University:
California State University, Northridge
Course:
PHYS 100B | General Physics II
Academic year:

2008
Views:

188

Pages:

36
Author:

Alejandro Callahan

Chapter 19 Electric Charges, Forces, and Fields Outline 19-1 Electric Charge 19-2 Insulators and Conductors 19-3 Coulomb’s Law (and net vector force) 19-4 The Electric Field 19-5 Electric Field Lines 19-6 Shield and Charging by Induction 19-7 Electric Flux and Gauss’s Law 19-1 Electric Charge Objectives: • What is electric charge • How it is created • Electric charge properties The effect of electric charge have been known for a long time. Figure 19-1 Charging an Amber Rod (a). Uncharged amber exert no force on papers. (b). Rod amber is rubbed against a piece of fur. (c). Amber become charged and then attracts the papers. Figure 19-2 Likes Repel, Opposites Attract Amber (-1) + glass (+) Amber (-) + Amber (-) : Repel : Attract Conclusions: There are two type of charges (in amber and glass), which is illustrated in Fig. 19-2 • • Positive charge (+) Negative charge (-) Properties • The “like” or same charges repel. • The opposite or different charges attract. • Object with zero net charge are said to be electrically neutral. A good example of neutral object is the atom, as shown in Fig. 19-3. Figure 19-3 The Structure of an Atom :proton and neutron. All electrons have the same electric charge. Electron is negative, and is defined with a magnitude e, giving by: Magnitude of an Electron’s Charge, e e = 1.60 × 10−19 C (19 − 1) SI unit: coulomb, C The electron has a mass: M e = 9.11× 10−31 kg (19 − 2) In contrast, the charge on proton (one constituent of the nucleus) is positive (+e). The mass of proton is m p = 1.673 × 10−27 kg (19 − 3) The other constituent of the nucleus is the neutron, which has zero charge. Its mass is mn = 1.675 × 10 −27 kg (19 − 4) Charge Separation /Transfer Figure 19-4 Charge Transfer (a) Initially, an amber rod and a piece of fur is electrically neutral: no more negative/positive charge! (b) Charges is transferred from one to the other. (c) At the end, the fur and the amber have charges of equal magnitude, but opposite sign. Conservation of Electric Charge The total electric charge of the universe is constant: No physical process can increase or decrease the total amount of electric charge. Some concepts Due to the movement of electrons, charge is transferred from one object to another. Positive Ion: The atom that loses an electron is said to be a positive ion; Negative Ion: The atom that receives an extra electron is said to be a negative ion; 19-2 Insulators and Conductors • Insulator: Charges cannot move about freely on it. Examples: amber, glass, wood, rubber. • Conductor: Charges can move freely on it. Examples: different metals and metal wires. • Semiconductor: Materials have properties of electric conduction between insulator and conductor. And semiconductors have wide application in electronics! Figure 19-6 Charging a Conductor (a). An uncharged metal sphere is touched by a charged rod, and some charge is transferred to it. (b). Charges move freely on a conductor, and spread out uniformly because of the same charge repel. 