LECTURE 1
CHARGE AND COULOMB’S LAW
Lecture 1
2
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Reading chapter 21-1 to 21-3.
¤ Electrical charge
n Positive and
negative charge
n Charge quantization and conservation
¤ Conductors
and insulators
¤ Induction
¤ Coulomb’s
law
Demo: 1
3
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Charged rods on turntable
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Charge can be positive or negative.
Like charges repel each other.
Opposite charges attract each other.
Charge quantization
4
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The unit of charge is the coulomb (C).
Charge is quantized which means it always occurs in an integer multiple of
e = 1.602×10-19 C.
Each electron has a charge of negative e.
Each proton has a charge of positive e.
Point of interest:
¤
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Quarks can have smaller charges than an electron but they do not occur as free particles.
The charge of an up quark is +2/3 e.
The charge of a down quark is -1/3 e.
A proton consists of 2 up quarks and 1 down quark, total charge +e.
A neutron consists of 1 up quark and 2 down quarks, total charge 0.
Conservation of charge
5
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The total charge is conserved.
When we charge a rod, we move electrons from one place to another
leaving a positively charged object (where we removed electrons)
and an equal magnitude negatively charged object (where we added
electrons).
Point of interest:
¤ Detecting anti-neutrinos: ν e
+ p → n + e+
(p) and positron (e+) have the same positive charge.
¤ The anti-neutrino ( ν ) and neutron (n) are not charged.
e
¤ The proton
Example: 1
6
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How many electrons must be
transferred to a body to result
in a charge of q = 125 nC?
Conductors and insulators
7
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In a conductor charged particles are free to move within the object.
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Metals are good electrical conductors because typically the outermost electron (conduction
electron) from the atom disassociates from a particular atom and is free to move about the
metal.
In an insulator charged particles are not free to move within the object.
Semiconductors have properties intermediate between conductors and insulators;
their properties change with their chemical composition.
Photoconductive materials become conductors when light shines on them.
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Photocopier and laser printer use photoconductive material.
Quiz: 1
8
Demo: 2
9
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Electroscope
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A needle electroscope has a part of the conductor (the
needle) free to rotate.
Because like charge repels, the needle is repelled from the
rest of the conductor.
Conducting and Non-conducting “T”
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Demonstration of a conductor (aluminum) and an insulator
(plastic) using an electroscope
Quiz: 2
10
Induction
11
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If we bring a charged object next to a metal, the electrons in the
metal will either
¤ move towards
a positively charged object.
¤ move away from a negatively charged object.
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Due to conservation of charge, this must leave a positive charge
where the electrons have moved from.
Charging by induction
12
Demo 3
13
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Induction spheres
¤ Demonstration
induction
of charging a conductor by
Coulomb’s law
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The force by charged object 1 on charged object 2 is given by
where q1 and q2 are the charges on the two objects, k = 8.99×109 N m2/C2 (Coulomb’s constant), r12 is the
distance between the objects, and 𝑟̂#$ is a unit vector pointing from object 1 to 2.
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The forces on the two charges are action-reaction forces.
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Newton’s law of gravity (𝐹 = 𝐺
() (*
)
+*
and Coulomb’s law of electric force look similar.
Demo: 4
15
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Metal rod on pivot
¤ Demonstration
of attraction between a charged object of either sign and a
conductor due to induction in the conductor
Example: 2
16
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Two small spheres, each with mass m = 5.0 g
and charge q, are suspended from a point by
threads of length L = 0.30 m. What is the
charge on each sphere if the threads make an
angle θ = 20º with respect to the vertical?
Quiz: 3
17