electric charge

polarization

Polarization is the movement of electrons within a neutral object due to the electric field of a nearby charged object. It occurs without direct contact between the two objects. You can see how it happens in Figure 23.8. When the negatively charged plastic rod in the figure is placed close to the neutral metal plate, electrons in the plate are repelled by the positive charges in the rod. The electrons move away from the rod, causing one side of the plate to become positively charged and the other side to become negatively charged. Polarization may also occur after you walk across a wool carpet in rubber-soled shoes and become negatively charged. If you reach out to touch a metal doorknob, electrons in the neutral metal will be repelled and move away from your hand before you even touch the knob. In this way, one end of the doorknob becomes positively charged and the other end becomes negatively charged.

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conduction

Another way electrons may be transferred is through conduction. This occurs when there is direct contact between materials that differ in their ability to give up or accept electrons. For example, wool tends to give up electrons and rubber tends to accept them. Therefore, when you walk across a wool carpet in rubber-soled shoes, electrons transfer from the carpet to your shoes. You become negatively charged, while the carpet becomes positively charged. Another example of conduction is pictured in Figure 23.7. The device this girl is touching is called a van de Graaff generator. The dome on top is negatively charged. When the girl places her hand on the dome, electrons are transferred to her, so she becomes negatively charged as well. Even the hairs on her head become negatively charged. As a result, individual hairs repel each other, causing them to stand on end. You can see a video demonstration of a van de Graff generator at this URL: .

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friction

Did you ever rub an inflated balloon against your hair? You can see what happens in Figure 23.6. Friction between the rubber of the balloon and the babys hair results in electrons from the hair "rubbing off" onto the balloon. Thats because rubber attracts electrons more strongly than hair does. After the transfer of electrons, the balloon becomes negatively charged and the hair becomes positively charged. As a result, the individual hairs repel each other and the balloon and the hair attract each other. Electrons are transferred in this way whenever there is friction between materials that differ in their ability to give up or accept electrons.

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transfer of electric charges

Atoms are neutral in electric charge because they have the same number of electrons as protons. However, atoms may transfer electrons and become charged ions, as illustrated in Figure 23.5. Positively charged ions, or cations, form when atoms give up electrons. Negatively charged ions, or anions, form when atoms gain electrons. Like the formation of ions, the formation of charged matter in general depends on the transfer of electrons either between two materials or within a material. Three ways this can occur are friction, conduction, and polarization. In all cases, the total charge remains the same. Electrons move, but they arent destroyed. This is the law of conservation of charge.

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static electricity and static discharge

Polarization leads to the buildup of electric charges on objects. This buildup of charges is known as static electricity. Once an object becomes charged, it is likely to remain charged until another object touches it or at least comes very close to it. Thats because electric charge cannot travel easily through air, especially if the air is dry. Consider again the example of your hand and the metal doorknob. When your negatively charged hand gets very close to the positively charged doorknob, the air between your hand and the knob may become electrically charged. If that happens, it allows electrons to suddenly flow from your hand to the knob. This is the electric shock you feel when you reach for the knob. You may even see a spark as the electrons jump from your hand to the metal. This sudden flow of electrons is called static discharge. Another example of static discharge, on a much larger scale, is lightning. You can see how it occurs in Figure 23.9. At the URL below, you can watch a slow-motion lightning strike. Be sure to wait for the real-time lightning strike at the very end of the video.

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electric charge and electric force

Electric charge is a physical property of particles or objects that causes them to attract or repel each other without touching. All electric charge is based on the protons and electrons in atoms. A proton has a positive electric charge, and an electron has a negative electric charge (see Figure 23.2). When it comes to electric charges, opposites attract. In other words, positive and negative particles are attracted to each other. Like charges, on the other hand, repel each other, so two positive or two negative charges push apart from each other. The force of attraction or repulsion between charged particles is called electric force. It is illustrated in Figure 23.3. The strength of electric force depends on the amount of electric charge and the distance between the charged particles. The larger the charge or the closer together the charges are, the greater is the electric force.

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electric fields

Electric force is exerted over a distance, so charged particles do not have to be in contact in order to exert force over each other. Thats because each charged particle is surrounded by an electric field. An electric field is a space around a charged particle where the particle exerts electric force on other particles. Electric fields surrounding positively and negatively charged particles are illustrated in Figure 23.4 and at the URL below. When charged particles exert force on each other, their electric fields interact. This is also illustrated in Figure 23.4.

