The neutral metal sphere is polarized when a charged rod is brought near it. The sphere is then grounded, meaning that a conducting wire is run from the sphere to the ground. Since the earth is large and most ground is a good conductor, it can supply or accept excess charge easily. In this case, electrons are attracted to the sphere through a wire called the ground wire, because it supplies a conducting path to the ground. The ground connection is broken before the charged rod is removed, leaving the sphere with an excess charge opposite to that of the rod.
Again, an opposite charge is achieved when charging by induction and the charged rod loses none of its excess charge. Figure 4. Charging by induction, using a ground connection.
Neutral objects can be attracted to any charged object. The pieces of straw attracted to polished amber are neutral, for example. If you run a plastic comb through your hair, the charged comb can pick up neutral pieces of paper. Figure 5 shows how the polarization of atoms and molecules in neutral objects results in their attraction to a charged object. Figure 5. Both positive and negative objects attract a neutral object by polarizing its molecules.
There is a slight shift in the distribution of the electrons orbiting the molecule, with unlike charges being brought nearer and like charges moved away. Since the electrostatic force decreases with distance, there is a net attraction.
When a charged rod is brought near a neutral substance, an insulator in this case, the distribution of charge in atoms and molecules is shifted slightly. Opposite charge is attracted nearer the external charged rod, while like charge is repelled. Since the electrostatic force decreases with distance, the repulsion of like charges is weaker than the attraction of unlike charges, and so there is a net attraction. Thus a positively charged glass rod attracts neutral pieces of paper, as will a negatively charged rubber rod.
Some molecules, like water, are polar molecules. Polar molecules have a natural or inherent separation of charge, although they are neutral overall. Polar molecules are particularly affected by other charged objects and show greater polarization effects than molecules with naturally uniform charge distributions. Water molecules are polarized, giving them slightly positive and slightly negative sides. As the water flows downward, due to the force of gravity, the charged conductor exerts a net attraction to the opposite charges in the stream of water, pulling it closer.
Make sparks fly with John Travoltage. Bring his hand close to the door knob and get rid of the excess charge. Skip to main content. Electric Charge and Electric Field. Search for:. Conductors and Insulators Learning Objectives By the end of this section, you will be able to: Define conductor and insulator, explain the difference, and give examples of each. Describe three methods for charging an object. Explain what happens to an electric force as you move farther from the source.
Define polarization. Check Your Understanding Can you explain the attraction of water to the charged rod in Figure 6? Figure 6. Click to run the simulation. Conceptual Questions An eccentric inventor attempts to levitate by first placing a large negative charge on himself and then putting a large positive charge on the ceiling of his workshop.
Instead, while attempting to place a large negative charge on himself, his clothes fly off. If you have charged an electroscope by contact with a positively charged object, describe how you could use it to determine the charge of other objects. Specifically, what would the leaves of the electroscope do if other charged objects were brought near its knob? When a glass rod is rubbed with silk, it becomes positive and the silk becomes negative—yet both attract dust.
Does the dust have a third type of charge that is attracted to both positive and negative? Why does a car always attract dust right after it is polished? Note that car wax and car tires are insulators.
Describe how a positively charged object can be used to give another object a negative charge. What is the name of this process? What is grounding? What effect does it have on a charged conductor? The effects of excess charge on the body are often demonstrated using a Van de Graaff generator. When a student places their hand upon the static ball, excess charge from the ball is shared with the human body.
Being a conductor, the excess charge could flow to the human body and spread throughout the surface of the body, even onto strands of hair. As the individual strands of hair become charged, they begin to repel each other. Looking to distance themselves from their like-charged neighbors, the strands of hair begin to rise upward and outward - a truly hair-raising experience.
Many are familiar with the impact that humidity can have upon static charge buildups. You have likely noticed that bad hair days, doorknob shocks and static clothing are most common during winter months. Winter months tend to be the driest months of the year with humidity levels in the air dropping to lower values. Water has a tendency to gradually remove excess charge from objects. When the humidity is high, a person acquiring an excess charge will tend to lose that charge to water molecules in the surrounding air.
On the other hand, dry air conditions are more conducive to the buildup of static charge and more frequent electric shocks. Since humidity levels tend to vary from day to day and season to season, it is expected that electrical effects and even the success of electrostatic demonstrations can vary from day to day. Predicting the direction that electrons would move within a conducting material is a simple application of the two fundamental rules of charge interaction.
Opposites attract and likes repel. Suppose that some method is used to impart a negative charge to an object at a given location. At the location where the charge is imparted, there is an excess of electrons. That is, the multitude of atoms in that region possess more electrons than protons.
Of course, there are a number of electrons that could be thought of as being quite contented since there is an accompanying positively charged proton to satisfy their attraction for an opposite.
However, the so-called excess electrons have a repulsive response to each other and would prefer more space. Electrons, like human beings, wish to manipulate their surroundings in an effort to reduce repulsive effects. Since these excess electrons are present in a conductor, there is little hindrance to their ability to migrate to other parts of the object. And that is exactly what they do.
In an effort to reduce the overall repulsive effects within the object, there is a mass migration of excess electrons throughout the entire surface of the object. Excess electrons migrate to distance themselves from their repulsive neighbors.
In this sense, it is said that excess negative charge distributes itself throughout the surface of the conductor. But what happens if the conductor acquires an excess of positive charge? What if electrons are removed from a conductor at a given location, giving the object an overall positive charge?
If protons cannot move, then how can the excess of positive charge distribute itself across the surface of the material? While the answers to these questions are not as obvious, it still involves a rather simple explanation that once again relies on the two fundamental rules of charge interaction.
Suppose that a conducting metal sphere is charged on its left side and imparted an excess of positive charge. Of course, this requires that electrons be removed from the object at the location of charging. A multitude of atoms in the region where the charging occurs have lost one or more electrons and have an excess of protons. The imbalance of charge within these atoms creates effects that can be thought of as disturbing the balance of charge within the entire object. The presence of these excess protons in a given location draws electrons from other atoms.
Electrons in other parts of the object can be thought of as being quite contented with the balance of charge that they are experiencing. Yet there will always be some electrons that will feel the attraction for the excess protons some distance away.
In human terms, we might say these electrons are drawn by curiosity or by the belief that the grass is greener on the other side of the fence. In the language of electrostatics, we simply assert that opposites attract - the excess protons and both the neighboring and distant electrons attract each other.
The protons cannot do anything about this attraction since they are bound within the nucleus of their own atoms. Yet, electrons are loosely bound within atoms; and being present in a conductor, they are free to move. These electrons make the move for the excess protons, leaving their own atoms with their own excess of positive charge.
This electron migration happens across the entire surface of the object, until the overall sum of repulsive effects between electrons across the whole surface of the object are minimized. Use your understanding of charge to answer the following questions.
When finished, click the button to view the answers. One of these isolated charged spheres is copper and the other is rubber. The diagram below depicts the distribution of excess negative charge over the surface of two spheres. Label which is which and support your answer with an explanation.
See Answer Answer: A is rubber and B is copper. Sphere A shown a non-uniform distribution of excess charge; so sphere A must be made of an insulating material such as rubber.
Sphere B shows a uniform distribution of excess charge; one would reason that it is made of a conductor such as copper. Which of the following materials are likely to exhibit more conductive properties than insulating properties?
Aluminum and silver are metals, making them good conductors. The human body is a fairly good conductor.
When wet, its an even better conductor. A and B are characteristic of positive and negative objects. As for C, both insulators and conductors can be charged.
As for D, this has nothing to do with the conductive properties of materials. As for E, neutrons are located in the nucleus and are "out of the way" of mobile electrons.
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