chapter 18 electric forces and electric fields. 18.1 the origin of electricity the electrical nature...
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Chapter 18
Electric Forces and Electric Fields
18.1 The Origin of Electricity
The electrical nature of matter is inherentin atomic structure.
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18.1 The Origin of Electricity
In nature, atoms are normallyfound with equal numbers of protonsand electrons, so they are electricallyneutral.
By adding or removing electronsfrom matter it will acquire a netelectric charge with magnitude equalto e times the number of electronsadded or removed, N.
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18.2 Charged Objects and the Electric Force
LAW OF CONSERVATION OF ELECTRIC CHARGE
During any process, the net electric charge of an isolated system remainsconstant (is conserved).
18.2 Charged Objects and the Electric Force
Like charges repel and unlike charges attract each other.
18.3 Conductors and Insulators
Not only can electric charge exist on an object, but it can also movethrough and object.
Substances that readily conduct electric charge are called electricalconductors.
Materials that conduct electric charge poorly are called electricalinsulators.
18.5 Coulomb’s Law
COULOMB’S LAW
The magnitude of the electrostatic force exerted by one point chargeon another point charge is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance betweenthem.
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18.5 Coulomb’s Law
Example 4 Three Charges on a Line
Determine the magnitude and direction of the net force on q1.
18.5 Coulomb’s Law
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18.6 The Electric Field
DEFINITION OF ELECRIC FIELD
The electric field that exists at a point is the electrostatic force experiencedby a small test charge placed at that point divided by the charge itself:
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SI Units of Electric Field: newton per coulomb (N/C)
18.6 The Electric Field
Electric fields from different sourcesadd as vectors.
18.6 The Electric Field
Example 10 The Electric Field of a Point Charge
The isolated point charge of q=+15μC isin a vacuum. The test charge is 0.20m to the right and has a charge qo=+0.80μC.
Determine the electric field at point P.
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18.6 The Electric Field
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18.6 The Electric Field
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The electric field does not depend on the test charge.
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18.6 The Electric Field
THE PARALLEL PLATE CAPACITOR
Electric field produced by a parallel platecapacitor
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18.7 Electric Field Lines
Electric field lines or lines of force provide a map of the electric fieldin the space surrounding electric charges.
18.7 Electric Field Lines
Electric field lines are always directed away from positive charges andtoward negative charges.
18.7 Electric Field Lines
Electric field lines always begin on a positive chargeand end on a negative charge and do not stop in midspace.
18.7 Electric Field Lines
The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge.
18.7 Electric Field Lines
18.9 Gauss’ Law
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18.9 Gauss’ Law
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18.9 Gauss’ Law
GAUSS’ LAW
The electric flux through a Gaussiansurface is equal to the net charge enclosed in that surface divided bythe permittivity of free space:
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SI Units of Electric Flux: N·m2/C
PROBLEMS TO BE SOLVED
• 18.20(21) [(a)0.166N,along +y-axis,(b)110.67m/s2,along +y-axis]; 18.35(36)[(a)8.02×105N/C,(b)1.28×10-13N]; 18.53(53)[32.55×10-9C].
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