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Electricity and Magnetism
Cheat Sheet
Table of Contents:
Charge
Vocabulary
Section Notes
Capacitors
Vocabulary
Section Notes
Electric Currents
Vocabulary
Section Notes
Series and Parallel
Vocabulary
Section Notes
Charge
Vocabulary
Ion ­ when an electron is removed from an atom (normal atoms have same number of electrons and protons)
Insulators ­ charge can be rubbed on or off their surfaces, but it tends to stick there and will not move easily
through the materials (examples: rubber, glass, plastic)
Conductors ­ electrons can move around freely and easily (examples: copper, silver, aluminum, and other metals)
Electricity ­ a flow of electrons through a conductor
Electron ­ a subatomic particle with a negative charge; they are mobile, can flow from one atom to another
Electrostatic ­ the charges are stationary
Coulomb ­ the unit of charge
Volt ­ Joule per Coulomb; the unit of electric potential energy (letter symbol: V, symbol: the source, ­| l­
[battery])
Net Charge ­ the sum of the negative and positive charges
Electrical Ground ­ a reservoir of positive and negative charges
Field Lines ­ give a very convincing picture of electric fields
Section Notes
● Like charges attract and unlike charges repel
● Two kinds of charges: negative and positive
○ Electrons have negative charge
○ Protons have positive charge
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○ Neutrons have neutral charge
A charged object will attract neutral objects
Electrical force grows weaker with distance
Electrons are elementary units of charge, and are transferred from one object to another
Coulomb’s Law: F = K (Qq / r2)
○ Q and q are the values of the charges
○ r2 is the distance between charges
○ k is a constant (in standard units, k = 9 x 109)
A single electron’s charge: ­e = 1.6 x 10­19 coulombs
Charge Conservation: the net charge in an isolated system cannot change
A charge fills space with an electric field
○ When another charge is in the electric field, electric forces act on it
Force exerted: work = force x distance
Electric Potential = Potential Energy / Charge
○ Units: energy per charge = joule per coulomb = volt
Gravitational Potential: the ability of the gravitational field to transmit energy to any mass with a height
h using the equation potential energy / mass = gravity x height
Capacitors
Vocabulary
Capacitor ­ consists of two conductors separated by an insulator (symbol: ­| |­)
Capacitance ­ is the constant of proportionality in Q = CV; number depends on characteristics of capacitor
Farads ­ the unit of measurement for capacitance
Discharging ­ neutralizing the plates of a capacitor
Electron Volt ­ the energy of one electron if the plates are charged to one volt (example: 100 V = 100 eV)
Section Notes
● Capacitors are charged by removing charge from one plate and placing it on the other
● When the switch is closed, electricity flows and charge is pumped into the capacitor
○ The amount of charge pumped is proportional to the battery’s voltage
○ Q = CV, or Charge = Constant x Voltage
● The higher the capacitance, the more charge the capacitor can hold
● Capacitance is directly proportional to area of plates
● Capacitance is inversely proportional to distance between plates
● When a capacitor is charged, positive and negative charges face each other and hold each other in place
across the insulator
○ There is also an electric field
● To convert electron volt to joules, use the definition of electric potential
○ 1 eV = charge of electron x 1 volt
○ 1 eV = 1.6 x 10­19C x 1 J / C
○ 1 eV = 1.6 x 10­19 Joules
Electric Currents
Vocabulary
Voltage Drop ­ the energy per charge that is going into heating and activating the item being powered (the
difference in potential)
Current ­ the flow of charge (letter symbol: i)
Ampere ­ Coulomb per second, the unit of measurement for currents
Conventional Current ­ the flow of positive charges from the positive terminal of the battery to the negative
Real Current ­ the flow of negative charges from the negative terminal of the battery to the positive
Ohm ­ unit of resistance
Multimeter ­ measures voltage, currents and resistance
Watt ­ voltage x current, Joule per second; the unit of measurement of power
Resistance ­ resistance to current (letter symbol: R, symbol: ­/\/\/\/­)
Section Notes
● Ohm’s Law: i = V / R, or Current = Voltage / Resistance
○ The higher the voltage, the more current flows through a given resistance
● Resistivity is high in insulators and low in conductors, and rises with temperature
● By definition, power = watt = Joule / second
○ Power = (Joules / Coulomb) x (Coulomb / second)
○ Power = volts x amperes
Series and Parallel
Vocabulary
Circuit ­ a closed path in which electricity can flow
Series Circuit ­ when the component are connected together one after the other
The Loop Theorem / Kirkhhoff’s First Law ­ the total energy consumed must equal the energy produced by
the battery
Parallel Circuit ­ when each component is connected directly to the battery
The Junction Theorem / Kirkhhoff’s Second Law ­ the current flowing into any junction in the circuit must
equal the sum of the currents flowing out
Section Notes
● In a series circuit, the sum of the voltage drops across the components must equal the battery voltage
○ Each component gets the same current
● In a parallel circuit, the current has to divide to flow through every component
○ The total resistance of the circuit is a fraction of the series circuit
○ So each component gets the same voltage, yet draws a current i inversely proportional to ir’s
resistance (i = V / R)
● In series, you just add the resistance to get the total resistance
● In parallel, you add the reciprocal of each resistor to get the total resistance