Electric Charge and Electric Field
3 Conductors and Insulators
Learning Objectives
- Define conductors and insulators, explain the difference between them, and give examples of each.
- Describe the three common methods of charging an object.
- Explain how the electric force changes with distance from a charged object.
- Define polarization and describe how it occurs.
Figure 3.1 shows a familiar example of conductors and insulators working together in an everyday electrical device.
Conductors and Insulators
Some materials allow electric charge to move through them easily, while others strongly resist the movement of charge. This difference plays a central role in every electrical device, from household wiring to medical equipment.
Materials that allow charge to move freely are called conductors. In metals, some electrons are only loosely bound to their atoms and can move throughout the material. These free electrons respond readily to electric forces, allowing charge to flow. Although these moving electrons frequently collide with atoms in the material and lose some energy, they are still able to travel through the conductor. In special materials known as superconductors, electric charge can move with essentially no energy loss.
Not all conductors rely on electrons. In liquids such as salt water, charge is carried by ions—atoms or molecules that have gained or lost electrons and therefore carry a net electric charge. This is why salty water conducts electricity much better than pure water. The movement of ions is also essential in the human body, where electrical signals in nerves and muscles depend on the flow of sodium, potassium, calcium, and chloride ions.
Materials that do not allow charge to move easily are called insulators. In these materials, electrons remain tightly bound to their atoms and cannot move freely. As a result, electric charge stays localized rather than flowing through the material. Common insulators include glass, rubber, plastics, ceramics, and dry wood. Pure water is also an insulator, although even small amounts of dissolved salts greatly increase its conductivity.
Figure 3.2 shows how an electroscope can be charged by contact with a charged object.
Charging by Contact
One of the simplest ways to charge an object is by contact, also called charging by conduction. In this process, a charged object physically touches another object, allowing electrons to move between them until the charges redistribute.
Figure 3.2 shows an electroscope being charged by contact with a positively charged glass rod. Because the glass rod is an insulator, it must actually touch the electroscope for charge to be transferred. The positive rod attracts electrons from the conducting electroscope. As electrons leave the electroscope and move onto the rod, the electroscope is left with an overall positive charge.
The two thin metal leaves now carry charges of the same sign. Since like charges repel, the leaves push away from one another and separate. This separation provides a simple visual indication that the electroscope has become charged. A negatively charged object could similarly transfer electrons to the electroscope, leaving it with a net negative charge.
Charging by Induction
Unlike charging by contact, charging by induction does not require the charged object to touch the object being charged. Instead, a nearby charged object causes charges within a conductor to rearrange, and this rearrangement can produce a permanent net charge under the proper conditions.
Figure 3.3 illustrates one method of charging by induction. Two neutral metal spheres are initially touching one another. When a positively charged rod is brought near one sphere, electrons move toward the rod, leaving the other sphere with an excess of positive charge. If the spheres are separated before the rod is removed, each sphere retains a net charge even though neither sphere was ever touched by the charged rod.
Another common method of charging by induction uses a grounded connection, as shown in Figure 3.4. A charged rod is brought near a neutral conductor, causing charges inside the conductor to separate. While the rod remains nearby, the conductor is connected to the Earth through a conducting wire. Because Earth is an enormous conductor, it can easily supply or absorb electrons. After the ground connection is removed and then the charged rod is taken away, the conductor is left with a net charge opposite in sign to that of the inducing rod.
Notice that in both induction methods, the charged rod never loses its own excess charge. It simply causes charges already present in another object to redistribute. Figure 3.4 summarizes the induction process when grounding is used.
Polarization
A charged object can also attract a neutral object without transferring any charge at all. This occurs through polarization, the slight separation of positive and negative charges within an otherwise neutral object.
When a charged object is brought near a neutral material, the electrons inside atoms or molecules shift slightly. In an insulator, the electrons cannot move freely from atom to atom, but they can move a tiny distance within each atom or molecule. This produces a small separation of charge, with opposite charges slightly closer to the external charged object and like charges slightly farther away.
Because the electric force becomes weaker with increasing distance, the attractive force on the nearby opposite charges is slightly stronger than the repulsive force on the more distant like charges. The result is a net attractive force. This explains why a charged comb can pick up small pieces of paper or why rubbed amber attracts bits of straw. Figure 3.5 illustrates how polarization produces this net attraction.
Some molecules, such as water, are already polar molecules. Their positive and negative charges are naturally separated even when no external electric field is present. These molecules are especially easy to orient in an electric field and therefore exhibit stronger polarization effects than nonpolar molecules.
Check Your Understanding
A thin stream of water bends toward a charged rod, as shown in Figure 3.6. If the water is electrically neutral overall, why is it attracted to the rod?
Figure 3.6 shows a charged rod attracting a thin stream of water through polarization.
Figure 3.6: A charged rod attracts a thin stream of water because the polar water molecules rotate slightly, producing a stronger attraction than repulsion.
Answer
Water molecules are polar molecules, meaning they have a slight separation of positive and negative charge even though each molecule is electrically neutral overall. When a charged rod is brought nearby, the water molecules rotate so that the side with opposite charge faces the rod.
