Download Electricity and Magnetism

General Physical
Science
Chapter 8
Electricity and Magnetism
Electrical Charge
Electrical
charge is one of the 7
fundamental quantities.
Two types of electrical charge
– Positive (+)
– Negative ((-)
– 3 types of subsub-atomic particles
Protons
(+)
((-)
Neutrons (no charge)
Electrons
Electrical Charge
The
magnitude of the proton charge
is equal to the magnitude of the
electron charge.
– Charges will exactly cancel.
Coulomb
– Unit of electrical charge
– +q = greater + charge = electron
deficient
– -q = greater - charge = electron rich
1
Electrical Force
Exists between two charged particles
Law of charges
– Unlike charges attract and like charges
repel.
Magnitude of the force = Coulomb’
Coulomb’s
Law
– The force of attraction or repulsion between
two charged bodies is directly proportional
to the product of the two charges and
inversely proportional to the square of the
distance between them.
Coulomb’
Coulomb’s Law
Mathematically
F = kq1q2 / r2
– F = Force (N)
– k = proportionality constant
9.0
x 109 N m2 / C2
Similar
to gravitational law,
however…
however…
– Much higher value for the constant.
– Based on charges, not masses.
– can be an attraction or repulsion
Static Charge
Excess or deficiency of electrons
– Excess; negative charge
– Defiency;
Defiency; positive charge
Walk across carpet in winter
Polarization
– separation of charge
– rubbing rubber balloon in hair
Induced polarization
– A charge causes charge separation
– reason balloon sticks to wall
2
Photocopier
Se drum + charge
Light reflected off paper
removes some of the
drum charge
Toner has a - charge
– sticks to + areas on
drum
Paper + charge gets
toner from drum
Toner ‘fused’
fused’ by heat
Electrical Charge and Electrical
Force
Learning
Goals
– Describe electrical charge in terms of
protons and electrons
– State similarities and differences
between Newton’
Newton’s Law of Universal
Gravitation and Coulomb’
Coulomb’s Law
Questions:
Problems:
11-4
1, 3
Electrical Current
Current
– Rate of flow of electrical charge
– Measured in Amperes (A)
1
A = 1C / 1s
I=q/t
General:
– I=Current
– q=charge
– t=time
q
=Ixt
3
Current
Conductors
– current flows readily
Does
not ‘flow’
flow’ in the traditional sense
– metals are good electrical conductors
– ‘loose’
loose’ electrons
Also
reason for good thermal conductivity
Insulators
– current d/n flow
– electrical wire coating
Current
Semiconductors
– intermediate between conductor and
insulator
– allows some (but not free) current flow
– graphite (resistors)
– Silicon/Germanium mixtures
computer
the
chips
‘semiconductor’
semiconductor’ industry
Example
A
current of 0.50 A flows in a wire
for 2.0 minutes. (a) How much (net)
charge goes past a point in the wire
in this time? (b) How many
electrons make up this charge?
What do we know?
– a) 0.50 A (C/s), 2.0 min (120 s); q = I
xt
– q = 0.50 C/s x 120 s = 60C
4
Example
60
C
Charge on a single electron is 1.6 x
10-19C
How many electrons = 60 C
– 60 C = x(1.6 x 10-19C)
– x = 60 C / 1.6 x 10-19C
– x = 3.8 x 1020 electrons
– 380,000,000,000,000,000,000
electrons!
Voltage
Greater separation
of charges means
more work
required to
separate more
charges.
Electrical potential
energy
– Voltage (potential)
– Work per unit
charge
– PE per unit charge
Voltage
Measure
of electrical potential is the
volt. (Work per unit charge)
–1 V = 1 J / 1 C
Measurement
of the ‘friction’
friction’ for
electrical current flow is called
resistance.
– Ohm - Ω
Ohms
Law - relationship between
voltage, current, and resistance
– V = IR
5
Current Flow
Voltage
- water pressure; Current =
water flow; Resistance = water
wheel
Current Flow
You
must have a complete circuit for
electricity to flow
By convention, current flow is the
way that positive charges would
flow.
– convention was developed before our
understanding of protons and neutrons.
– No practical difference because of this
convention.
Electrical Power
Power
= current x voltage
– P = IV
But
V = IR
– P = I2R
Heat
loss
– All metals have some resistance
Light
bulbs (watts)
– Lower T, lower resistance
– Superconductivity at very low
temperatures.
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Example
Find
the current and resistance of a
60W, 120V light bulb in operation.
What do we know?
– Power = 60W, Voltage = 120 V
– P = IV means 60 = I x 120
– I = 60/120 = 0.50 A
– P = I2R means 60 = 0.502 x Ω
– Ω = 60 / 0.502 = 240Ω
240Ω
Current, Voltage and Electrical
Power
Learning
Goals
– Define current.
current.
– Define voltage,
voltage, and state how Ohm’
Ohm’s
law relates it to current and resistance.
– Explain electrical power and identify the
parameters used to describe it.
Questions:
55-8
Exercises: 55-23 odd
Simple Electrical Circuits
Two
types of current
– Direct (battery)
– Alternating (wall outlet)
US
Two
ac typically 60 Hz
basic types of electrical circuits
– Series
– Parallel
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Series Circuit
Hooked up as a
chain
– Same current at all
points.
– Total resistance
sum of individual
resistances.
– Total voltage drop
sum of individual
drops.
– Rt = ΣRi
Parallel Circuit
Hooked up like a
rope.
– Voltage across each
resistance the
same.
