1. Electric Charges and Forces
Chapter 25
Electric
Charges and Forces
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2. Workbook 25.4
25.4 Two lightweight balls hang straight down when both are neutral. They are close enough together to interact, but not close enough to touch. Draw picture showing how the balls hang if:
Both are touched with a plastic rod that was rubbed with wool.
The two balls of part a are moved farther apart.
Ball A is touched by a plastic rod that was rubbed with wool and ball B is touched by a glass rod that was rubbed with silk.
Both are charged by a plastic rod, but ball A is charged more than ball B.
Ball A is charged by a plastic rod. Ball B is paper (neutral).
Ball A is charged by a glass rod. Ball B is paper (neutral).
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3. Hanging Charged Masses
Construct a free-body diagram for the hanging charged object (it is not moving).
Apply Newton's second law to your diagram.
Rank the forces (descending):T , F (Q on q), w
What happens to orientation of rope if Q  Q/2?
What happens to orientation of rope if w  w/2?
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4. Workbook 25.5
25.5 Four lightweight balls 1, 2, 3, 4 are suspended by threads. Ball 1 has been touched by a plastic rod that was rubbed with wool. When the balls are brought close together, without touching, the following observations are made:
Balls 2, 3, and 4 are attracted to ball 1.
Balls 2 and 4 have no effect on each other.
Ball 2 is attracted to ball 3.
What are the charge states (glass, plastic, neutral) of balls 1, 2, 3, and 4? Explain.
Answer:
Charge
A plastic plastic plastic neutral
B plastic neutral neutral glass
C plastic neutral glass neutral
D plastic neutral neutral neutral
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5. 25.6 Charged plastic and glass rods hang by threads.
Workbook 25.6
25.6 Charged plastic and glass rods hang by threads.
An object repels the plastic rod. Can you predict what it will do to the glass rod? If so, what? If not, why not? Explain.
A different object attracts the plastic rod. Can you predict what it will do to the glass rod? If so, what? If not, why not? Explain.
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6. Coulomb’s law
Point particles
Along the line between
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7. Atom
Positive core
Electron cloud
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8. Triboelectric series +++ (positive) Human Hands (too moist) Rabbit Fur
Glass
Human Hair
Nylon
Wool
Fur
Lead
Silk
Aluminum
Paper
Cotton
Steel (neutral)
Wood
Amber
Hard Rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Polyurethane
Polyethylene (like Scotch tape)
Polypropylene
Vinyl (PVC)
Silicon
Teflon
- - - (negative)
(List is adapted from Nature’s Electricity, by C. K. Adams)
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9. Insulators and Conductors
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10. Electroscope
Predict what will happen if a charged rod is brought near an electroscope.
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11. Electroscope
Predict what will happen if a charged rod is brought near an electroscope with an insulator below the top ball.
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12. Workbook 25.12 Answer: A Leaves separate more B Leaves separate less
25.12 A negatively charged electroscope has separated leaves.
Suppose you bring a negatively charged rod close to the top of the electroscope, but not touching. How will the leaves respond? Use both charge diagrams and words to explain.
How will the leaves respond if you bring a positively charge close to the top of the electroscope, but not touching? Use both charge diagrams and words to explain.
Answer:
A Leaves separate more
B Leaves separate less
C No change
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13. Can.ans
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14. Workbook 25.19
25.19
A positively charged rod is held near, but not touching, a neutral metal sphere.
Add plusses and minuses to the figure to show the charge distribution on the sphere.
A B C D
b) Does the sphere experience a net force? If so, in which direction?
no charge
no force
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15. Board.ans
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16. Board2.ans
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17. Is there a net force on the atom? If so, in which direction? Explain.
Workbook 25.21
25.21
The figure shows an atom with four protons in the nucleus and four electrons in the electron cloud.
Draw a picture showing how this atom will look if a positive charge is held just above the atom.
Is there a net force on the atom? If so, in which direction? Explain.
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18. Coulomb’s law
Point particles
Along the line between
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19. Workbook 25.22
25.22 For each pair of charges, draw a force vector on each charge to show the electric force acting on that charge. The length of each vector should be proportional to the magnitude of the force. Each + and – symbol represents the same quantity of charge.
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20. Workbook (Ans)
25.22 For each pair of charges, draw a force vector on each charge to show the electric force acting on that charge. The length of each vector should be proportional to the magnitude of the force. Each + and – symbol represents the same quantity of charge.
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21. What is the magnitude of the electric force between them?
Force magnitude: Consider two small balls (1: q1=10 nC, m1=20 mg; 2: q2=5 nC, m2=10 mg) separated by 3 cm.
What is the magnitude of the electric force between them?
What is the magnitude of the gravitational force between them?
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22. Electric Force – Superposition (graphical addition)
Into which quadrant does the vector point that represents the total force on charge #3?
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23. Electric Force – Superposition (adding components)
Find the total force on charge #3.
(Hint: Use superposition.)
K = 8.99 x 109 Nm2/C2
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24. Electric Force – Superposition (answer)
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25. Unit vector Rectangular: always same direction
Polar: different directions for each spot
Pick a source charge location. (ball)
Students make x, y vector components.
Students make r unit vector.
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27. Problem 25. 62: What are the electric fields at points 1 and 2
Problem 25.62: What are the electric fields at points 1 and 2? Give your answer as a magnitude and direction.
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28. ANSWER Problem 25. 62: are the electric fields at points 1 and 2
ANSWER Problem 25.62: are the electric fields at points 1 and 2? Give your answer as a magnitude and direction.
E=1.8x105 N/C, 60o north of west
Ex=-9.0x104 N/C, Ey=1.56x105 N/C
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