Electrostatics: Coulombs Law, Equilibrium, Electric Fields...the amount of electric charge lost by...
Transcript of Electrostatics: Coulombs Law, Equilibrium, Electric Fields...the amount of electric charge lost by...
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Electrostatics: Coulombs Law, Equilibrium, Electric Fields
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Atomic Structure
•Matter is made up of atoms.
Proton (positive charge)
neutron (neutral)
electron (negative charge)+
+
+
–
––
+
–
atom nucleus
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Atomic Structure and Charge
•# electrons = # protons neutral
•# electrons > # protons negative charge(gain of e-)
•# electrons < # protons positive charge
(loss of e-)
Charge is Quantized: charges must be whole numbers and cannot exist in fraction form.
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Triboelectric Series: Relative ranking of common materials based on their electron affinity
*electron donating materials (+, glass) * electron accepting materials (-, teflon)
• Glass
• Human Hair
• Nylon
• Silk
• Fur
• Aluminum
• Paper
• Cotton
• Rubber
• PVC
• Teflon
+ + + + +
+ + + +
+ + +
+ +
+
-
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Electric Charge is Conserved
•When two materials interact, the amount of electrons gained by one material must be equal to the amount of electric charge lost by the other material. • Net charge remains unchanged.
• Law the Conservation of Charge
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Charge and Mass of Atomic Particles
Charge Mass
•Electron -1.60 x 10-19 C 9.109 x 10-31 kg
•Proton +1.60 x 10-19 C 1.673 x 10-27 kg
•Neutron 0 1.675 x 10-27 kg
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Insulators
•Insulators: materials that do NOT allow electrons to flow through them easily.
• Insulators can be easily charged by friction as the extra electrons gained CANNOT easily escape.
• Some common insulators are glass, air, plastic, rubber, and wood.
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•Conductors: materials that allow electrons to flow through them easily.
• Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape.
• common conductors are copper, aluminum, gold, and silver.
Conductors
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Charging objects
Polarization: the process of separating opposite charges within an object, net charge of material remain the same
• positive charge becomes separated from the negative charge
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3 Ways to Charge an Object
1. Friction: rubbing two different materials together creates polarization
• objects made of different materials will hold onto their electrons with different strengths.
• As objects rub past one another the electrons with weaker bonds are “ripped” off of one material and collected on the other material.
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2. Induction - Bring a charged object (rod) close to a neutral one (ball) without contact
• electrons in the ball will be repelled by the rod leaving a positive side (polarization)
• The now positive side of the ball will be attracted to the negative rod
+ + + + +
+ + + +
- - -
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3 Ways to Charge an Object
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Induction is a temporary change if no grounding occurs
electrons are not transferred
Induction is a permanent change if grounding occurs
electrons are transferred
• charge induced is opposite
3 Ways to Charge an Object
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3. Conduction is a permanent charge with contact between 2 objects
electrons are transferred
• Charge conducted is the same
• After conduction the balls will repel each other
Lost
Gained El Electrons
3 Ways to Charge an Object
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What is grounding?
• Removing a static charge by producing a path to the ground
• Electrons move from a negatively charged objects to the ground until the object is neutral
• Electrons move from ground to neutralize positively charged objects
• The earth both accepts and gives electrons while remaining overall neutral
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Electric Charge: Section 2
• Symbol: Q or q
• Unit: coulomb (C)
• 1 C = the charge on 6.24 x 1018 electrons
Extra info to help you with problems
• 1 electron = -1.60 x 10-19 C
• A coulomb is a huge charge so charge is usually stated in µC (1x10-6 C) or nC (1x10-9 C)
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Coulomb’s LawCharged objects can exert a force on each other. This force can
be either attractive force or a repelling force.
3 factors affecting the magnitude of the force between two charged objects:
1) Charge on the objects (magnitude of charge)
2) Distance between objects
3) Material separating objects
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F: electrical force
Q1: charge 1
Q2: charge 2
d: distance between charges
k: constant depending on materials separating objects
- For air, k = 8.99 x 109 N·m2/C2, this is the value we will use for calculations
Coulomb’s Law
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How are charge and electrical force related?
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How is electrical force related to distance between the charges?
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When using this equation:
• A positive force (F) signifies repulsion• Both charges (Qs) must be positive or both negative
• A negative force (Fe) signifies attraction• One charge (Q1 or Q2) must be positive and the other
negative
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Sample Problemsa. What is the electrostatic force between
two objects, +13 μC and -22 μC which are 0.055m apart
(μC = x 10-6 C)
b. Is it an attraction or a repulsion?
Attraction (Q1 and Q2 are opposite signs)
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Problem Set 11) What is the electrostatic force between two objects of -14x 10-6 C and -22 x 10-6 C that are 0.86 m apart?
2) The electrostatic force between two objects is +6.4 N when the charges are -4.5 x 10-6C and -8.8 x 10-6C. How far apart are the objects?
3) A force of -1.1N is exerted between two charged objects when they are 43 cm apart. The charge on one object is -5.7 x 10 -6 C. What is the charge on the other object?
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Finding Net Force with more that one charge:Superposition
• See beige box 17 B on page 649 in your text book
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Final Electricity Test Includes
• Ohms Law
• Resistance Rules
• Equivalent (total) Resistance
• Complex Circuit Resolution
• Electrostatics Concepts
• Coulombs Law (force)