19-3 Coulomb’s Law Coulomb’s law for the magnitude of the electrostatic force between two point charges q1 q2 F =k , 2 r with k = 8.99 × 109 N .m 2 / C 2 SI unit: newton, N (19 − 5) Figure 19-7 Forces Between Point Charges Note: the mass of proton is ~ 2000 times that of the electron ! Note: the mass of proton is ~ 2000 times of the electron ! An Example: Calculate the magnitude of electric force between a electron and a proton at a distance of r= 5.29 x 10 -11 m. According to Coulomb’s law q1 q2 F =k r2 = (8.99 × 109 N .m 2 / C 2 ) = 8.22 × 10−8 N −1.60 × 10−9 C 1.60 × 10−9 C (5.29 × 10−11 m) 2 An Example 19-9: The attraction electrostatic force between the point charges +8.44c10-6 C and Q has a magnitude of 0.975 N when the separation between the charges is 1.31 m. Find the sign and magnitude of the charge Q. Negative charge and The magnitude of the charge Q is Q= 2 (1.31 m ) ( 0.975 N ) 2 r F −5 2.21 10 C = = × 9 2 2 −6 kq (8.99 ×10 N ⋅ m / C )(8.44 ×10 C ) Example 19-1: The Bohr Orbit In the Borh’s Hydrogen model, the electron is imagined to move in a circular orbit about a stationary proton. The force responsible for the electron circular motion is the electric force between the electron and the proton. Given that the radius of the electron’s orbit is 5.29x10-11m, and its mass is me=9.11x10-31km. Find the electron’s speed. Solution 1. Set the coulomb force between the electron and proton equal to the centripetal force required for the electro’s circular orbit: q1 q 2 = m e a cp k 2 r e2 v2 k 2 = me r r 2. Solve for the speed of the electron, v: v = e k m er 3. Substitute numerical data: v = (1 . 6 0 × 1 0 − 1 9 C ) 8 . 9 9 × 1 0 9 N .m 2 / C 2 (9 .1 1 × 1 0 −31 k g )(5 .2 9 × 1 0 −11 m ) = 2 .1 9 × 1 0 6 m / s Superposition The electric force, like all forces, is a vector. Hence, a charge experiences forces due to two or more charges is the vector sum of all the forces. For examples, in Fig. 19-8, the total force on charge 1 is the vector sum of the forces due to charges 2, 3 ,4 Vector sum/ Net force: JG JG JG JG F 1 = F 12 + F 13 + F 14 Figure 19-8 Superposition of Forces Example 19-15: Net Force For three charges, given q= +12 uC and d= 16 cm. (a) Find the direction and magnitude of the net electrostatic force exerted on the point charge q2 in the following figure. (b) How will your answer change if the distance d were tripled? Note: 1 uC=10-6 C Solution (a) Write Coulomb’s law using vector notation: G G G q1q3 q1q2 F2 = F21 + F23 = − k 2 xˆ + k 2 xˆ d d Substitute the charge magnitudes given in the figure G k k F2 = 2 [ −q1q2 + q1q3 ] xˆ = 2 ⎡⎣ − q ( 2.0q ) + ( 2.0q )( 3.0q ) ⎤⎦ xˆ d d = 2 kq 4.0] xˆ = [ 4.0] 2 [ d (8.99 ×10 N ⋅ m 9 2 / C2 )(12 × 10−6 C ) ( 0.16 m ) 2 2 xˆ = ( 200 N ) xˆ The net electrostatic force on q2 is 200 N = 0.20 kN toward q3 (b) If the distance d were tripled, the magnitude would be cut to a ninth and the direction would be unchanged. Example 19-21: Superposition (a) Find the direction and magnitude of