Download Ch 21 PPT

Ch 21
Electric Fields
Evidence of Electric Fields
• Sailors in the time of
Columbus would
occasionally witness the
ends of their masts
glowing. It would appear
to be on fire but not
burning. Sailors learned
that this meant there was
an approaching
lightening storm.
Other Evidence of Electric Fields
21.1
Creating and Measuring Electric
Fields
Electric Fields
• Michael Faraday developed the concept of an
electric field
• Faraday discovered that a charged object creates
an electric field in all direction around it
Electric Fields
• Electric field (E): a vector quantity that relates
the force exerted on a test charge to the size of
the test charge
F
E
q'
• Magnitude of an electric field is measured in
newtons per coulomb (N/C)
Example
• A negative charge of 2.0 x 10 -8 C experiences a
force of 0.060 N to the right in an electric field.
What are the field magnitude and direction?
Electric Field Lines
• The direction of an electric field is in the
direction of the force on a positive test charge
▫ Which direction would a positive charge go if put
near that charge
▫ Has to be a small charge so that it doesn’t mess
with the field of the charge being tested
Electric Field Lines
• Direction shown by direction of arrows
• Strength shown by spacing between lines
Electric Field Lines
Electric Field Lines
• Note: field lines do not really exist
21.2
Application of Electric Fields
Electric Potential Energy
• Just like gravitational potential energy increases
with distance, so essentially does electric
potential
• It requires work to separate two charges and the
work is stored as potential difference
W
V 
q'
Electric Potential Difference
W
V 
q'
• Unit is joules per coulomb
• 1 joule per coulomb is 1 volt
• Electric Potential Difference in a Uniform Field
V  Ed
Example
• Two parallel plates are given opposite charges.
A voltmeter measures the electric potential
difference to be 60.0 V. The plates are 3.0 cm
apart. What is the magnitude of the electric field
between them?
Example
• Two large, charged parallel plates are 4.0 cm
apart. The magnitude of the electric field
between the plates is 625 N/C.
▫ What is the electric potential difference between
the plates?
▫ What work will you do to move a charge equal to
that of one proton from the negative to the
positive plate?
Storing Charges
• A device used to store charge is called a
capacitor.
▫ A device designed to have a specific capacitance
 Capacitance (C): the ratio of an object’s store charge
to its potential difference
 Measured in farads, F
q
C
V
Example
• A sphere has an electric potential difference
between it and Earth of 60.0 V when it has been
charged to 3.0 x 10 -6 C. What is its capacitance?
Homework
• P502: 2, 6, 12, 23, 25, 32, 33