ELECTRIC FORCE AND WORK
Fields
• Electricity and gravity both act at distance
• Have regions (called “fields”) where they
– alter space
– change how objects within the field behave
• More mass = stronger gravitational field, and
more change caused by gravity
• More charge = stronger electric field, and greater
change caused by electricity
• Fields are vectors (with magnitude and direction)
http://physics.bu.edu/~duffy/py106/Electricfield.html
ELECTRIC FORCE AND WORK
• Electricity, like gravity, can act over distance
http://www.physicsclassroom.com/class/circuits/u9l1a.cfm
Gravitational force
Law of Gravitation: Gravity is directly
proportional to mass, and inversely proportional
to distance between objects. It always acts
toward center of mass (attractive)
• The more massive the object(s), the greater the
gravitational force
• The more distance separating objects, the smaller the
force
Newton’s Law of Universal Gravitation
F=G
F = force
.
M1 m2
d2
m1 = mass of one object
m2 = mass of second object
G = 6.672 × 10-11 N.m2/kg2
(G is a constant of gravity)
d = distance
Electric force
• Acts between objects with charge, just as gravity acts
between objects with mass
• Force can be either attractive or repulsive
• Lines of force drawn away from + charges and toward –
charges
• As though protons push away, and electrons attract
http://www.physicsclassroom.com/class/circuits/u9l1a.cfm
Electric charges
• Electrically charges form fields
• Charges either – or +. Identical charges repel;
opposite charges attract
• Amount of electric charge is measured in
coulombs
• 1 C = 6.25 * 10 18 electrons
http://demo.webassign.net/ebooks/cj6demo/pc/c18/read/main/c18x18_7.htm
Moving charges
• Moving a charge in uniform field
creates temporary imbalance in
placement of + and – charges, and
causes imbalance
• Greater imbalance (called
potential difference, or voltage) =
greater force:
– needed to hold charges in place, or
– they exert when return to their
balanced state
http://www.vislab.uq.edu.au/research/molecular_modeling/pic
s/DielectricBoundary/DielectricBadLinesFixed.png
Strength of electric field
• Place the letters of the
particles in order from
the one feeling the
strongest effects of
electric force to the
one feeling the
weakest effects.
Volts and voltage
http://www.klimaco.com/caraudiopages_pix/vcr1.gif
• In electric systems, potential
difference is “electric
potential” measured in volts
• Voltage is like force in
mechanical systems or
pressure in fluid systems
• Voltage causes charge to
move in electric systems. It
is the prime mover in electric
systems.
Current
• Flow of charge is
current
• Flows until + and –
charges equalized (no
potential diff.)
• Current measured in
amperes
• 1 A = 1 C/sec
http://www.can-do.com/uci/lessons99/electricity.html
Electric system components
• Voltage source
(battery, generator)
• Control element
(switch, rheostat)
• Load (light bulb, TV,
whatever uses
electricity)
• Conducting path
(wires)
• Path must be closed
for current to flow
http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEE
TS-Module-01-3-111-3-120.htm
AC and DC
• Direct current – moves in 1 direction. Battery is
voltage source fr. Negative cathode -> positive anode.
Small appliances
• Alternating current – oscillates (back and forth) in wire.
Generator is source. Cycle occurs 60 times/sec (60 Hz)
in US. This is frequency. Large appliances use AC
http://www.allaboutcircuits.com/vol_2/chpt_1/1.html
http://www.toptenz.net/wp-content/uploads/2010/09/DirectCurrent-Alternating-Current.png
Circuits
• Series (1 path for
electrons to take)
• Parallel (more than 1
path for electrons to
take)
http://www.physics247.com/physics-tutorial/series-circuits.shtml
http://www.allaboutcircuits.com/vol_1/chpt_5/3.html
Electric work
• Cannot see
electric work,
only results of it
http://www.ramelectronics.net/howto-pc-tv.ep
http://3.bp.blogspot.com/wgZ6IeuVmX4/Tb3VYpk0_fI/AAAAAAAAAZ8/US1X5RnRxTM/s1600/electroni
c+devices.jpg
http://sustainabledesignupdate.com/2006/09/start-saving-energy/
Electric work
• Measured in Joules
• 1 Joule = 1 volt * 1 Coulomb (J=1vq), how much
voltage makes given amount of charge move
• Efficiency of electric devices always = work out
work in
and is always less than 100%
• Energy not put to useful work is lost as thermal
energy
Electric rate
• Rate at which voltage
makes charge move is
current. Measured in
amperes (A or I)
• 1 A = 1 coulomb / 1
sec
http://www.allaboutcircuits.com/vol_6/chpt_2/4.html
http://www.equipmentexplained.com/phy
sics/electricity/basic_electricity/basic_elec
tricity.html#current
Coulomb’s Law
F = K . q1q2
d2
F = force
q = charge
d = distance
K = constant of charge
Electric forces either attract
or repel
http://www.jaiom18.com/wp-content/uploads/2010/08/Coulomb’sinverse-square-law1.png
http://guweb2.gonzaga.edu/faculty/cronk/chemistr
y/images/Coulomb_Law.gif
Coulomb’s Law
• What happens to electric force as charge
increases? As distance increases?
http://1.bp.blogspot.com/-nVsgQF0s4WQ/TWdHsobJ4zI/AAAAAAAAADY/tNC5JLsCSso/s1600/line.gif
Coulomb’s Law
• Electric force is directly
proportional to charge, and
inversely proportional to
distance between objects
(inverse square law)
http://academic.greensboroday.org/~regesterj/potl/E&M/EFields/super6fieldlines.PNG
http://www.kirksville.k12.mo.us/khs/Teacher_Web/alternative/electricfield.gif
Electric field lines
• Describe the lines of
force in each diagram
http://www.desktopclass.com/wp-content/uploads/2011/06/linespm.jpg
http://www.berkeleyscience.com/images/forcefields.gif