Transcript Electricity and Magnetism Chapter 7 Section 1

Electricity and Magnetism
Chapter 7 Section 1
Chapter 8 Sections 2 & 3
Chapter 7 Section 1
Electric Charge
Positive and Negative Charge
Atoms have particles protons, neutrons, and
electrons
p+ = + charge no = no charge e- = - charge
Same # of p+ and e- = atoms are electrically
neutral
Transferring Charge
Some electrons are bound tighter to some
objects
e- bound to shoe more than carpet
Shoe gains e- = - charged
Carpet loses e- = + charged
Static Electricity: accumulation of excess
electric charge
Conservation of Charge
Def: charge can be
transferred from
object to object, but it
cannot be created or
destroyed.
Charges Exert Forces
Clingy clothes from a
dryer
Opposites attract
Force depends on
distance and size of
charge
Positive clothes like
negative clothes
Like charges repel
Opposite charges attract
Like charges repel
Electric vs. Gravitational Force
Electric force from p+ and e- in H atom is 1039
times stronger than gravitational force
Most things gravity force is stronger than
electrical
Conductors and Insulators
Conductor: material where e- move easily
Metals best conductors – copper
Insulators: material where e- do NOT
move easily
Plastics, wood, rubber, glass
Charging Objects
Rubbing materials can transfer e- - one is
(+) the other is (-)  charging by contact
Charge by Induction
Def: rearrangement of electrons on neutral
object by nearby charged object.
Lightning
Static discharge: transfer of charge
because of a buildup of static electricity
Clouds have movement of air and rain 
areas of + and – charge build
Lightning is discharge between cloud and
ground
Thunder
Lightning rips e- from atmosphere and
produces heat increase 25,000oC
Atoms expand quickly and release sound
waves
Earth is struck 100 times every second
Grounding
Harmful effects from charge build up
If there is a path to earth – relieves build
up
Grounded – it will transfer any excess
charge to earth (big neutral object)
Electroscope
Chapter 8 Section 2
Electricity and Magnetism
Basics of Magnets
Magnetic Field: exerts a force other
magnetic objects
Magnetic Poles: where magnetic force is
strongest
Field goes from North to South
Earth’s Magnetic Field
Compass
Compass needle is small magnet – lines up with
earth’s magnetic field
From north magnetic pole to south magnetic
pole
Electric Current and Magnetism
Moving electric currents create magnetic
field in direction of current
Current increases = increase magnetic
field
Electromagnets
Def: temporary magnet made by wrapping
a wire coil carrying a current around an
iron core
Magnetic field in coil is stronger than
straight wire
Solenoid: single wire wrapped in
cylindrical coil
Electromagnets
Solenoid
Electromagnet
Properties of Electromagnets
Temporary b/c when current stops –
magnet stops
Increase strength – more turns in solenoid
or more current
Useful b/c magnetic strength can be
changed – used for work – motors and
speakers
Making Sound
CD changing musical
info  changing
electrical current
Electromagnet is
attracted or repelled
by permanent magnet
Moves speaker cone
to make music
Rotating Electromagnets
Reverse current to change poles 
electromagnet rotates
Galvanometers
Def: use electromagnet to measure
electric current
Used in gauges in cars
Electric Motors
Def: device that changes electrical energy to
mechanical energy
Simple Electric Motor
3 main parts: wire coil, permanent magnet and
battery
Also have brushes – conducting pads connected
to battery and a commutator – split conducting
metal ring
Spinning Motor
Commutator rotates with coil as current runs through
Step 1:
Step 2:
brushes not in
contact with commutator
Inertia keeps coil spinning
Spinning Motor
Step 3:
commutator reverses
current – keeps spinning
Step 4:
commutator
reverses current
again - keeps
spinning
Chapter 8 Section 3
Producing Electric Current
Mechanical to Electrical Energy
Moving wire loop through magnetic field 
electric current
Moving magnet through wire loop 
electric current
Electromagnetic induction: creating a
current by changing magnetic field
Generators
Def: uses electromagnetic induction to transform
mechanical energy to electrical.
Move a coil in a permanent magnet
Switching Current Directions
Each half-turn poles of coil switch  current
switches direction
Coil keeps rotating current periodically changes
direction
US generators rotate 60 times/sec = 3,600
rotations/min
Using Generators
This type of generator is used in cars =
alternator
Lights, accessories, spark plugs
Running engine turns coil
Electricity to Your Home
Huge generators in power plants rotate the
permanent magnet and the coil stays still
Electricity is generated in the coil
A Turbine: (large wheel rotates when pushed by
water, wind or steam) spins the magnet
Some use thermal energy – heat water – turn
turbine (mechanical energy) – spin generator
(electrical energy)
Pg. 241
Direct and Alternating Currents
Current in battery vs. electric generator
Battery – Direct Current (DC): flows in
one direction through wire
Wall Outlet – Alternating Current (AC):
reverses direction of current in a regular
pattern
60 cycles/s = 60 Hz  changes direction
120 times each second
Transmitting Electrical Energy
Electricity to your house
Goes through power lines – energy 
heat due to resistance in wires
Reduce heat/resistance  send at high
voltage 150,000 V
BUT can’t use that high of voltage  goes
through transformer
Transformers
Def: increases or decreases the voltage of an
alternating current.
Has primary coil and secondary coil wrapped
around same iron core
Current goes in primary coil  magnetizes core
As AC current changes direction the poles of
magnet change
Changing poles  creates electric current in
secondary coil
Types of Transformers
Step-Up: increases voltage – outgoing is higher
Primary coil smaller than secondary
Step-Down: decreases voltage – outgoing is
lower
Primary coil is larger than secondary
Transmitting AC
Generator  step-up tran.  high voltage
lines  step-down tran.  house