the net force exerted on the point charge q3 in the figure. Let q= +2.4 uC and d= 33cm. (b) How would your answer change if the distance d were double? Picture the problem Solution 1. (a) Find G k q2 q3 k ( 2.0q )( 3.0q ) 6.0kq 2 F2 = xˆ = xˆ = xˆ 2 2 2 d d d G 2. Find F4 G k q3 q4 k ( 3.0q )( 4.0q ) 12kq 2 F4 = yˆ = yˆ = yˆ 2 2 2 d d d G 3. Find F1 G k q1 q3 k ( q )( 3.0q ) ⎛ xˆ yˆ ⎞ ˆ ˆ F1 = − cos 45°x − sin 45°y ) = − ⎜− ⎟ 2 ( 2 2 2 ⎝ ⎠ 2d 2d ( ) 3.0 2kq 2 = ( −xˆ − yˆ ) 2 4d ( ) 4. Find the vector sum of the three forces: G ⎛ 6.0kq 2 3.0 2kq 2 ⎞ ⎛ 12kq 2 3.0 2kq 2 − Fnet = ⎜⎜ ⎟⎟ xˆ + ⎜⎜ 2 − 2 2 4d 4d 2 ⎝ d ⎠ ⎝ d 3.0kq 2 ⎡ 2.0 − 2 4 xˆ + 4 − 2 4 yˆ ⎤ = 2 ⎣ ⎦ d ( = G Fnet ) ( ) 3.0 ( 8.99 × 109 N ⋅ m 2 / C 2 )( 2.4 × 10−6 C ) ( 0.33 m ) = ( 2.3 N ) xˆ + ( 5.2 N ) yˆ ⎞ ⎟⎟ yˆ ⎠ 2 2 ( ) ( G F 5. Find the direction of net from the +x axis: θ = tan −1 Fnet, y Fnet, x = tan −1 5.2 N = 66° 2.3 N G 6. Find the magnitude of Fnet Fnet = Fnet2 , x + Fnet2 , y = ( 2.3 N ) + ( 5.2 N ) = 5.7 N 2 ) ⎡ 2.0 − 2 4 xˆ + 4 − 2 4 yˆ ⎤ ⎣ ⎦ 2 7. (b) If the distance d were doubled, the magnitude of the force would be cut to one-fourth and the direction would be unchanged. Superposition For case of more than two forces, the net force vector is JG JG JG JG F net = F 1 + F 2 + F 3 + .... The X and Y components of the net force are Fnet , x = F1, x + F2, x + F3, x + .... Fnet , y = F1, y + F2, y + F3, y + .... The magnitude and direction of the net force are Fnet = Fnet2 , x + Fnet2 , y θ = tan −1 ( Fnet , y / Fnet , x ) Summary 1) There are two kinds of charge: positive (+) and negative (-) charges. 2) Like charges repel; different charges attract. 3) Electric charge conservation: the total charges are constant in the universe. 4) Insulator, semiconductor, and conductor. 5) Coulomb’s law q1 q2 F =k , 2 r (19 − 5) 6) Superposition (based on Coulomb’s law): net vector force. Example 19-3: Superposition Three charges, each equal to +2.90 uC, are placed at three corners of a square 0.500 m on a side, as shown in the diagram. Find the magnitude and direction of the net force on charge number 3 Picture the problem Solution JG 1. Find the magnitude of F 31 F31 = k q1 q 3 ( 2r )2 = ( 8 . 9 9 × 1 0 N .m 9 = 0 .1 5 1 2 ( 2 .9 × 1 0 −6 C ) 2 /C )× 2 ( 0 .5 0 0 m ) 2 2 ( 2 .9 × 1 0 −6 C ) 2 /C )× ( 0 .5 0 0 m ) 2 2 N JG 2. Find the magnitude of F 32 F32 = k q2 q3 (r )2 = ( 8 . 9 9 × 1 0 N .m 9 = 0 .3 0 2 N 2 JG JG 3. Calculate the two orthogonal components of F 31 and F 32 F3 1, x = F3 1 c o s 4 5 .0 ° = ( 0 .1 5 1 N ) ( 0 .7 0 7 ) = 0 .1 0 7 N F3 1, y = F3 1 s i n 4 5 .0 ° = ( 0 .1 5 1 N ) ( 0 .7 0 7 ) = 0 .1 0 7 N F 3 2 , x = F 3 2 c o s 0 ° = ( 0 . 3 0 2 N ) (1) = 0 . 3 0 2 F32 , y = F32 s in 0 ° = ( 0 .3 0 2 N )( 0 ) = 0 N N JG 4. Calculate the net force F 3 F3 , x = F3 1, x + F3 2 , x = 0 .1 0 7 N + 0 .3 0 2 N = 0 .4 0 9 F3 , y = F3 1, y + F3 2 , y = 0 .1 0 7 N + 0 N = 0 .1 0 7 F3 = F 3 2, x + F 3 2, y = 5. Find the direction of θ 3 = ta n −1 ( F3, y F3,x N N ( 0 .4 0 9 ) 2 + ( 0 .1 0 7 N ) 2 = 0 .4 2 3 JG F3 ) = ta n −1 ( 0 .1 0 7 N ) = 1 4 .7 ° 0 .4 0 9 N N