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instructional diagrams

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questions

If you get a shock when you touch a metal doorknob, static discharge has occurred.

-->  a. true

b. false

Static electricity occurs whenever

a. electric fields interact.

b. particles exert electric force.

-->  c. charges build up on an object.

d. electrons flow away from an object.

All electric charge is based on the protons and electrons in atoms.

-->  a. true

b. false

Both lightning bolts and electric current occur because of

a. polarization.

b. static discharge.

c. static electricity.

-->  d. moving electric charges.

Positive and negative particles always repel each other.

a. true

-->  b. false

Electric field lines around a positively charged particle

a. repel the particle.

b. attract the particle.

c. point toward the particle.

-->  d. point away from the particle.

If you rub a piece of wool cloth on a balloon, friction causes the

a. wool to transfer electrons to the balloon.

b. balloon to become negatively charged.

c. balloon to attract the wool.

-->  d. all of the above

Charged particles must be in contact in order to exert electric force over each other.

a. true

-->  b. false

When charged particles exert force on each other, their electric fields interact.

-->  a. true

b. false

The movement of air molecules, water droplets, and ice particles inside a cloud causes the

-->  a. transfer of electric charges within the cloud.

b. top of the cloud to become negatively charged.

c. bottom of the cloud to become positively charged.

d. all of the above

When electrons move from one object to another, the total charge remains the same.

-->  a. true

b. false

Rubber attracts electrons less strongly than wool does.

a. true

-->  b. false

Electrons can be transferred between objects only when the objects are touching.

a. true

-->  b. false

Electric charges cannot travel easily through the air, especially if the air is dry.

-->  a. true

b. false

During a thunderstorm, negative charges become concentrated at the tops of clouds.

a. true

-->  b. false

A charged particle exerts force over a distance because the particle has an electric field.

-->  a. true

b. false

buildup of electric charges on an object

a. electric charge

b. conduction

c. electric field

d. polarization

e. electric force

-->  f. static electricity

g. static discharge

The law of conservation of charge states that charges cannot move without being destroyed.

a. true

-->  b. false

transfer of electrons within an object

a. electric charge

b. conduction

c. electric field

-->  d. polarization

e. electric force

f. static electricity

g. static discharge

force of attraction or repulsion between charged particles

a. electric charge

b. conduction

c. electric field

d. polarization

-->  e. electric force

f. static electricity

g. static discharge

When you touch a van de Graaff generator, electrons flow to you from the device.

-->  a. true

b. false

transfer of electrons through direct contact between objects

a. electric charge

-->  b. conduction

c. electric field

d. polarization

e. electric force

f. static electricity

g. static discharge

Polarization is the transfer of electrons between oppositely charged objects.

a. true

-->  b. false

The ground beneath a storm cloud becomes positively charged by conduction.

a. true

-->  b. false

space around a charged particle where the particle exerts electric force

a. electric charge

b. conduction

-->  c. electric field

d. polarization

e. electric force

f. static electricity

g. static discharge

physical property that causes particles to attract or repel each other without touching

-->  a. electric charge

b. conduction

c. electric field

d. polarization

e. electric force

f. static electricity

g. static discharge

sudden flow of electrons from an object that has a buildup of charges

a. electric charge

b. conduction

c. electric field

d. polarization

e. electric force

f. static electricity

-->  g. static discharge

Moving electric charges create

a. lightning bolts.

b. electric current.

c. friction.

-->  d. two of the above

Electric field lines around a negatively charged particle

a. point away from the particle.

b. circle around the particle.

-->  c. point toward the particle.

d. repel the particle.

The strength of electric force is determined by the

a. amount of electric charge.

b. conservation of electric charge.

c. distance between charged particles.

-->  d. two of the above

The formation of charged matter depends on the

-->  a. transfer of electrons.

b. loss of protons.

c. discharge of current.

d. buildup of neutrons.

Ways that charges can be transferred include

a. friction.

b. conduction.

c. polarization.

-->  d. all of the above

If you rub a balloon on your hair, the balloon and hair attract each other because

-->  a. electrons are transferred from the hair to the balloon.

b. hair is negatively charged and the balloon is positively charged.

c. the hair attracts electrons more strongly than the balloon does.

d. two of the above

A buildup of electric charges on an object occurs because of

a. static electricity.

b. static discharge.

c. electric current.

-->  d. polarization.

diagram questions

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