Transcript Electricity and Magnetism

Electricity and Magnetism
The Sciences chapter 5
Electricity
• If you walk across a rug and pick up a
charge from the carpet, are you negatively
or positively charged?
Electricity describes charged particles
• Rub a balloon with wool,
picks up electrons
• Rub glass with silk, loses
electrons
• Objects attract or repel
when “electrically charged”
• “static electricity”
Figure 5-1
The two kinds of electrical charges. Opposite charges
attract, while like charges repel.
Electricity describes charged particles
• These charged particles can be at rest
(“static electricity”)
• or they may be moving
(“current electricity”)
Ben Franklin & Electric Charge
• 1746, 1st to use
“negative” and
“positive” in e’statics
• 1752 famous kite
experiment
• Invented lightning rod
Lightning
• Result of charges that
become separated in
thunder clouds
• Negative charge on
cloud induces positive
charge on ground
• Discharge causes
lights and heats air
The movement of electrons
• What were Franklin’s “negative fluids”?
• Need to know atomic structure to
understand electricity
Draw an atom. Label its parts.
What do the different parts do?
• Protons
• Neutrons
• Electrons
• Every atom has a
positively charged
nucleus surrounded by
negatively charged
electrons.
• The electrons of all
atoms are identical.
Each has the same
quantity of negative
charge and the same
mass.
• Protons and neutrons
compose the nucleus.
Protons are 1800X
more massive than
electrons, but carry an
amount of positive
charge equal to the
negative charge of
electrons
• Neutrons have slightly
more mass than
protons and have no
net charge.
Static Electricity
Coulomb’s Law
• The electric force between two charged
particles varies directly as the product of
their charges and inversely as the square
of the separation distances.
• force (newtons) = k x 1st charge x 2nd charge / distance2
Coulomb’s Law
• …observed that if 2 electrically charged
objects are moved farther away from each
other, the force between them gets smaller, just
like gravity.
• If the distance between the 2 objects is doubled,
the force decreases by a factor of 4.
• (inverse square relationship)
Electrical Field
• A kind of “aura” or “force field” around
every electric charge
• Extends radially away
from the proton and
in opposite direction
about the electron.
…distinguish between
conductors and insulators
• Conductors
• Insulators
• Material that electrons
are able to pass
through
• Metals, ionic solutions
• Material that electrons
do not through easily
• Glass, wood, rubber,
plastics
Electrical potential and electric current
• Movement of electric charge creates electric
current
• Charges move as current only when energy
is supplied to them
• Circuit
• Switch
• Voltage
• Current
If we use the water analogy…
• Voltage = the pressure
• Current = the rate of flow
Voltage
•
•
•
•
•
The push that makes electrons move.
1.5 V “D” cell or 6 V lantern battery
Higher voltage = greater push on electrons
Water analogy
Voltage causes current.
Electric Current
• Voltage creates current
• Current is the amount of charge passing a
point in a circuit in a second
• Metric unit = Ampere (A)
• Measures by an ammeter
• Different devices often carry different
amounts of current
…distinguish between
DC
and
AC
• Direct current
• Current moves in one
direction
• From dry cells or
batteries
• Alternating current
• Pumped to us by Cobb
EMC
• Oscillates back and
forth at 60 Hz
• wall sockets
Ohm’s Law
• How is current related to voltage?
• Direct relation between the two led to
discovery of “resistance”
• Voltage / Current = Resistance (V / I = R)
Resistance
• measures how hard it is for current to move
through a conductor (unit = Ohm).
• Easier for electrons to move through thick
wires than thin wires
• Light bulb filament, thin, high resistance,
heats up and glows
…distinguish between
Series Circuits & Parallel Circuits
• Only one path for
current flow
• Same amount of
current thru entire
circuit
• Cheap string of
decorative lights
• Alternate paths for
current flow
• Current divides up
among the paths
• Wiring system for
lights and elec outlets
in homes & buildings
Electrical Safety
•
•
•
•
Fuses
Circuit breakers
Ground-fault interrupter
“Atoms Family” ElecSafety link
Electric Power
• Power = energy used / time
• Also calculated as product of current and voltage
• Watts = amp’s x volts
• Ex: 60W bulb draws .5 A on a 120V line
120W lamp draws 1A on a 120V line
 If a 120V line to a socket is limited (by a fuse)
to 15A, will it operate a 1200W dryer?
Solution…
• If a 120V line to a socket is limited (by a
fuse) to 15A, will it operate a 1200W dryer?
• 1200W = A x 120V
A = 10, yes
Review Questions from chap 5:
Electricity and Magnetism
• Try discussion questions 1-11, p. 114.
• Problems 2,3,5,6,7, p. 114.
• Answers are posted in study guide.