Chapter 19 Electric Charges, Forces, and...

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Chapter 19 Electric Charges, Forces,

and Fields

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Overview of Chapter 19

• Electric Charge !

• Insulators and Conductors !

• Coulomb’s Law !

• The Electric Field !

• Electric Field Lines !

• Shielding and Charging by Induction

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19-1 Electric Charge

The effects of electric charge were first observed as static electricity:

After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects

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19-1 Electric ChargeCharging both amber and glass rods shows that there are two types of electric charge:

• like charges repel • opposites attract

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19-1 Electric Charge

All electrons have exactly the same charge; the charge on the proton has the same magnitude but the opposite sign:

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19-1 Electric Charge

The electrons in an atom are in a cloud surrounding the nucleus and can be separated.

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19-1 Electric Charge

When an amber rod is rubbed with fur, some of the electrons on the atoms in the fur are transferred to the amber:

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19-1 Electric Charge

• We find that the total electric charge of the universe is a constant: Electric charge is conserved.

!• Electric charge is quantized in units of e. !

• The atom that has lost an electron is now positively charged – it is a positive ion !

• The atom that has gained an electron is now negatively charged – it is a negative ion

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19-2 Insulators and Conductors

• Conductor: A material whose conduction electrons are free to move throughout. Most metals are conductors. !•Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.

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19-2 Insulators and Conductors

If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

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19-2 Insulators and Conductors

Semiconductors: Have properties intermediate between conductors and insulators; their properties change with their chemical composition. !Photoconductors: materials become conductors when light shines on them.

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19-3 Coulomb’s Law

Coulomb’s law gives the force between two point charges:

The force is along the line connecting the charges •Attractive if the charges are opposite.. •Repulsive if the charges are like..

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19-3 Coulomb’s LawThe forces on the two charges are action-reaction forces.

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19-3 Coulomb’s LawIf there are multiple point charges, the forces add by superposition.

Treat forces of charges as vector sum, figure out overall direction and magnitude of force…

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19-4 The Electric FieldDefinition of the electric field:

Here, q0 is a “test charge” – it serves to allow the electric field to be measured…

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19-4 The Electric FieldIf we know the electric field, we can calculate the force on any charge:

•Direction of the force depends on the sign of the charge…

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19-5 Electric Field Lines

Electric field lines are a convenient way of visualizing the electric field. !Electric field lines: 1. Point in the direction of the field vector at every point 2. Start at positive charges or infinity

3. End at negative charges or infinity

4. Are more dense where the field is stronger

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19-5 Electric Field Lines

Lines from positive charges point away, lines from negative charges point towards…

•Direction defined as where a positive charge would move…

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19-5 Electric Field Lines

Combinations of charges. Use the afore rules to determine how these look….

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19-5 Electric Field Lines

A parallel-plate capacitor consists of two conducting plates with equal and opposite charges. Here is the electric field…

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19-6 Shielding and Charge by Induction

Excess charge on a conductor is free to move

• Charges will move so that they are as far apart as possible. •Called Induction

!Means that excess charge on a conductor resides on its surface…

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19-6 Shielding and Charge by Induction

When electric charges are at rest, the electric field at the center of the conductor is zero…

•This is shielding….

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19-7 Electric Flux and Gauss’s Law

Electric flux is a measure of the electric field perpendicular to a surface:

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19-7 Electric Flux and Gauss’s LawGauss’s law states that the electric flux through a closed surface is proportional to the charge enclosed by the surface:

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19-7 Electric Flux and Gauss’s LawGauss’s law can be used to find the electric field in systems with simple configurations.

For example, we know charge per unit area, σ, for

!φ= E(2A) = σA/ε0

!E = σ/2ε0

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Answer: a) 2.06 x 10-13 C

Answer: b) -7.5 x 10-18 C

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Answer: 1.37 m

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Find the total electric force on q2 in terms q and triangle length d.

Answer: FT = √3 kq2/d2

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Answer: a) 6.74 x 104 N/C

Answer: b) 1.69 x 104 N/C

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Answer: a) -3.0 x 107 N/C

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Using Gaus’s law, a) show that the electric field due to a point charge is E = kq/r2 . Note ε0 = 1/4πk. b) Does the electric field at surface change if the point charge becomes at small sphere?

Answer: b) No