Download Magnetism

Electricity
• Electric Charge Rules:
–
–
–
–
More protons than electrons: + charge
More electrons than protons: - charge
Like charges repel; opposite charges attract
In electricity, ONLY e- move, not protons!
• Strength of electric force due to amount of
excess + or – charge
• Electric Field: region around charged particles
in which others will attract or repel
• Definition: the buildup of –charge on
an object
• Method of Charging:
– Friction-movement of electrons from
one objects surface to another
– Conduction-transferring electrons
from one object to another by direct
contact
• Ex. Many metals allow electrons to flow
Static Electricity cont.,
• Voltage– Energy or push that makes electrons move
– The h the voltage, the h energy each
electrons carries
– Measured in units called volts (V)
• Electric Current: flow of e- through a
wire
– Symbol for current is I
– Current is measured in amperes, or
amps (A) for short
• h current = h e-
Flow of Electricity cont.,
• Resistance: opposition to flow of
electricity
– Symbol for resistance is R
– Resistance is measure in ohms, which is
represented by Ω
– Poor conductors have high resistance (or
low conductivity)
• Ex. Iron
– Good conductors have low resistance (or
high conductivity)
• Ex. Copper
– Long and thin wires have more resistance
than short and thick wires
– Any device that you plug in is called a
resistor (load)
• Ohm’s Law:
– current in a wire =voltage/resistance (I=V/R)
• See Ex. Problems on pg. Ω
• 453
• Current Directions:
– Direct Current (DC): electrons flow in the same
direction
• Ex. Batteries (e- flow through and terminal)
– Alternating Current (AC): Electrons constantly change
their direction of flow
• Ex. Electricity from power plants (via generators)
• What’s a Circuit?
– Provides a complete, closed path for an electric
current to flow
– Circuit works when path is closed; doesn’t work when
path is open
• Parts of a Circuit:
– A source of e- (i.e. battery, outlet)
– A load (resistance) which is the device that uses the
electricity
– Wires carry the electrons
– A switch, which opens or closes the circuit
•Only one path for a
current to flow
•All parts of the circuit are
connected to one after the
other
–Ex. Old Christmas Lights
•Same current (I) passes
through each resistor
•Current stays the same
•Voltage drops after each
resistor
•Different parts of circuit
are on separate branches
•Several paths for
electrons can be takes
–Ex. New Christmas Lights
•Voltage stays the same
•Current decreases
through each branch
Electrical Power
• Measure of the rate at which electricity
does work or provides energy
• Power=voltage*amperes
– Units: Watts=volts*amperes
– 1 kilowatt (kW)=1000 Watts
• Electric Energy: energy=power*time
(E=P*t)
– Units: kilowatt-hours=kilowatts*hours
– Your electric meter at home reads in
kilowatt-hours; this number is multiplied by
cost/kilowatt-hr to determine your bill
Power Distribution Grid
• At the power plant:
– Generators produce 3-phase AC electricity
via generators (electromagnetic
conduction)
– Electric goes through a step-up
transformer, were voltage is increased for
travel over electric wires (transmission
substation)
– Voltage is usually stepped up to between
155,000-765,000 volts!
Power Distribution Grid cont.,
• Transmission of Electricity:
– High voltage lines are those huge steel
towers (3 wires for 3 phases+ground)
– Electricity usually travels up to 300
miles from power plant
• Power Substation: can do several diff.
things
– Have ‘buses’: split the power off into
many directions
– Decrease the voltage by using a stepdown transformer
• Before Going Into the House:
– Multi-phase wires are tapped down to
single phase (1 wire) @ 120 V
• You often see three wires going to houses
(single phase electric + ground)
– Transformer drum step-down 7200
volts to 240 volts for house
– Some neighborhoods have
underground wires, so you might see a
green transformer box (these are step
down voltage)
• At the House:
– Electric wires travel through watt-hour
meter (charges you for electric)
– Two 120-volts wires then travel to circuit
breaker (box in garage or basement),
where it’s split and wired to all of the outlets
in the house
Magnetism
Magnetism
• All magnets have North and South
poles
• Rule for magnetic poles: like poles
repel/opposites attract
• Magnetic Field=region in which
magnetic forces can act
– Earth has one around it; strongest at poles
– Magnetic lines of force define these areas
– Electrons in atoms spin, causing the
magnetic field
Magnetic Materials
• Naturally occurring (ex. Magnetite)
• Some materials that are non-magnetized can
become so: iron, nickel are examples
Earth as a Magnet
• Magnetosphere: region of magnetic field
around the earth
• Earth poles: Geographic north pole is actually
Magnetic south pole and vice versa
Compasses
• Needle of a compass is actually magnetic
• Compass needle points to north pole (magnetic)
Electromagnetism
• Electricity and magnetism are inseparable
• Electromagnet: made by winding wire around an iron core
– Will have N and S poles
– Polarity of magnet depends on polarity of electric current
• Magnetism can induce electricity
• Electricity can induce magnetism
Electromagnets
•Electric Wire wrapped around an iron core
•These are temporary magnets; can be turned
on and off
–Ex. Junkyards, doorbells, telephones, electric
motors
Electric Motor
•Converts electric energy to mechanical
energy (motion)
•Operates on the idea of reversal of polarity
•Uses a stationary permanent magnet, a
rotating electromagnet, and a polarityreversing switch (conductor)
Electric Generator
• Converts mechanical energy into electrical energy
• Loop of wire (conductor) spins inside a magnetic
field to create electricity; known as an electormagnetic induction
• Most of the power you use everyday comes from
generators
Galvonometer
• Device that detects electric current (basically a coil
of wire connected to an electric circuit and a needle)
• Current deflects needle towards direction of current