# CHAPTER 9 - Physics

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16 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

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CHAPTER 9

ELECTROMAGNETIC

Electromagnetism

Properties of Charges:

(1) Charges come in two flavors:
positive and negative

(2) Like charges repel; unlike charges attract

(3) Charge is discrete: comes in increments of the

amount of charge on the electron.

(4) Charge is
conserved
.

Force Fields

m
1

m
2

Earth

moon

r

E
g
=

m
1

r
2

G

Gravitational Force Fields

F
g
=

m
1
m
2

r
2

G

Electric Force Fields

q
2

Charge 2

r

F
E
=

q
1
q
2

r
2

k

q
2

Charge 1

E
E
=

q
1

r
2

k

+q

The electric field lines of a
POSITIVE

-
q

The electric field lines of a
negative

charge point
inwards:

-
q

+q

When more than one charge is present, the field lines
are continuous and can make complex shapes:

+q

+q

Two Positive charges:

Stationary charges produce
electric

fields.

+q

N

S

Magnetic fields:

the
permanent magnet

Magnetic fields:

the coil

NOTE:

Moving charges are called
currents

(like water
currents).

(1) Stationary charges produce
electric
fields

(2) Moving charges produce
electric + magnetic

fields

James Clerk Maxwell: the “Isaac Newton”
of electromagnetism.

Changing
electric

fields produce
magnetic

fields

Changing
magnetic

fields produce
electric

fields

q

???

A disturbance in the field travels at the
speed of light: c = 3 x 10
8

m/s

Light is an electromagnetic wave!!!

Speed of light was first measured from Astronomical
observations: It looked like it too Jupiter’s moon more
time to orbit when Earth is at B than when it is at A

Anytime a charge is
ACCLERATED
, light

THE COMPLETE ELECTROMAGNTIC SPECTRUM

ELECTRONIC WAVES:

they are created by human beings by causing
electrons to vibrate in electric circuits

Created by the random thermal
motion of molecules

VISIBLE, ULTRAVIOLET AND X
-

Light
created by electrons moving within an individual atom

Created within atomic nuclei by
high energy nuclear processes caused by the strong
forces that hold the nucleus together

An x
-
ray machine!

REVIEW

1.
Do the electric circuits in your home produce
magnetic fields?

2.
List three characteristics that all
electromagnetic waves have in common
.

3.
What evidence is there that light is a wave.

4.
By tapping her finger on the surface of a pond,
Lisa creates waves. She taps at a frequency of
3 Hz. The speed of the wave is 2 m/s. Find the
wavelength of the wave. If she doubles the
frequency of her tapping, what happens to the
wavelength of the waves?

5.
A radar transmission is reflected off the surface
of the moon and returns to Earth in 6 seconds.
How far is it to the surface of the moon?