Because the opposite charges are slightly closer to the rod than the like charges, the attractive force is stronger than the repulsive force. The result is a net force pulling the stream of water toward the charged rod, causing it to bend.
Interactive Exploration: John Travoltage
Have you ever experienced a small shock after walking across a carpet and touching a metal doorknob? This common phenomenon is caused by the buildup and sudden discharge of static electric charge. In this simulation, you'll investigate how electric charges are transferred by friction and how excess charge is released when a conductive path becomes available.
Experiment by rubbing John's foot on the carpet to build up electric charge. Then move his hand toward the metal doorknob and observe what happens when the accumulated charge suddenly flows to the conductor. As you explore, think about how charge is transferred, why the spark occurs, and what conditions are required for an electrical discharge.
Accessibility note: If you are unable to use the interactive simulation, read the guided exploration questions first and compare your predictions with the discussion that follows. Your instructor may also provide screenshots or a demonstration of the simulation.
Guided Exploration
As you interact with the simulation, try to answer the following questions:
- Rub John's foot on the carpet several times. What happens to the number of excess electrons on his body?
- Move John's hand toward the metal doorknob. At what point does a spark occur?
- Why does the spark happen suddenly instead of continuously as the hand approaches the doorknob?
- What happens to the excess charge on John's body after the spark?
- Repeat the experiment by accumulating different amounts of charge before touching the doorknob. How does the amount of accumulated charge affect the discharge?
- Based on your observations, explain why you are more likely to experience a static shock after walking across a carpet on a dry winter day than on a humid summer day.
After completing the exploration, compare your observations with the concepts presented in this chapter. Notice that friction transfers electrons between different materials, creating an imbalance of charge. When a charged object comes close to a conductor, the electric field can become strong enough to ionize the surrounding air, allowing the excess charge to flow suddenly in the form of a visible spark.
Section Summary
- A conductor allows electric charge to move freely, while an insulator strongly resists the movement of charge.
- In metals, electric current is carried by free electrons. In conducting liquids, such as salt water, charge is carried by ions.
- Objects can be charged in three common ways:
- by friction, through the transfer of electrons between materials,
- by contact (conduction), by touching a charged object, and
- by induction, without direct contact by using charge separation.
- Charging by contact leaves an object with the same sign of charge as the object that touched it.
- Charging by induction, together with grounding, leaves an object with the opposite sign of charge from the inducing object.
- Grounding connects an object to Earth, allowing excess charge to flow to or from the Earth's large reservoir of charge.
- Polarization is the separation of positive and negative charge within a neutral object. Polarization explains why charged objects can attract neutral objects.
- Polar molecules, such as water, have a permanent separation of charge and therefore respond strongly to electric fields.
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. Explain.
- 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? Explain.
- 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? On a charged insulator?
Problems & Exercises
- Suppose a speck of dust in an electrostatic precipitator has [latex]1.0000\times10^{12}[/latex] protons in it and has a net charge of [latex]-5.00\,\text{nC}[/latex], a very large charge for a small speck. How many electrons does it have?
- An amoeba has [latex]1.00\times10^{16}[/latex] protons and a net charge of [latex]0.300\,\text{pC}[/latex].
- How many fewer electrons are there than protons?
- If you paired them up, what fraction of the protons would have no electrons?
- A [latex]50.0\,\text{g}[/latex] ball of copper has a net charge of [latex]2.00\,\mu\text{C}[/latex]. What fraction of the copper's electrons has been removed? Each copper atom has 29 protons, and copper has an atomic mass of 63.5.
- What net charge would you place on a [latex]100\,\text{g}[/latex] piece of sulfur if you put an extra electron on 1 in [latex]10^{12}[/latex] of its atoms? Sulfur has an atomic mass of 32.1.
- How many coulombs of positive charge are there in [latex]4.00\,\text{kg}[/latex] of plutonium, given its atomic mass is 244 and that each plutonium atom has 94 protons?
Glossary
- free electron
- an electron that is not tightly bound to an individual atom and can move freely through a conductor
- a material that allows electric charge to move easily through it
- insulator
- a material that strongly resists the movement of electric charge
- grounded
- connected to the Earth through a conductor, allowing excess charge to flow to or from the Earth
- charging by induction
- the process of charging an object without direct contact by using charge separation and, when necessary, grounding
- polarization
- the separation or slight redistribution of positive and negative charge within an otherwise neutral object
- electrostatic repulsion
- the force that causes objects with like electric charges to push away from one another
- polar molecule
- a molecule with a permanent separation of positive and negative charge
a material that allows electric charge to move easily through it
an electron that is not tightly bound to an individual atom and can move freely through a conductor
a material that strongly resists the movement of electric charge
the process of charging an object without direct contact by using charge separation and, when necessary, grounding
connected to the Earth through a conductor, allowing excess charge to flow to or from the Earth
the separation or slight redistribution of positive and negative charge within an otherwise neutral object
a molecule with a permanent separation of positive and negative charge
the force that causes objects with like electric charges to push away from one another
