Transcript excess number of electrons creates a negative charge

Chapter 20 Electricity
Chapter 21 Magnetism
Prentice Hall 2006
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Assignments for Chapter 20
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Define vocabulary terms in 20.1-3 (24)
Write the key concepts in 20.1-3 (14)
611/1-3 Math Practice re Power
613/5-8
625/1-10(complete sentences), 14,15,17,18,27,29,30
Workbook pages: 239-248
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20.1 Electric Charge
and Static Electricity
A. Electric charge – an electrical property of
matter that creates a force between
objects.
*based on number of electrons and
protons in the objects
*excess number of electrons creates
a negative charge - atom gains
electrons
*excess number of protons creates a
positive charge - atom loses electrons
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Electric Charge and Force
srikant.org/core/node8.html
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Electric Force Fields
www.stkate.edu/physics/phys112/index.html
The strength of an
electric field
depends on the
a) Amount of charge
that produced the
field and
b) The distance from
the charge.
Potential difference
is the difference in
electrical potential
difference
between 2 places
in an electric field.
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20.1 Electric Charge and Static
Electricity, continued
*equal number of electrons and protons
creates a neutral charge
*unit of charge is a coulomb
>electron charge is –1.6 x 10-19 coulombs
>proton charge is +1.6 x 10-19 coulombs
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devices used to detect charges:
Van deGraaf Generators
Electroscopes
Pith balls
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Van de Graaf Generator
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•
www.columbia.edu/.../rce/main/demo/eandm.ht
ml
Electroscopes
physicslearning.colorado.edu/PIRA/PiraSubTOC....
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Pith Balls
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Discussion of prior slide
1) The neutral pith ball has equal number of (+) and (-) charges
2) The negative (-) charges on the pith ball are repelled away form the
rod. The positive (+) charges on the pith ball are closer to the
negative (-) charges on the rod than the pith balls positive (+)
charges. The pith balls positive charges are therefore attracted more
strongly than the pith balls negative (-) charges are repelled. Hence
there is an overall attraction
3) When rod makes contact, surface negative (-) charges are able to
move onto the pith ball, giving it an overall negative charge.
4) The pith ball is then repelled away from the rod.
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Static Electricity
• Study of the behavior of electric charges
(at rest) including how charge is
transferred between objects by
– Friction (like walking across a carpet)
– By conduction (contact)
– By induction (bringing a charged object
near a neutral object)
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Electrical Potential Energy
Potential difference (measured in
volts) between two points is what
causes electricity to move.
Lightning is a natural result of this.
Batteries provide a potential difference
this across their terminals and produce
direct current.
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DC circuit is produced by a battery.
Potential Difference (voltage drop) is
maintained across the + and - terminals
of a battery.
What is the difference between the
circuit at left and the one above?
The circuit at left is in series
The circuit above is in parallel
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20.2 For current to flow:
• You must have
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Source of voltage
Complete path or circuit
Conductor with low resistance
Device to use the energy (i.e., light bulb)
• Current flows from positive to negative
– This is considered conventional current
– Direction of positive charge movement that is
equivalent to actual motion of charge in the material
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Types of Current
• Direct current (DC) - flows in one direction
only
– Flashlight and car batteries produce DC
• Alternating current (AC) - flows back and
forth
– This is produced by an electrical generating
power plant that sends electricity to places
like businesses, schools, and homes
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Resistances
• Conductors have low resistances and
allow charges to flow easily
– Samples: metals
• Insulators have high resistances because
electrons are tightly bound to its atoms
– Samples: plastics, dry wood
• Superconductors have little or no
resistance below their critical temperature
– Samples: some metals (Nb, Sn, Hg,…)
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Ohm’s Law
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V = IR
V, potential difference in volts, v
I, current in amperes, a
R, resistance in ohms, W
* Resistance is affected by a material’s
V
thickness, length, and temperature.
*Increasing the thickness of a metal wire
I R
will reduce its resistance.
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Problem samples
Find the resistance in a circuit with an 8.0
volt battery and 0.2 amp flowing when the
current is on.
