Transcript CH32 Electrostatics Wrap-Up

CH32 Electrostatics Wrap-Up
Physics (B)
Fall, 2010
Electric Charge-REVIEW
• Charged objects exert electric forces on each
other:
Opposite (+ & -) charges attract;
Like (+ & +) (- & -) charges repel.
• Like energy and momentum, electric charge is
a conserved quantity:
Charge cannot be created or destroyed,
but it can be transferred, or MOVED between
objects.
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Units for Electric Charge
• Electric Charge: Symbol = q
Metric Unit: Coulomb, C
• A Coulomb is defined as the number of electrons
passing through a current of 1 Amp each second:
C = A·s.
• The conversion between C and e was determined by
Robert A. Millikan:
1 e = 1.60×10–19 C.
• It is much easier to measure charge flowing in a
current than to count protons and electrons
individually!
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Coulomb’s Law
•
DO NOT WRITE THIS!!!!! States the force between
two charges varies directly as the product of the charges
and inversely as the square of the distance between
them*
*for charged particles that are small compared with the distance between them
•
Or, in terms of Math equation:
Fe = k q1q2
d2
(Eq. 7)
Where Fe = the Electric Force (N)
k = Coulomb’s Constant, = 9.0 X 109 Nm2/C2
q1 = charge of particle #1 q2=charge of particle #2
d2 = distance between two particles (m)
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Coulomb’s Law Example
Two pellets, each with a charge of 1μC, are separated by a distance of 0.30m. What is the electric
force, Fe, between the two pellets?
Step1: Given:
q1=1.0 X 10-6 C
k = 9.0 X 109 Nm2/C2
q2=1.0 X 10-6 C
d = 0.30m
Step 2: Unknown:
Fe=?
Step 3: Write Formula:
Fe = k q1q2
d2
Step 4: Plug-In Values:
Fe = (9.0 X 109) (1.0 X 10-6)(1.0 X 10-6)
Fe = 0.1N
(0.30)2
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Electric Charge
• There are three material types related to
Electric Charge:
1. Conductors-material where electric charge
flows easily
2. Insulators-material where flow of e- charge is
reduced or prevented
3. Semiconductors-material where flow can be
made to behave like BOTH (e.g. PV cells in
solar panels)
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Let’s Go Practice!