1. Electric Charge and Electric Field
Chapter 16
Electric Charge and Electric Field
© 2008
Giancoli, PHYSICS,6/E © Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey

2. Structure of the atom (protons, neutrons and electrons)
Nucleus contains protons and neutrons
Protons have positive charge, neutrons are neutral
Mass of proton ≈ mass of neutron
Mass of proton (and neutron)  1800x mass of electron
Electrons have negative charge and are attracted to nucleus
Charge of electron is equal in magnitude to that of proton
Normal atom is neutral
Ion is atom that has gained or lost one or more electrons
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3. Conductors and Insulators
Conductor: metal atoms in solids have one or more “free” electrons per atom which move freely through the material
Insulator: no free electrons so it will not conduct electricity
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4. Static Electricity Static Electricity:
Rubbing certain materials together can separate electrons from their atoms
Removing electrons from a material makes it positive
In solids, it is always the free electrons that move
Electrical charge on the plastic rod induces a separation of charge in scraps of paper and thus attracts them.
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5. Induced Charge
(a) If you bring a + charge near a conductor, it will attract electrons to it leaving the other half of the metal positive.
(b) If they touch, then electrons move to the positively charged object, leaving the conductor positively charged.
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6. Coulomb’s Law Forces are equal in magnitude but opposite in direction
For spherical charges, r is the center to center distance
This equation gives the magnitude of the force--you have to figure the direction from the signs of the charges
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7. Coulomb’s Law C is Coulomb -- the unit of charge
k  9.0  109 N·m2 / C2
C is Coulomb -- the unit of charge
e = 1.60x10-19 C electronic charge (positive)
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8. Coulomb’s Law
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9. Example 1. Particles of charge Q1 = +5. 00 C, Q 2 = -6
Example 1. Particles of charge Q1 = C, Q 2 = -6.0 C and Q3 = +8.0 C are placed in a line separated by 0.40 m between each pair. Calculate the force on Q2. _ Q2 + Q1 + Q3
This is the magnitude, we get direction from charges.
Force is directed to the right.
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10. Example 2. Particles of charge Q1 = +5. 00 C, Q 2 = -6
Example 2. Particles of charge Q1 = C, Q 2 = C and Q3 = C are placed on the corners of a square of side m as shown below. Calculate the force on Q2 (Magnitude and direction). + Q1
Note that the charges and distances are the same as in Example 1, so we do not need to use Coulombs Law again.  _ Q2 + Q3
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11. The Electric Field Graphical representation of electrical forces
Electrical force “acts at a distance” like gravity
Electric field E surrounds every charge
We investigate the field with a small positive charge called a “test charge” q
The field is given by:
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12. The Electric Field Units are N/C
E is a vector = direction of force experienced by positive test charge
The magnitude of q is so small that it does not disturb the charges that cause the field
To plot the field, move the test charge around the charges that cause the field
Since q is positive the field points away from a + charge and towards a - charge
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13. Field of a Point Charge Q
This is the field created by a point charge or a spherical charge distribution Q
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14. Electric Field is a Vector
Field thus points toward a negative charge and away from a positive charge
Since test charge is positive, the direction of the electric field is the direction of the force felt by a positive charge
If there are two or more charges creating the field then the field at any point is the vector sum of the fields created by each of the charges
The test charge does not contribute to the field and it is too weak to cause any of the charges creating the field to move.
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15. E1 E2 Example 3A. A +100 C point charge is separated from a
-50 C charge by a distance of 0.50 m as shown below. (A) First calculate the electric field at midway between the two charges. (B) Find the force on an electron that is placed at this point and then calculate the acceleration when it is released. + Q1 _ Q2 E1 E2
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16. Example 3B. A +100 C point charge is separated from a
-50 C charge by a distance of 0.50 m as shown below. (A) First calculate the electric field at midway between the two charges. (B) Find the force on an electron that is placed at this point and then calculate the acceleration when it is released. + Q1 _ Q2
In part A we found that E = 2.1x107 N/C and is directed to the right.
( to left )
( to left )
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17. Example 4. A +100 C point charge is separated from a
-50 C charge by a distance of 0.50 m as shown below. Sketch the electric field at the point x as shown. + Q1 _ Q2
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18. Electric Field Lines Graphical way of showing the electric field.
You have seen graphical representations of the earth’s magnetic field-the electric field maps are similar.
Sometimes called lines of force.
Arrow on field line gives direction of force.
The closer together the lines of force are, the stronger the electric field.
Electric field lines are directed out from positive charges (a) and in toward negative charges (b).
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19. Electric Field of Point Charges

20. Electric Field of Point Charges

21. Electric Field of Parallel Plates
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22. Electric Fields and Conductors
In the static situation, the field outside the conductor is perpendicular to the surface of the conductor
if the field had a component parallel to the surface, it would cause the electrons in the conductor to move until there was only a perpendicular component.
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23. Electric Fields and Conductors
If a conductor is placed in an electric field, the electrons will rearrange themselves until the field inside the conductor is zero
The field inside a hollow conductor shell is zero (Fig 16-33)
This makes a metal car a relatively safe place in an electrical storm.
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