R=V
I
R = 8.0 v
0.2 a
R = 40. W
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• Practice Problems
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Practice Problems, answers
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1. R=V/I = 24v/0.80a = 3.0 x 101 W
2. R=V/I = 120v/0.50a = 240 W
3. V = IR = (0.50a)(12W) = 6.0 v
4. I = V/R = 1.5 v/ 3.5 W = 0.43a
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20.3 Circuits
• Two main types
– Series where there is one path for current flow
– Parallel where there is more than one path for
current to flow. Most circuits in your home are of
this type - parallel.
– Series
Parallel
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Sw.1
4
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Answers to the previous 4 slides
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Frame 25: no, no, yes
Frame 26: 1-abc; 2-ab; 3-a; 4-abc; 5-abc
Frame 27: 1-a; 2-c; 3-a
Frame 28: 4-d; 5-d; 6-c
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Electrical Safety
• These devices are used make
electricity safer:
– Circuit breaker (if too much current flows,
this opens the circuit - page 609)
– Fuse (if too much current flows, the wire in
this melts and opens the circuit - page 612)
– Ground-fault circuit interrupter (this
automatically opens the circuit if it senses
unequal currents - see page 613)
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Schematic Diagrams
• The figures on the prior slide are schematic
diagrams where symbols represent the parts of
a circuit. See page 374 in book.
lamp
Wires, Connected, Crossing
Wires, Not Connected,
Crossing
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Power = Current x Voltage
P = IV, unit: watt = amp volt
An electric space heater requires 29 amp of
120 v current to adequately warm a room.
What is the power rating of the heater?
P = IV
P = 29a (120v)
P = 3.5 x 103 watt
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Your assignment for 20.3
• 611/1-3 math practice
• 613/5-8
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20.4 Electronic Devices
• Electronic Signal information sent as
patterns in controlled
flow of electrons
through a circuit.
• Analog signals are
produced by
continuously
changing voltage or
current (page 619)
• Vacuum tubes can
be used to change
AC into DC, increase
signal strength, turn
current on or off.
• Semiconductors are
made from crystalline
solids and conduct
current under certain
circumstances
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Other electrical information
• Transistors are solid • A diode maintains
state components
proper voltage levels
with 3 layers of
in the circuits in a
semiconductors
mobile phone
• Transistors amplify • An Integrated Circuit
a mobile phone’s
is a thin slice of
incoming signal
silicon that contains
many solid-state
components
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21.1&2 Magnetism &
Electromagnetism
• Define vocabulary terms in 21.1-3 (15)
• Write the key concepts in 21.1-3 (10)
• 651/1-10 (complete sentences), 14, 15,17,
30
• Workbook pages: 251-258
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21.1&2 Magnetism &
Electromagnetism
Review:
1. Like magnetic poles _____
2. Unlike magnetic poles ____
3. Magnetic field lines run from ___ to ___
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answers
• Like poles repel
• Unlike poles attract
• Magnetic field flows from N to S
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Earth’s Magnetic Field
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Electricity can produce magnetism
and …
• Magnetism can
produce electricity!!!
• When electricity flows
through a conductor, a
magnetic field is
produced
• Right hand rule for a
straight conductor
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21.3 Electrical Energy Generation
and Transmission
• When a coil of wire moves in a magnetic
field, electricity will flow in the coil (aka:
electromagnetic induction)
• This is how our electricity is generated
• Faraday’s Law - an electric current can be
produced in a circuit by a changing
magnetic field.
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Right hand rule used in designing
and understanding of generators
I current flow
B magnetic field direction
F resulting force acting on wire
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Faraday’s Law
• Just by moving a magnet in a coil of wire
or [as in fig. (c), the switch is turned on or
off] electricity is produced. This is how
electricity is generated.
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Transformers
These change the voltage and current of electricity by having different numbers
of turns on the primary and secondary coils
Before electricity
gets to your house
it is stepped down
by a passing through
a step-down
transformer.
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More ElectroMagnetic Information
• The stronger the
magnetic force of a
magnet, the more
effect it has on
another magnetic or
on moving charges
• Magnetic fields are
produced by moving
electric charges
• AC and DC
generators are similar
except DC generators
use commutators to
produce direct current
• Heat from fossil fuels
spins a turbine (coil of
wire that spins in a
magnetic field) that
generates electrical
energy
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