and conductors coulomb’s law the field modelphy21bio/presentations/phy1022ch26.pdf6 phys 1022:...

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PHYS 1022: Chap. 26, Pg 2

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and Conductors •  Coulomb’s Law The Field Model

Topics: •  Developing a Charge Model •  Charge •  Insulators

PHYS 1022: Chap. 26, Pg 4 New Topic

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  Charging by contact  direct transfer

  Charging by induction  no contact

 Can happen to both conductors and insulators

  separation of charge is called polarization

  There are two kinds of charge:  positive (+): glass rubbed by silk  negative (–): wax or plastic rubbed by wool   like charges repel  unlike charges attract


1.  positive (+) 2.  negative (–)

3.  neutral

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3.  neutral



3.  neutral

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PHYS 1022: Chap. 26, Pg 10

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PHYS 1022: Chap. 26, Pg 12

  Electric force is a non-contact force (acts at a distance)

The unit for charge is Coulomb (C) The smallest charge: e = 1.6 x 10-19 C.

Two 1-C charges separated by 1 m has Coulomb force of 9x109 N!

k = 9 x 109 N m2/C2

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PHYS 1022: Chap. 26, Pg 13

-q

+q1

+q2

  If there are several charges, the net force on any one of them is the vector sum of the individual forces due to all the others.

  Superposition principle

PHYS 1022: Chap. 26, Pg 14

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PHYS 1022: Chap. 26, Pg 15

1.  left 2.  right

3.  zero

PHYS 1022: Chap. 26, Pg 16

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PHYS 1022: Chap. 26, Pg 17

PHYS 1022: Chap. 26, Pg 18

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PHYS 1022: Chap. 26, Pg 19

PHYS 1022: Chap. 26, Pg 20

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PHYS 1022: Chap. 26, Pg 21

1 cm

1 nC +

+ +

1 cm

1 cm 2 nC 2 nC


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PHYS 1022: Chap. 26, Pg 23

  Recall our discussion of the two kinds of forces:   Contact force:

»  You push on the desk »  The ground pushes on the chair »  Tension in a rope

Hmm...

Faraday and Newton had some trouble with this one

PHYS 1022: Chap. 26, Pg 24

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PHYS 1022: Chap. 26, Pg 25

These isolated Temperatures make up a Scalar Field

(you learn only the temperature at a place you choose)

PHYS 1022: Chap. 26, Pg 26

That would require a VECTOR field. (you learn how fast the wind is blowing AND in what direction)

You may prefer to know which way the wind is blowing …

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PHYS 1022: Chap. 26, Pg 27

  Definition of the Electric Field:

E ≡ F / qtest

PHYS 1022: Chap. 26, Pg 28

€

Fg =G Mr2m = mg

M m

Q q

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PHYS 1022: Chap. 26, Pg 29

+Q

-Q +Q

+Q +4Q

-Q +Q

PHYS 1022: Chap. 26, Pg 30

  Example: electric dipole  Point charges q1=+12nC and

q2=-12nC are placed 10 cm apart. This combination of two charges with equal magnitude and opposite sign is called an electric dipole.

 Compute the electric field at points a, b, and c.

  Superposition principle   The field at a given point is a vector sum of the electric field caused by each

charge individually.

and conductors coulomb’s law the field modelphy21bio/presentations/phy1022ch26.pdf6 phys 1022:...

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