Transcript Electric & Gravitational Fields and Electric Potentials

Warm Up 3
• 36 The illustration shows the transcription process.
What is the main purpose of the structure labeled
W?
• F Carrying instructions for protein synthesis
• G Transforming into a protein
• H Replacing damaged DNA
• J Passing traits to offspring
Electric & Gravitational Fields
and Electric Potentials
Gravitational vs Electrical Force
Discussion
1. Gravitational Force acts between 2 _____.
2. Electrical Force acts between 2 _____.
3. In which direction does the gravitational
force act? How about the electrical force?
4. Does gravity get stronger or weaker from the
center of a mass? How about electrical force?
5. Do these 2 forces seem similar?
Gravity and Electricity
Gravitational Force
Electrical Force
Force between 2 masses
Attraction only
Weaker over distance
Cause things to orbit
Force between 2 charges
Attraction & Repulsion
Weaker over distance
Cause electrons to orbit
Gm1m 2
F
2
d
kq1q2
F 2
d
G = 6.67 x 10-11 Nm2/kg2
k = 8.99 x 109 Nm2/C2
Force Fields in Physics
• Force Fields are just areas with forces.
– Gravitational Field – Area where the force of
gravity pulls on an object
– Electric Field – Area where an electric force can
push or pull an object
Repulsion or Attraction?
• Visualization of an electric field
– Polarized thread in oil
Attraction or Repulsion?
• Line model
Force Fields in Physics
• Force fields are drawn with lines
– The lines represent the direction positive charge
would accelerate if placed in the field.
– The result is lines start from a positive charge and
end at a negative charge
Force Fields in Physics
• Force fields are drawn with lines
– The density of lines represent magnitude
Electrical Potential Energy
• Think about trying to bring together 2
magnets at their North poles.
• How much force does it take for you to bring
them together? At the beginning versus when
they are really close?
Electrical Potential Energy
• Electrical Potential Energy – Energy required
to keep a charge from moving in an electric
field.
• Strong field + strong charge + short distance =
high Electrical Potential Energy
• When the charge is released, that potential
energy is transformed to kinetic energy
Electric Potential
• Electric potential = “electric potential
energy”/”charge”
• SI unit for Electric Potential is the “volt”, V
J
V 
C
Misconceptions of the Volt
• High Voltage is deadly
– Van de Graaff generators can easily generate
50,000 V
– High potential energy but very little charge
Sample Problem
• If 20. J of work is required to move 4.0 C of
charge in a uniform electric field, the electric
potential will be _____.
•
•
•
•
A) 80. V
B) 5.0 V
C) 0.0 V
D) 100 V
Exit Ticket 2
• Consider two charged objects. One carries a
charge of 1.8 x10-6 C When the two are
separated by a distance of 0.9m, there is a
force of 2.7 N between them. What is the
charge on the second object?
Warm Up 4
Electric Potential Energy
• Electric Potential Energy (ePE) is due to a
charge’s location in an electric field
• ePE is the amount of work needed to move a
charge to that location
– Similar to gravitational potential energy depends
on the height of an object
• The amount of charge you are moving does
not matter
Lightning
• High Electric Potential is created by friction
and air currents in the clouds
• Clouds become polarized, usually bottom of
the cloud is negative
• Polarized cloud induces the ground to be
polarized
• Lightning occurs to neutralize the charge
imbalance
Electric Shielding
• Why is it safe to drive stay inside a car during a
lightning storm?
• Excess charges only coat the surface of an
object
– Object’s normal electrons force the extra
electrons to stay outside
• Inside of the car remains neutral
– Same with the inside of a Van de Graaff generator
Lab – Mapping Electric Potential
• Groups of up to 3
• You will measure the electric potential difference
with a volt meter
• Measure 36 (6 x 6) points around the positive
terminal
• Measure 6 points from the negative terminal to
the positive terminal
• Draw contour map at 0.5 V intervals
• Turn in drawing