Transcript Electric Charge

Whiteboard Work
Identify similarities and differences between
the electric force (Colomb’s law) and the
gravitational force (Newton’s gravitational
formula).
Announcements
• No pre-lab for next week
• Exam 4 today
Electricity and Magnetism
Underlying a whole lot of
phenomena
Outline
• Stationary charges
– forces, potential, fields
• Moving charges
– current, resistance, circuits
• Magnetism
– another effect of moving charges
• Magnetic Induction
– Pushing charges with magnetism
Electric Charge
Objectives
• Determine electric force using Coulomb’s
Law.
• Explain forces in terms of electric fields.
• Determine energies from electric potential.
Electric Forces Between
Objects
always between objects
Coulomb’s Law
F=
kq1q2
d2
2
Nm
k = 8.992  109 2
C
C = coulomb (unit of electric charge)
Coulomb’s Law
F=
kq1q2
d2
Force is attractive for opposite charges
Force is repulsive for like charges
Proportional to the inverse square of the
separation
Poll Question
A hydrogen atom consists of a positive proton and
a negative electron. How does the force between
the electron and proton change when the electron
moves twice as close?
A. The force becomes twice (2) as much.
B. The force becomes half (1/2) as much.
C. The force remains the same.
D. The force becomes four times (4×) as much.
E. The force becomes one-fourth (1/4) as much.
Charge Polarization
tells us something about matter
Scenario
A bag contains equal numbers of positive
and negative charges. The charges can
move around inside the bag, but they cannot
leave the bag. The bag is placed near a
very large, immobile + charge.
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+
Quick Question
What sort of force exists between the +
charges in the bag and the large + charge?
A. The + charges are attracted to the charge.
B. The + charges are repelled by the charge.
C. The + charges are neither attracted nor repelled.
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+
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Quick Question
What sort of force exists between the –
charges in the bag and the large + charge?
A. The – charges are attracted to the charge.
B. The – charges are repelled by the charge.
C. The – charges are neither attracted nor repelled.
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+
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Quick Question
In which direction do the + charges in the
bag accelerate due to the large + charge?
A. Toward the charge.
B. Away from the charge.
C. The + charges will not accelerate.
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+
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Quick Question
In which direction do the – charges in the
bag accelerate due to the large + charge?
A. Toward the charge.
B. Away from the charge.
C. The – charges will not accelerate.
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+
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Poll Question
After the charges re-distribute, which force
to the external + charge will be stronger?
A. The attraction to the – charges.
B. The repulsion to the + charges.
C. The attraction and repulsion will exactly cancel.
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+
+
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Poll Question
What sort of force exists between the bag
overall and the large + charge?
A. The bag is attracted to the charge.
B. The bag is repelled by the charge.
C. The bag is neither attracted nor repelled.
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+
+
+
–
–
–
–
+
Group Poll Question
What sort of force on the bag will exist if the
external charge is negative?
A. The bag is attracted to the charge.
B. The bag is repelled by the charge.
C. The bag is neither attracted nor repelled.
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+
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–
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+
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Electric Fields
around electric charges
Electric Field
• Field E relates the electric force F on an
object to its charge q
F = qE
• Field is a vector
Electric field
• Magnitude is the force in N on a +1 C
charge
• Direction is the direction of the force
exerted on a positive charge
• Vectors point away from positive charges
and toward negative charges
• Unit = N/C
Poll Question
A positive charge experiences a force F to
the right in an electric field. How does the
force change if the field strength doubles?
A.
B.
C.
D.
E.
F.
The force becomes 1/4 what it was (F/4).
The force becomes 1/2 what it was (F/2).
The force remains the same (F).
The force becomes 2 what it was (2F).
The force becomes 4 what it was (4F).
The force reverses direction (–F).
Fields and Newton’s Third Law
• Field notation is unilateral
– Remember that forces are always between
objects
• A charge’s field acts on other charges
– Never on itself
Visualizing Fields
• Field vectors
• Field lines
Field Vectors
Board Work
Draw field vectors to describe the electric
field of a single positive charge.
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Field Lines
Field Lines
• Magnitude of the force on a charge is
greater where field lines are close together
• Direction of the force is parallel to field
lines
– Force on a positive charge is along field lines
– Force on a negative charge is opposite field
lines
Poll Question
Green arrows are
field lines.
Particles A–D have
the same charge.
Which experiences
the greatest force
from the field?
C
A
B
D
E. All four forces are equal.
Poll Question
Green arrows are
field lines.
Particles A–D have
the same charge.
Which experiences
the greatest force
from the field?
C
A
B
D
E. All four forces are equal.
Board Work
Draw field lines to describe the electric field
of a single positive charge.
+
Electric Potential
potential energy per charge
Electric Potential
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•
Field:force as potential:potential energy
Field is force per charge
Potential is potential energy per charge
Unit = J/C = volt = V
Electric Potential
What is the work required to move a
charged object through a field?
higher e
Dx
lower e
Work = qE·Dx = D(PE) = qDe
e = electric potential = PE/unit charge
Electric Field and Potential
Potential and Field
• Equipotential surfaces are always
perpendicular to electric field lines/vectors.
Why?
• Potential changes rapidly where field is
strong. Why?
It makes sense from the relationships between
• Electric field and electric force
• Electric potential and electric potential energy
• Work and potential energy
• Force and work
Reading for Next Time
• Electric current
• Main Ideas
– Ohm’s law
– Meanings of the quantities in Ohm’s law