Which Universal Force Can Both Repel and Attract

Learning Objectives

By the terminate of this section, yous will be able to:

• Define conductor and insulator, explicate the difference, and requite examples of each.
• Describe 3 methods for charging an object.
• Explain what happens to an electric force as yous move further from the source.
• Define polarization.

Figure 1. This power adapter uses metal wires and connectors to comport electricity from the wall socket to a laptop estimator. The conducting wires permit electrons to move freely through the cables, which are shielded by prophylactic and plastic. These materials act equally insulators that don't allow electric charge to escape outward. (credit: Evan-Amos, Wikimedia Commons)

Some substances, such as metals and salty water, let charges to motion through them with relative ease. Some of the electrons in metals and similar conductors are not leap to individual atoms or sites in the cloth. These complimentary electrons can movement through the fabric much as air moves through loose sand. Whatsoever substance that has costless electrons and allows charge to motility relatively freely through information technology is called a conductor. The moving electrons may collide with fixed atoms and molecules, losing some energy, simply they can move in a conductor. Superconductors allow the movement of charge without any loss of energy. Salty water and other similar conducting materials contain costless ions that can move through them. An ion is an atom or molecule having a positive or negative (nonzero) total charge. In other words, the total number of electrons is not equal to the full number of protons.

Other substances, such equally glass, do non let charges to move through them. These are called insulators. Electrons and ions in insulators are bound in the structure and cannot move hands—every bit much as 10²³ times more slowly than in conductors. Pure water and dry out tabular array table salt are insulators, for example, whereas molten salt and salty water are conductors.

Charging past Contact

Effigy 2 shows an electroscope existence charged by touching it with a positively charged glass rod. Because the drinking glass rod is an insulator, information technology must actually touch the electroscope to transfer accuse to or from it. (Note that the extra positive charges reside on the surface of the glass rod equally a result of rubbing it with silk before starting the experiment.) Since only electrons move in metals, nosotros meet that they are attracted to the elevation of the electroscope. There, some are transferred to the positive rod by bear on, leaving the electroscope with a cyberspace positive charge.

Figure 2. An electroscope is a favorite musical instrument in physics demonstrations and educatee laboratories. It is typically fabricated with gold foil leaves hung from a (conducting) metallic stalk and is insulated from the room air in a glass-walled container. (a) A positively charged glass rod is brought about the tip of the electroscope, attracting electrons to the summit and leaving a net positive charge on the leaves. Like charges in the light flexible golden leaves repel, separating them. (b) When the rod is touched against the ball, electrons are attracted and transferred, reducing the net charge on the glass rod only leaving the electroscope positively charged. (c) The excess charges are evenly distributed in the stem and leaves of the electroscope once the drinking glass rod is removed.

Electrostatic repulsion in the leaves of the charged electroscope separates them. The electrostatic force has a horizontal component that results in the leaves moving apart besides as a vertical component that is balanced by the gravitational strength. Similarly, the electroscope can be negatively charged by contact with a negatively charged object.

Charging past Induction

It is not necessary to transfer excess charge directly to an object in order to charge it. Figure 3 shows a method of induction wherein a charge is created in a nearby object, without straight contact. Hither we see two neutral metal spheres in contact with one another just insulated from the residue of the world. A positively charged rod is brought virtually one of them, attracting negative accuse to that side, leaving the other sphere positively charged.

This is an example of induced polarization of neutral objects. Polarization is the separation of charges in an object that remains neutral. If the spheres are now separated (earlier the rod is pulled away), each sphere will have a internet charge. Annotation that the object closest to the charged rod receives an contrary accuse when charged past consecration. Annotation likewise that no charge is removed from the charged rod, so that this process can exist repeated without depleting the supply of excess charge.

Figure 3. Charging by consecration. (a) Two uncharged or neutral metal spheres are in contact with each other just insulated from the rest of the world. (b) A positively charged drinking glass rod is brought near the sphere on the left, attracting negative charge and leaving the other sphere positively charged. (c) The spheres are separated before the rod is removed, thus separating negative and positive accuse. (d) The spheres retain cyberspace charges after the inducing rod is removed—without ever having been touched by a charged object.

Another method of charging by induction is shown in Figure 4. The neutral metallic sphere is polarized when a charged rod is brought near it. The sphere is and then grounded, meaning that a conducting wire is run from the sphere to the ground. Since the globe is large and most basis is a expert 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 footing. The ground connection is broken before the charged rod is removed, leaving the sphere with an excess charge opposite to that of the rod. Once more, an reverse charge is achieved when charging by consecration and the charged rod loses none of its excess charge.

Effigy 4. Charging by induction, using a basis connectedness. (a) A positively charged rod is brought near a neutral metal sphere, polarizing it. (b) The sphere is grounded, assuasive electrons to be attracted from the globe'southward ample supply. (c) The ground connexion is broken. (d) The positive rod is removed, leaving the sphere with an induced negative charge.