– Current will vary in
each branch.
– 1/Rt = Σ(1/Ri)
– For two resistors:
Rt=
(R1R2) / (R1+R2)
Example
Three resistors have values of 6.0Ω,
6.0Ω,
6.0Ω, and 3.0Ω.
3.0Ω. What is their total
resistance when connected in parallel,
and how much current will be drawn
from a 12V battery if it is connected to
the circuit?
What do we know?
– 6.0Ω,
6.0Ω, 6.0Ω, and 3.0Ω
3.0Ω
– 1/Rt=1/R1 + 1/R2 + 1/R3 = 1/6 + 1/6 + 1/3
= 2/3
– Rt = 3/2Ω
3/2Ω
– V = IR means I=V/R = 12/1.5 = 8.0A
8
Parallel Circuits
Total resistance
always less than
the lowest
resistance
Household wiring
– all in parallel
– rest will work if one
fails
– Christmas lights
‘Shunt
resistor’
resistor’
Electrical Safety
Fuses
– Edison base
end
can
similar to lightbulb
switch fuse ratings
– TypeType-S
bases
different
switch fuse ratings
cannot
– Circuit breakers
– Fuses/CB always on ‘hot’
hot’ side
9
Electrical
Safety
Case
grounding
– ‘3rd’ wire
Polarized
Plug
Simple Electrical Circuits and
Electrical Safety
Learning
Goals
– Define voltage and state how Ohm’
Ohm’s
Law relates it to current and resistance
– Explain electrical power and identify the
parameters used to describe it.
Questions:
99-13
Magnetism
Lodestones
– 6th Century BC
– Magic
Artificial
Magnets
– Chinese
– Made from natural magnets
10
Magnetism
Two
regions of magnetic strength
– North pole (N) points north
– South pole (S) points south
Law
of Poles
– Like poles repel and unlike poles attract.
– Attraction/repulsion inversely related to
the square of the distance between the
poles.
Magnetism
All
magnets contain both N and S
pole
– Dipoles
Magnetic
field
– Imaginary lines indicating the direction
a compass would point if it were placed
near a magnet.
– Lines of force
Magnetism
Use metal filings to
‘see’
see’ magnetic
field.
– iron filings become
magnetized
induced
magnetism
Screwdrivers
– serve to show lines
of force.
11
Magnetism
Source
of magnetism is
moving/spinning electrons!
– Can generate magnetic field with
moving electric field
Strength
directly proportional to current
– Electromagnets
Electromagnetic Fields
Magnetism
Ferromagnetic
materials
– internal magnetic
domains
– Random
arrangement means
nonnon-magnetic
– Can induce
alignment with
external magnet
Tends
to randomize
with removal of
external magnet.
12
Magnets
– ‘Soft’
Soft’ iron temporary
magnetism
– ‘Hard’
Hard’ magnetic
material =
permanent magnet
‘Hard’
Hard’
iron
Co, some other
elements
Ceramics
Ni,
– Electromagnets
Magnets
Curie Temperature
– will convert a permanent
magnet to a nonnonmagnetic material.
Earth’
Earth’s magnetic field
– Proposed by William
Gilbert
– Aurora Borealis and
Aurora Australis
– Origin unknown
Theorize
due to Earth’
Earth’s
rotation
Earth’
Earth’s Magnetic Field
Approximates bar
magnet
Known to ‘shift’
shift’
with time
Magnetic and true
poles d/n coincide
– must apply
magnetic correction
for ‘true’
true’ direction
13
Isogonic Lines in Aviation
Magnetism
Learning
Goals
– State the law of poles and describe
the magnetic field.
– Identify the cause of magnetism, and
tell why some materials can be
magnetized and others cannot.
– Analyze some aspects of Earth’
Earth’s
magnetic field.
Questions:
1414-18
Electromagnetism
Interaction
of electrical and magnetic
effects.
– Moving electric fields generate magnetic
fields
– Magnetic fields deflect a moving electric
charge.
Telephone
receiver.
14
Telephone Receiver
Microphone
– Diaphragm that can
vibrate and change
resistance (carbon
mic)
mic)
Varies electric
current
Speaker
– electromagnetic coil
– current varies
strength
– diaphragm vibrates
Magnetic Force on a Moving
Electric Charge.
Motors and Generators
No
moving current - no force
current - force on wire
Moving
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Motors and Generators
Brushes
– reverse current
– cause attraction to
become repulsion
and repulsion to
become and
attraction.
Generators
Same basic
principle as motors
AC - rotate a wire
loop between
magnets
– Faraday
How do we
transmit?
Electrical Transmission
Transformers
– Cause change in
voltage
– windings
– iron core
Use to decrease
current
– Decreases I2R loss
16
Transformers
For
a transformer:
– V2 = (N2 / N1) x V1
N1
= number of windings in primary coil
= number of windings in secondary coil
V1 = voltage at primary coil
V2 = voltage at secondary coil
N2
Will
not work with dc
Electronics
Emission
and control of electrons
Television
Diodes and Transistors
Integrated circuits
Electromagnetism
Learning
Goals
– Identify some electromagnetic
interactions and applications.
– Distinguish between motors and
generators.
generators.
– Explain the principle and use of
transformers.
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Electromagnetism
Questions:
1919-24
Exercises: 25, 27
Key Terms; Matching, Multiple
Choice, and FillFill-inin-thethe-Blank
Questions; Visual Connection and
Applying your Knowledge
General Physical
Science
Chapter 8
Electricity and Magnetism
18