Download Electricity and Magnetism

Electricity and Magnetism
I.
Electric Charges:
A. Atom review: Protons (+), Neutrons (0), and Electrons (-) charged.
B. Atoms are electrically neutral (# protons = # electrons).
C. Ions – charged atoms.
D. Charge rule: Opposites ATTRACT, likes REPEL.
E. Net charge: (+) or (-) charge remaining after other charges cancel.
II.
III.
IV.
V.
VI.
Static Electricity:
A. Static electric charge – net (+) or (-) charge on a body. It can be
transferred.
B. 3 ways static charge is acquired: friction, conduction, &
induction.
Friction: one body LOSES electrons, the other GAINS them when
objects are RUBBED together.
Conduction: transfer of charge by DIRECT CONTACT.
A. Charge flows from (-) to (+).
B. Neutral objects acquire static charge of charged object.
C. Static “fly away” static hair is caused when hairs become (-)
charged and REPEL because they are of like charge.
Induction: the rearrangement of charge on a body due to a NEARBY
charge. Electrons move toward (+) object. **Charge flows from (-)
to (+)!
Voltage (Potential Difference): tendency of a charge to move
between 2 regions.
A. Greater charge = larger potential difference.
B. When potential difference exists between 2 regions, charge flow
occurs.
C. Example: Lightning occurs when charged clouds (by friction)
induce an opposite charge on the ground and a potential difference
exists between the 2 regions and a charge flow (lightning) occurs.
VII. Electric Current: sustained charge flow.
A. A current measures the amount of charge (rate) passing a point in
a given time.
B. Conductors - substances through which electrons flow easily.
C. Volt – unit of potential difference / voltage.
D. A voltage source such as a battery / dry cell maintains potential
difference.
E. Dry cell vs. wet cell
F. Amperes (amps) – unit of measure of current.
G. Ammeter – instrument to measure current.
H. Circuit – path through which charge flows. To sustain an electric
current you need:
1. voltage source (battery)
2. conductive path (wire)
3. CLOSED circuit
VIII. Electrical Circuits
A. Series - current has only one path to travel.
1. If one part is disconnected, circuit is OPEN (lights go out).
2. Series circuit diagram:
B. Parallel - current has more than one path it can travel.
1. If one part is disconnected, circuit is still CLOSED (lights
remain lit).
2. Parallel circuit diagram:
IX.
X.
XI.
KEY:
Bulb
Conductor (wire)
Battery
Insulators – substance through which charge flow is difficult (rubber,
glass, wood).
A. Semiconductor – acts as either a conductor or an insulator (ex:
computer chips).
B. Air is fair insulator but not when ARCING occurs:
1. Arcing – electrons break free and create a conducting path.
2. Examples:
a. Getting “shocked” by doorknob: the potential difference
between hand / knob is large and the electrons arc across air
space.
b. Lightning.
How potential difference, current, resistance relate:
A. Largest voltage (pot. Diff.) = largest current.
B. Circuit with a resistor = smaller current.
C. Resistance – affects current, measures how difficult it is to move a
charge through a substance.
1. Resistor – anything that impedes (slows) charge flow by
converting electrical energy to other forms of energy like
heat, light, or KE.
2. Shorter wires have LESS resistance, thinner wires have
MORE resistance
D. Current depends on voltage and resistance of medium.
Ohm’s Law – current is directly proportional to potential difference
(voltage).
A. Doubling voltage, doubles current and doubling resistance cuts
current in half.
B. Formulas:
I=V/R
I = current (unit – amps / A)
V=IxR
V = voltage / pot. Diff. (unit – volts /V)
R=V/I
R = resistance (unit – Ohms / )
C. Examples:
I=V/R
V=IxR
R=V/I
I = 12 V / 6 
V = 12 A x 2
R = 15 V / 3A
I = 2 amps
V = 24 volts
R=5
XII. Superconductors – conductor with zero resistance to the flow of
electrons.
A. Delivering electrons through superconducting lines would save
money.
B. Uses of superconductors: super computers, electricity
transmission, and electric trains.
XIII. Alternating and Direct Current:
A. Direct Current (DC) – electrons move in one direction in the
current. Ex: battery-operated appliances.
B. Alternating Current (AC) – electrons constantly reverse directions
in the current. Ex: electric appliances.
1. Flat prong in plugs carries AC.
2. Third prong (round) is the ground – prevents shock by directing
current to the Earth.
XIV. Electric Power / Cost of Energy:
A. Power: Rate at which electrical energy is converted into other
forms of energy. The unit of power – watt.
B. Formulas:
1. P = I x V
P = Power (watts)
2. I = P / V
I = Current (amps)
3. V = P / I
V = Voltage or potential difference (volts)
C. The cost of Energy:
1. Formula: E = P x t
E = Energy (KWH)
P = Power (watts / KW)
T = Time (hours-H)
2. 1000 watts = 1 kilowatt (KW)
3. ***To calculate the cost of energy:
a. Convert power to kilowatts: P(KW) = P(W) / 1000
b. Find the energy used: E(KWH) = P(KW) x t(H)
c. Calculate cost: cost = E(KWH) x Rate (cents/KWH)
d. Example: A 500 Watt appliance is used for 12 hours.
Calculate the cost of energy if the rate is $ .08 / KWH.
a. P = 500 watt / 1000 = .5 KW
b. E = P x t
E = .5 KW x 12 H
E = 6 KWH
c. Cost = E x R
Cost = 6 KWH x $.08 Cost = $ .48
XV. Magnets/ Magnetic Fields/ Uses
A. Magnetic domains- atoms behave like tiny magnets,
each with a north and south pole
B. Compass needles are small thin magnets and always
point one end north.
C. Like poles repel
D. Unlike poles attract
E. If a magnet is cut in half, the ends become new north and south poles.
F. Magnetic fields
1. the region around a magnet where magnetic forces act.
2. the field is strongest around the ends of the magnet, and these are
the areas call magnetic poles