Download Electricity Study Guide

Electricity Study Guide
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Static electricity​: A stationary electric charge (remaining in one area, as opposed to
current electricity)
○ Positive or negative charge assigned arbitrarily by Ben Franklin - glass has a
positive charge when rubbed and plastic/amber has a negative charge when
rubbed
Rules of charge
○ Likes repel, unlikes attract
○ No net charge can be created or destroyed
On the atomic level:
○ Negative net charge means an excess of outer electrons
○ Positive net charge means a deficiency of outer electrons
○ Static charge​: Result of a transfer of outer electrons between materials (not
continuous)
○ Side note: Water molecules are polar, and their charge is not distributed
uniformly. Thus, on humid days, charge can “leak” due to the attraction of extra
electrons to the positive ends of water molecules and water’s neutralization of
positively charged objects. Thus, static electricity is more noticeable on dry days
with less water molecules in the air
Materials:
○ Conductors​: Materials with lots of loosely bound electrons free to move about
within material so electricity can run through - include metals, aqueous salts,
acids/bases
○ Insulators​: Materials with tightly held electrons not free to move, electricity can
not run through - include wood, glass, plastic, nonpolar covalent molecules
○ Semiconductors​: Have a few loose electrons - include Silicon, Germanium
doped with arsenic, Boron, Antimony
Charging an object
○ Rubbing two objects together transfers electrons from one to the other
○ Can bring neutral object in contact (conduction) or in close proximity (induction)
with a charged object, in both cases no charge is created/destroyed
■ Conduction​: Physical contact with a charged object allows the transfer of
electrons to/away from neutral object
● Net charge acquired by the neutral object is equal to that of the
charging object
■ Induction​: Close proximity to a charged object causes the separation of
charges in a neutral object
● Net charge acquired by the neutral object is opposite to that of the
charging object
● Can create a net charge same as that of the charging object if the
object is grounded (connected with a conducting wire to the
ground)
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Kq q
Coulomb’s Law​: F E = | r12|| 2 |
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F​e​ = Electrostatic force between charges
K = 9*10​9​ Nm​2​/C​2
q = Charges measured in coulombs (C)
■ Charge of a proton: 1.6*10​-19​ C
■ Charge of an electron: -1.6*10​-19​ C
○ r = Distance between charges (m)
Types of circuits:
○ Series​: Current follows one path
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Paralle​l: Circuit divided into two or more paths
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Combination​: Series in parallel or parallel in series (pictured)
Current​: The amount of charge to pass a given point per unit time, measured in
coulombs/second or amperes (A)
○ I = Q/t
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I = Current in amperes (A)
Q = Amount of charge (C)
● Lowercase for static charge, uppercase for flowing charge
■ t = Time (s)
Resistance​: A measurement of the difficulty in forcing electric current through a circuit impedes charge flow and is measured in ohms (Ω)
ρL
○ R= A
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R = Resistance, measured in ohms
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■ ⍴ (The Greek letter rho) = Resistivity, measured in Ohm*meters (Ωm)
■ L = Length in meters (m)
■ A = Cross-sectional area in square meters (m​2​)
In a series circuit, Req = R1 + R2 + ...
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■ Resistances in circuit all add up to equivalent resistance
In a parallel circuit, R1eq = R1 + R1 + ...
1
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2
Reciprocals of resistances add up to reciprocal of equivalent resistance
*If the circuit is parallel in series, first use reciprocals to find the resistance of the
“parallel” part, and add it to the overall “series” part. If the circuit is series in parallel, first
add to find the “series” resistance, and then find the resistance of the overall “parallel”
circuit using the reciprocals
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Voltage​: Electromotive force, or differences in electric potential energy measured in
volts (v), which are joules/coulomb (J/C)
○ Ohm’s Law​:​ R = ΔV
I or ΔV = I R
■ R = Resistance in ohms (Ω)
■ ΔV = Voltage in volts (v)
■ I = Current in amperes (A)
Kirchhoff’s Rules​:
○ Junction rule​: In series circuits, current is the same throughout. Charge is not
gained or lost, so I = I 1 = I 2 = I 3 ... . In parallel circuits, current is additive, so
I = I 1 + I 2 + I 3 ...
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Loop rule​: In series circuits, the voltage of each part of a circuit (each voltage
drop) adds up to the battery’s voltage. Energy is conserved, so
ΔV = ΔV 1 + ΔV 2 + ΔV 3 ... . In parallel circuits, voltage is the same throughout, so
ΔV = ΔV 1 = ΔV 2 = ΔV 3 ...
*Similar to calculating resistance, for combination circuits, individual “series” or “parallel”
parts should be considered. Practice doing this!
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Power​: The rate at which work is done, affecting the brightness of a bulb. Measured in
J/s, or watts (W)
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P = I 2 R , Can be manipulated using Ohm’s Law into P =
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P = Power in watts (W)
I = Current in amperes (A)
R = Resistance in ohms (Ω)
ΔV = Voltage in volts (v)
△V 2
R
or P = I △V
Equations to know:
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Kq q
Coulomb’s Law: F E = | r12|| 2 |
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Current: I = Q/t
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Resistance: R =
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Voltage (Ohm’s Law): R =
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Kirchhoff’s Rules: I = I 1 = I 2 = I 3 ... ​and​ ΔV = ΔV 1 + ΔV 2 + ΔV 3 ... (In ​series​),
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ρL
A
, Req = R1 + R2 + ... (In ​series​),
ΔV
I
1
Req
=
1
R1
or ΔV = I R
I = I 1 + I 2 + I 3 ​and ΔV = ΔV 1 = ΔV 2 = ΔV 3 ... (In ​parallel​)
Power: P = I 2 R or P =
△V 2
R
or P = I △V
+
1
R2
+ ... ​(In ​parallel​)