Chapter 25

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# Chapter 25 - PowerPoint PPT Presentation

Chapter 25. Electric Charges and Forces. 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:

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### Chapter 25

Electric

Charges and Forces

Physics 133

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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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?

Physics 133

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.
• Charge 1 2 3 4
• A plastic plastic plastic neutral
• B plastic neutral neutral glass
• C plastic neutral glass neutral
• D plastic neutral neutral neutral

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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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Coulomb’s law
• Point particles
• Along the line between

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Atom
• Positive core
• Electron cloud

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Wood

Amber

Hard Rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Polyester

Styrene (Styrofoam)

Polyurethane

Polyethylene (like Scotch tape)

Polypropylene

Vinyl (PVC)

Silicon

Teflon

- - - (negative)

+++ (positive)

Human Hands (too moist)

Rabbit Fur

Glass

Human Hair

Nylon

Wool

Fur

Silk

Aluminum

Paper

Cotton

Steel (neutral)

Triboelectric series

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Electroscope
• Predict what will happen if a charged rod is brought near an electroscope.

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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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Workbook 25.12
• 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.
• A Leaves separate more
• B Leaves separate less
• C No change

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Can.ans

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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.
• ABCD
• b) Does the sphere experience a net force? If so, in which direction?
• ABCD

- -

- -

- -

- +

- +

- +

+ -

+ -

+ -

no charge

no force

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Board.ans

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Board2.ans

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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.

Physics 133

Coulomb’s law
• Point particles
• Along the line between

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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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Workbook 25.22 (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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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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Electric Force – Superposition (graphical addition)

Into which quadrant does the vector point that represents the total force on charge #3?

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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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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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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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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

Physics 133