Lecture Outline

Chapter 15

College Physics, 7th Edition

Wilson / Buffa / Lou

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Units of Chapter 15

Electric Charge

Electrostatic Charging

Electric Force

Electric Field

Conductors and Electric Fields

Gauss’s Law for Electric Fields: A Qualitative Approach

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15.1 Electric Charge

Electric charge is a fundamental property of matter; electric charges may be positive or negative.

The atom consists of a small positive nucleus surrounded by a negative electron cloud.

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15.1 Electric Charge

Like charges repel; unlike charges attract.

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15.1 Electric Charge

SI unit of charge: the coulomb, C. All charges are integer multiples of the charge on the integer multiples of the charge on the electron electron (typically an easy AP test question):

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15.1 Electric Charge

Charge is conserved:

The net charge of an isolated system remains constant.

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15.2 Electrostatic Charging

Conductors transmit charges readily.

Semiconductors are intermediate; their conductivity can depend on impurities and can be manipulated by external voltages on purpose (computers).

Insulators do not transmit charge at all (grounding).

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15.2 Electrostatic ChargingAn electroscope may be used to determine if an object is electrically charged.

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15.2 Electrostatic Charging

Charge may also be moved within an object—without changing its net charge—through a process called polarization.

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15.3 Electric ForceThe force exerted by one charged particle on another is given by (Coulomb’s Law):

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15.3 Electric Force

If there are multiple point charges, the force vectors must be added to get the net force.

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15.4 Electric Field

Definition of the electric field:

The direction of the field is the direction the force would be on a positive charge.

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15.4 Electric FieldCharges create electric fields, and these fields in turn exert electric forces on other charges.

Electric field of a point charge:

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15.4 Electric FieldFor multiple charges, the total electric field is found using the superposition principle:For a configuration of charges, the total, or net, electric field at any point is the vector sum of the electric fields due to the individual charges.

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15.4 Electric Field

It is convenient to represent the electric field using electric field lines, or lines of force. These lines are drawn so the field is tangent to the line at every point – radial lines that are perpendicular to the circular lines.

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15.4 Electric Field

Rules for drawing electric field lines:

1. Closer lines mean a stronger field.

2. The field is tangent to the lines at every point.

3. Field lines start on positive charges and end on negative charges.

4. The number of lines entering or leaving a charge is proportional to the magnitude of the charge.

5. Field lines never cross.© 2010 Pearson Education, Inc.

15.4 Electric FieldElectric field lines of a dipole:

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15.4 Electric Field

Electric field lines due to like charges: (a) equal charges; (b) unequal charges.

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15.5 Conductors and Electric Fields

Excess charges on a conductor will repel each other, and will wind up being as far apart as possible – poles.

Any excess charge on an isolated conductor resides entirely on the surface of the conductor – Faraday’s Cage, current in a wire.

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15.5 Conductors and Electric Fields

The force from neighboring charges is less when the curvature of the surface is large:

Excess charge tends to accumulate at sharp points, or locations of highest curvature, on charged

conductors. As a result, the electric field is greatest at such locations.

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15.6 Gauss’s Law for Electric Fields: A Qualitative Approach

If a greater amount of charge is enclosed, more field lines cross the surface.

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15.6 Gauss’s Law for Electric Fields: A Qualitative Approach

Here, surface 1 surrounds the positive charge and has lines exiting it. Surface 2 surrounds the negative charge and has lines entering it. Surface 3 does not enclose any charge, and the same number of lines exit as enter.

Surface 4 encloses both charges; as they are equal in magnitude, the same number of lines exit the surface as enter it.© 2010 Pearson Education, Inc.

Summary of Chapter 15

Like charges repel; unlike charges attract.

Charge is conserved.

Electrons move freely inside conductors, but not inside insulators.

Objects may be charged electrostatically by friction, conduction, or induction.

Polarization is the separation of positive and negative charge within an object.

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Summary of Chapter 15

Coulomb’s law:

The electric field, a vector, is the force per unit charge. Electric fields from multiple charges add by superposition.

Electric field lines are used to represent the electric field.

A conductor has zero electric field inside and has all excess charge on its surface.

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Summary of Chapter 15

The electric field is always perpendicular to the surface of a conductor.

The charge density and electric field are greatest on a conductor where the curvature is largest.

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