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# 1 電荷與電場

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PowerPoint® Lectures for

University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson

1 電荷與電場

Revised by Sylveen H. Huang

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### 44. Particle Physics and Cosmology

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PowerPoint® Lectures for

University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman

### Chapter 21

Electric Charge and Electric Field

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### Electric charge

• Two positive or two negative charges repel each other. A positive charge and a negative charge attract each other.

Figure 21.1 below shows some experiments in electrostatics.

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### Conductors and insulators

A conductor permits the easy movement of charge through it. An insulator does not.

• Most metals are good conductors, while most nonmetals are insulators.

(See Figure 21.6 at the right.)

Semiconductors are intermediate in their

properties between good conductors and good

insulators.

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### Charging by induction

• In Figure 21.7 below, the negative rod is able to charge the metal ball without losing any of its own charge. This process is called charging by induction.

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### Electric forces on uncharged objects

• The charge within an insulator can shift slightly. As a result, two neutral objects can exert electric forces on each other, as shown in Figure 21.8 below.

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### Force between charges along a line

• Follow Example 21.2 for two charges, using Figure 21.12 at the right.

• Follow Example 21.3 for three charges, using Figure 21.13 below.

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### Vector addition of electric forces

• Example 21.4 shows that we must use vector addition when adding electric forces. Follow this example using Figure 21.14 below.

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### Electric field

A charged body produces an electric field in the space around it (see Figure 21.15 at the lower left).

We use a small test charge q0 to find out if an electric field is present (see Figure 21.16 at the lower right).

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### Superposition of electric fields

• The total electric field at a point is the vector sum of the fields due to all the charges present. (See Figure 21.21 below right.)

• Review Problem-Solving Strategy 21.2.

• Follow Example 21.8 for an electric dipole. Use Figure 21.22 below.

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### Electric field lines of point charges

• Figure 21.28 below shows the electric field lines of a single point charge and for two charges of opposite sign and of equal sign.

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### Electric dipoles

An electric dipole is a pair of point charges having

equal but opposite sign and separated by a distance.

• Figure 21.30 at the right illustrates the water

molecule, which forms an electric dipole.

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### Force and torque on a dipole

• Figure 21.31 below left shows the force on a dipole in an electric field.

electric dipole moment

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### • Follow Example 21.14 using Figure 21.33.

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