ap physics - chapter 18 powerpoint
TRANSCRIPT
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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coulombs
18.1 The Origin of Electricity
In nature, atoms are normallyfound with equal numbers of protonsand electrons, so they are electricallyneutral.
Neq
q = net electrical charge
N = Number of electrons (added or removed)
e = 1.60 x 10-19 C
18.1 The Origin of Electricity
Example 1 A Lot of Electrons
How many electrons are there in one coulomb of negative charge?
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18.2 Charged Objects and the Electric Force
It is possible to transfer electric charge from one object to another.
The body that loses electrons has an excess of positive charge, whilethe body that gains electrons has an excess of negative charge.
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).
Charges are equally shared when objects of different charges come intocontact with each other
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 an 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 3 A Model of the Hydrogen Atom
In the Bohr model of the hydrogen atom, the electron is in orbit about thenuclear proton at a radius of 5.29x10-11m. Determine the speed of the electron, assuming the orbit to be circular.
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18.5 Coulomb’s Law
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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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18.5 Coulomb’s Law
Find the net force on q1
18.5 Coulomb’s Law
18.6 The Electric Field
As we know, a charge can experience an electrostatic force due to the presence ofother charges. This surrounding Force/Coulomb is known as 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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FE
SI Units of Electric Field: newton per coulomb (N/C)
18.6 The Electric Field
Example 6 A Test Charge
The positive test charge has a magnitude of 3.0x10-8C and experiences a force of 6.0x10-8N.
(a) Find the force per coulomb that the test chargeexperiences.
(b) Predict the force that a charge of +12x10-8Cwould experience if it replaced the test charge.
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(b) N1024C100.12CN0.2 88 F
18.6 The Electric Field
It is the surrounding charges that create the electric field at a given point.
The electrostatic force points in the direction of attraction
The electric field always points away from the positive charge and towards the negative charge.
18.6 The Electric Field
Example 7 An Electric Field Leads to a Force
The charges on the two metal spheres and the ebonite rod create an electricfield at the spot indicated. The field has a magnitude of 2.0 N/C. Determinethe force on the charges in (a) and (b)
18.6 The Electric Field
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(b) N1048C100.24CN0.2 88 EqF o
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
Conceptual Example 12 Symmetry and the Electric Field
Point charges are fixed to the corners of a rectangle in twodifferent ways. The charges have the same magnitudesbut different signs.
Consider the net electric field at the center of the rectanglein each case. Which field is stronger?
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
18.7 Electric Field Lines
Conceptual Example 13 Drawing ElectricField Lines
There are three things wrong with part (a) of the drawing. What are they?
1.Field lines can never cross
2.The number of electric field lines that end on thenegative charges.
3.Spacing of electric field lines
18.8 The Electric Field Inside a Conductor: Shielding
The electric field just outside the surface of a conductor is perpendicular to the surface at equilibrium under electrostatic conditions.