6/6/2014
Electricity
Electrostatics
CH 22 – Electricity at Rest
• Lightning
• Holds atoms and molecules together
– Chemical bonding
• Lights, heaters, fans…
• Computers, radios, cell phones - electronics
Electrical Charges
Electrical Charges
• Atoms have a nucleus of positively charged
protons and neutral neutrons surrounded by a
cloud of negatively charged electrons
• Charge is measured in coulombs (C)
• Protons and electrons have the minimum
amount of charge
• Atoms normally have a equal number of
electrons and protons and so are electrically
neutral.
• We only feel charge or see sparks when there
is an excess of electrons or protons
• +1.6 x 10-19 C and -1.6 x 10-19 C, respectively
• It would take 6.25 billion billion electrons to make one
coulomb
Electric Forces
• Like sign particles repel – electrons to
electrons or protons to protons
• Unlike sign particles attract – electrons to
protons of vice versa
Electric Field
• An electric charge alters the space around it
making an electric field.
• Other charges can be thought of as interacting with
that field
• The field magnitude is the electric force
/charge
E = FE/q
• Its direction is the direction a small positive test
charge would take
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Electric Fields
Electric Potential Energy (EPE)
• Electrical charges pick up potential energy
when you lift them in an electrical field, just as
they pick up potential energy when you lift
them in a gravitational field.
• By doing work (lifting) you give the charge
electrical potential energy
• Increase the field, or the charge, or the
distance and you increase EPE
Electric Potential Energy
Electric Potential
• A doubly charged object lifted the same
distance in an electrical field will have double
the EPE
• When dealing with many charges in an
electrical field it is convenient to consider the
electrical potential energy per unit charge
• Then at any location, the electrical potential
energy per coulomb will be the same
Electric Potential
Electric Potential
• Electric potential =
Electrical potential energy/charge
V = EPE/q
1 volt = 1 joule/coulomb
• Electrical potential and voltage are used
interchangeably
• A 1.5 volt battery gives 1.5 joules of energy to
every coulomb of charge passing through it
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Electric Potential
• You can turn the electric potential equation
around to solve for EPE
EPE = Vq
• So to have significant energy, you must have
both voltage and charge
• A high voltage generator with very low charge
will shock but not harm you
Electrical Energy Storage
• A capacitor is a device
with two metal plates at
different electric
potential which store
charge
• The battery does work
pulling electrons from
one plate to the other,
setting up charge
separation
Capacitor
• It stores electrical
energy in the
electric field
between the plates
• Found in photoflash
units and
defibrillators where
a rapid release of
energy is required
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