger, there is a movement of electrons from the finger onto the pie tin. This electron movement is induced by the presence of the positively-charged acrylic board. Electrons, being negatively-charged, are attracted to positively-charged objects like the acrylic board. So the action of touching the finger - a large source of electrons - to the pie tin results in the movement of electrons into the pie tin. This is a charging step. The pie tin becomes negatively charged.
But what about the protons? Don't protons move during this step? No! As discussed above, protons are tightly bound in the nucleus of atoms. They are unable to move during electrostatic experiments. Charging processes are always explained in terms of electron movement.