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![Page 1: 5.1 Electric Forces & Fields Chapter 18. The Origins of Electricity In the mid 18 th century Ben Franklin created the idea of positive and negative electric.](https://reader035.fdocuments.us/reader035/viewer/2022062511/551a771855034643688b52ed/html5/thumbnails/1.jpg)
5.1 Electric Forces & Fields
Chapter 18
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The Origins of Electricity
• In the mid 18th century Ben Franklin created the idea of positive and negative electric charge.
• It wasn’t until 150 years later the electron was discovered.
• Franklin described an electric “fluid” that would flow depending on electric pressure.
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Electric Charge
• In 1909 Robert Millikan discovered charge was “quantized.”
• This means there is a smallest amount.• Thing about it like this, In order to have a
car, all the pieces must be there. If you try to take the engine out, it’s no longer a car.
• Electric charge is much the same.• The electron has a set charge, take some
away and it’s no longer an electron.
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The Millikan Experiment
• Click here to recreate the Millikan oil-drop experiment
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Charged Particles
• Protons (+e): Mass = 1.673 x 10-27 kg, Charge = 1.60 x 10-19 C
• Neutron: Mass = 1.675 x 10-27 kg, Charge = 0
• Electron (-e): Mass = 9.11 x 10-31 kg, Charge = -1.60 x 10-19 C
e = 1.60 x 10-19 C
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Neutral Objects
• If the number of electrons equals the number of protons the object is said to be electrically neutral.
• In general q (charge) = Ne, where N is an integer.
• Since protons are much more difficult to remove, most objects are charged by removing or adding electrons.
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Charged Objects
• When two dissimilar materials are rubbed together, electrons usually go from one to the other.
• Look on the triboelectric scale to see which way they go, positive or negative
• Also, charge is conserved.
• The net charge of an isolated system is constant.
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Like and Unlike charges
• Like charges repel each other
• Unlike (opposite) charges attract
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Conductors & Insulators
• Materials that have loose valence electrons are conductors.
• Materials with tightly held valence electrons are insulators.
• Can you think of some?
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Charging by Induction (Conductors)
Click on the picture to open an applet
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Example Question
• Two separated, identical conducting spheres are charged with 4 C and -12 C, respectively. If the spheres are allowed to touch and then separated again, what will be the charge on each sphere?
Answer:• The net charge is -8 C. So each sphere
will have -4 C of charge.
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Polarization (Insulators)
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Electroscope
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Van De Graff Generator
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Coulomb’s Law
• F = 1/(4o) q1q2 / r2
– F = Force (N)
– o = 8.85 x 10-12 (electric permittivity of a vacuum)
– q = charge (Coulombs)– r = distance between charges
• 1/(4o) = k
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Point Charges
• When more than two charges are acting on each other we sum the forces.
• Treat each pair independently, then add the forces.
q1 q2 q3
r1 r2
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Point Charges in 2D
• When more than two charges are acting on each other in 2D, sum the forces for x and y dimensions.
• Again, treat each pair independently, then add the forces.
q1 q2
q3
r1r 2
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Electric Field
• Just like mass creates gravitational fields, charges create electric fields
• With gravity the field strength is measure as Newton per kilogram
• What do you think Electric fields are measured in?
Newton's per Coulomb
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Measuring the Electric Field
• If the unit is Newton's per Coulomb, what is the equation?
E = F / qo = k q/ r2
• Simple enough, right.
• E = Electric Field
• F = Force
• qo = charge producing field
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Summing electric Fields
• It is the surrounding charges that create an electric field at a given point in space.
• Look at Example 8 – Figure 18.18
• Two charged objects contribute as follows to the net electric at point P: Ea = 3.00 N/C directed to the right, and Eb = 2.00 N/C directed downward. What is the net electric field at P?
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Solution
• We use the electric field vectors to determine the resultant.
• Add the vectors with the Pythagorean theorem.
• And find the angle with arctan.
• E = 3.61 N/C @ 33.7o
EA
EB
E
+
++
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Electric Field Rules
• Fields start at positive and end at negative, or start or end at infinity.
• This is by convention. The field is said to predict the movement of a positive charge.
• The density of lines should represent the strength of the field.
• A positive charge will have a velocity tangent to a field line.
• Field lines do not actually exist since the are an infinite number of paths a test charge can take.
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Picturing the Electric Field
Click on the picture to open an applet
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Electric Fields Inside Conductors
• Excess electric charge moves to the surface of a conductor
• At equilibrium the electric field inside a conductor is zero– This comes from the fact that free
electrons will not move inside the conductor.
• So the electric field lines don’t penetrate the conductor
• The electric field outside a conductor is perpendicular to the surface
Click here for more info
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Triboelectric Scale• Human hands (usually too
moist, though) (Very positive)• Rabbit Fur• Glass• Human hair• Nylon• Wool• Fur• Lead• Silk• Aluminum• Paper• Cotton• Steel (Neutral)
• Wood• Amber• Hard rubber• Nickel, Copper• Brass, Silver• Gold, Platinum• Polyester• Styrene (Styrofoam)• Saran Wrap• Polyurethane• Polyethylene (like Scotch
Tape)• Polypropylene• Vinyl (PVC)• Silicon• Teflon (Very negative )