Neutral objects can exist attracted to any charged object. The pieces of harbinger attracted to polished amber are neutral, for example. If you run a plastic comb through your pilus, the charged rummage can option 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.

Effigy v. Both positive and negative objects attract a neutral object past polarizing its molecules. (a) A positive object brought near a neutral insulator polarizes its molecules. In that location is a slight shift in the distribution of the electrons orbiting the molecule, with dissimilar charges being brought nearer and like charges moved away. Since the electrostatic force decreases with altitude, at that place is a net attraction. (b) A negative object produces the opposite polarization, simply again attracts the neutral object. (c) The same consequence occurs for a conductor; since the unlike charges are closer, there is a net attraction.

When a charged rod is brought nearly a neutral substance, an insulator in this case, the distribution of charge in atoms and molecules is shifted slightly. Contrary accuse 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 allure of dissimilar charges, and so there is a net attraction. Thus a positively charged drinking 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 accuse, although they are neutral overall. Polar molecules are particularly affected by other charged objects and evidence greater polarization effects than molecules with naturally uniform accuse distributions.

Bank check Your Understanding

Tin can y'all explain the attraction of water to the charged rod in Figure six?

Figure half-dozen.

Solution

Water molecules are polarized, giving them slightly positive and slightly negative sides. This makes water even more susceptible to a charged rod's attraction. As the water flows downward, due to the strength of gravity, the charged usher exerts a net attraction to the opposite charges in the stream of water, pulling it closer.

PhET Explorations: John Travoltage

Brand sparks fly with John Travoltage. Jerk Johnnie'south foot and he picks up charges from the carpet. Bring his hand close to the door knob and get rid of the excess accuse.

Click to run the simulation.

Section Summary

• Polarization is the separation of positive and negative charges in a neutral object.
• A conductor is a substance that allows charge to flow freely through its atomic structure.
• An insulator holds accuse within its atomic structure.
• Objects with like charges repel each other, while those with unlike charges attract each other.
• A conducting object is said to be grounded if information technology is continued to the Earth through a conductor. Grounding allows transfer of accuse to and from the earth'due south large reservoir.
• Objects can be charged by contact with another charged object and obtain the same sign charge.
• If an object is temporarily grounded, it tin can be charged by induction, and obtains the opposite sign charge.
• Polarized objects take their positive and negative charges concentrated in different areas, giving them a non-symmetrical charge.
• Polar molecules have an inherent separation of charge.

Conceptual Questions

1. An eccentric inventor attempts to levitate by get-go placing a big negative charge on himself and and then putting a large positive charge on the ceiling of his workshop. Instead, while attempting to place a large negative accuse on himself, his dress fly off. Explain.
2. If you have charged an electroscope past contact with a positively charged object, describe how yous could use information technology 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?
3. When a drinking glass rod is rubbed with silk, information technology 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? Explicate.
4. Why does a car ever attract dust right after it is polished? (Note that car wax and machine tires are insulators.)
5. Depict how a positively charged object can exist used to give another object a negative accuse. What is the name of this process?
6. What is grounding? What effect does it have on a charged conductor? On a charged insulator?

Problems & Exercises

1. Suppose a speck of dust in an electrostatic precipitator has 1.0000 × ten¹² protons in it and has a net charge of –5.00 nC (a very big charge for a small speck). How many electrons does it have?
2. An amoeba has 1.00 × ten^xvi protons and a internet charge of 0.300 pC. (a) How many fewer electrons are at that place than protons? (b) If y'all paired them upwards, what fraction of the protons would take no electrons?
3. A 50.0 one thousand brawl of copper has a net charge of two.00 μC. What fraction of the copper'due south electrons has been removed? (Each copper atom has 29 protons, and copper has an atomic mass of 63.five.)
4. What cyberspace charge would you identify on a 100 m slice of sulfur if you put an actress electron on 1 in 10¹² of its atoms? (Sulfur has an atomic mass of 32.one.)
5. How many coulombs of positive accuse are at that place in 4.00 kg of plutonium, given its atomic mass is 244 and that each plutonium atom has 94 protons?

Glossary

free electron: an electron that is gratuitous to move away from its atomic orbit

conductor: a material that allows electrons to move separately from their atomic orbits

insulator: a material that holds electrons securely inside their atomic orbits

grounded: when a usher is continued to the Earth, allowing accuse to freely flow to and from World'southward unlimited reservoir

induction: the process by which an electrically charged object brought almost a neutral object creates a charge in that object

polarization: slight shifting of positive and negative charges to opposite sides of an cantlet or molecule

electrostatic repulsion: the miracle of 2 objects with like charges repelling each other

Selected Solutions to Problems & Exercises

1. 1.03 × 10¹²

3. 9.09 × ten⁻¹³

5. 1.48 × x⁸ C

gentrymusection.blogspot.com

Source: https://courses.lumenlearning.com/physics/chapter/18-2-conductors-and-insulators/

Share this post