PowerPoint® Lectures for
University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson
1 電荷與電場
高二物理
Revised by Sylveen H. Huang
ELECTROMAGNETISM
21. Electric Charge and Electric Field 22. Gauss’s Law
23. Electric Potential
24. Capacitance and Dielectrics
25. Current, Resistance, and Electromotive Force 26. Direct-Current Circuits
27. Magnetic Field and Magnetic Forces
ELECTROMAGNETISM
27. Magnetic Field and Magnetic Forces 28. Sources of Magnetic Field
29. Electromagnetic Induction 30. Inductance
31. Alternating Current
32. Electromagnetic Waves
OPTICS
33. The Nature and Propagation of Light
34. Geometric Optics and Optical Instruments 35. Interference
36. Diffraction
MODERN PHYSICS
37. Relativity
38. Photons: Light Waves Behaving as Particles 39. Particles Behaving as Waves
40. Quantum Mechanics 41. Atomic Structure
42. Molecules and Condensed Matter 43. Nuclear Physics
44. Particle Physics and Cosmology
PowerPoint® Lectures for
University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman
Chapter 21
Electric Charge and Electric Field
Goals for Chapter 21
• To study electric charge and charge conservation
• To see how objects become charged
• To calculate the electric force between objects using Coulomb’s law
• To learn the distinction between electric force and electric field
• To calculate the electric field due to many charges
• To visualize and interpret electric fields
• To calculate the properties of electric dipoles
Introduction
• Water makes life possible as a solvent for biological molecules. What electrical properties allow it to do this?
• We now begin our study of electromagnetism, one of the four fundamental forces.
• We start with electric
charge and look at electric
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.
Laser printer
• A laser printer makes use of forces between
charged bodies.
Electric charge and the structure of matter
• The particles of the atom are the negative electron, the positive proton, and the uncharged neutron.
• Protons and neutrons make up the tiny dense nucleus which is surrounded by electrons (see Figure 21.3 at the right).
• The electric attraction between protons and
electrons holds the atom
together.
Atoms and ions
• A neutral atom has the same number of protons as electrons.
• A positive ion is an atom with one or more electrons removed.
A negative ion has gained one or more electrons.
Conservation of charge
• The proton and electron have the same magnitude charge.
• The magnitude of charge of the electron or proton is a natural unit of charge. All observable charge is
quantized in this unit.
• The universal principle of charge conservation states
that the algebraic sum of all the electric charges in any
closed system is constant.
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.
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.
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.
Electrostatic painting
• Induced positive charge on the metal object attracts the
negatively charged paint droplets.
Coulomb’s law
• Coulomb’s Law: The
magnitude of the electric force between two point charges is directly
proportional to the
product of their charges and inversely proportional to the square of the
distance between them.
(See the figure at the right.)
• Mathematically:
F = k|q
1q
2|/r
2= (1/4π
0)|q
1q
2|/r
2Measuring the electric force between point charges
• The figure at the upper right illustrates how
Coulomb used a torsion balance to measure the electric force between point charges.
• Example 21.1 compares the electric and
gravitational forces.
Follow it using Figure
21.11 at the lower right.
Ex.21.1 electric force vs. gravitational force
Force between charges along a line
• Read Problem-Solving Strategy 21.1.
• 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.
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.
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).
Definition of the electric field
• Follow the definition in the text of the electric field
using Figure 21.17 below.
Electric field of a point charge
• Follow the discussion in the text of the electric field of a point charge, using Figure 21.18 at the right.
• Follow Example 21.5 to calculate the
magnitude of the electric field of a
single point charge.
Electric-field vector of a point charge
• Follow Example 21.6 to
see the vector nature of the
electric field. Use Figure
21.19 at the right.
Example 21.6 Electric-field vector of a point
charge
Electron in a uniform field
• Example 21.7 requires us to find the force on a charge
that is in a known electric field. Follow this example
using Figure 21.20 below.
Example 21.7 Electron in a uniform field
Example 21.7
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.
Example 21.9 Field of a ring of charge
• On the ring axis
Example 21.9 Field of a ring of charge
• For x >> a
Example 21.10 Field of a charged line segment
Field of a charged line segment
• For x >> a
• For very long segment (a >> x )
→
→
Example 21.11 Field of a uniformly charged disk
•
Example 21.12 Field of two oppositely charged
infinite sheets
Electric field lines
• An electric field line is an imaginary line or curve
whose tangent at any point is the direction of the electric
field vector at that point. (See Figure 21.27 below.)
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.
Visualizing electric field
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.
Force and torque on a dipole
• Figure 21.31 below left shows the force on a dipole in an electric field.
electric dipole moment