Charging by Induction - Questions 3 Help

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Charging by induction is a charging process in which a charged object is brought near to a neutral object. The neutral object becomes polarized. The neutral object is then touched by a ground, This touching to the ground forces a movement of electrons between the neutral object and the ground. When the ground is pulled away, the neutral object is permanently charged.

There is just one question here but the order in which the answer options is presented will be scrambled. Here's the question:

A neutral metal sphere ischarged by induction. Step 1: A negatively-charged balloon is brought near the neutral metal sphere. Step 2: While the balloon is held near, the sphere is touched. The sphere becomes charged.

How did the sphere become charged? And what charge did it acquire?

Step 1: Polarization Step
Protons move from the sphere to the balloon.
Protons move from the balloon to the sphere.
Electrons move from the sphere to the balloon.
Electrons move from the balloon to the sphere.
Protons in the sphere move towards the balloon.
Protons in the sphere move away from the balloon.
Electrons in the sphere move towards the balloon.
Electrons in the sphere move away from the balloon.

Step 2: Charging Step
Protons move from the finger to the sphere. The sphere becomes positive.
Protons move from the sphere to the finger. The sphere becomes negative.
Electrons move from the sphere to the finger. The sphere becomes positive.
Electrons move from the finger to the sphere. The sphere becomes negative.
Protons move from the balloon to the sphere. The sphere becomes positive.
Protons move from the sphere to the balloon. The sphere becomes negative.
Electrons move from the sphere to the balloon. The sphere becomes positive.
Electrons move from the balloon to the sphere. The sphere becomes negative.

This charging by induction process occurs in two steps. The first step is referred to as the polarization step. When the positively-charged balloon is held near the neutral sphere, there is a separation of the positive charges from the negative charges within the sphere. The second step is the charging step. When the sphere 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 balloon forces (or induces) a movement of electrons within the metal sphere. Knowing which way electrons move within the sphere 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 balloon are oppositely-charged, the electrons in the sphere will be attracted to the balloon and move towards it.

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 charged object near a neutral object.

Why don't electrons move from the balloon onto the metal sphere? Electrons are free to move within conducting materials. The sphere is made of metal - a conductor. This allows the electrons to move from one location on the sphere to another location. But air separates the sphere from the balloon. Air is not a good conductor; it prevents the electrons from moving through it. The result is that the electrons do not move between the sphere and the balloon.

The polarization step polarizes the charges in the sphere. The mass migration of electrons from the right side of the sphere to the left side of the sphere leaves the left side with an overall negative charge and the right side with an overall positive charge. Yet because there has been no movement of electrons onto or off of the sphere, it is still neutral. The charging doesn't occur until the next step.

Charging Step
When the sphere is touched by the finger, there is a movement of electrons from the finger to the sphere. This electron movement is induced by the presence of the positively-charged balloon. Electrons, being negatively-charged, are attracted to positively-charged objects like the balloon. So the action of touching the finger to the sphere results in the movement of electrons from the finger into the sphere. This is a charging step. Because electrons enter the sphere, the sphere 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.

Try these links to The Physics Classroom Tutorial for more help with understanding charging by induction:

Charging by Induction (Tutorial page)

Charging a Single Sphere by Induction using a Negatively-Charged Object (animation)