Download Unit 7: Electrostatics and Electric Fields

Unit 7: Electrostatics and Electric
Fields
Electrostatics
A. Background
• The study of an objects electric
charge, field, and its potential
–Electronsnegative charges
that move/transfer
between objects
B. The structure of Matter
• Atoms make up ___________
• Protons and Neutrons are in the
_______________
• Electrons are in the electron cloud and
have the freedom to ________________
C. Quantities of Charge
• Electric charge, q, is a _____________________
• q is used to measure both the charges of
____________ and ___________
• Units for q is a coulomb (C )
• On ref table page 1  _________________ is
the charge of a proton or electron
+1.60 x 10-19 C is the charge of _____________(positive
charge)
-1.60 x 10-19 C is the charge of _____________(negative
charge)
• Other quantities to know:
– 1.60 x 10-17 C is the charge of 100 protons
– 2.4 x 10-19 C is the charge of 1.5 protons; this is
impossible because you cant split a proton in half!
• 1 coulomb (C ) =
____________________________________
D. Charged Objects
• Objects can gain or lose _____________
(negative charges) ________:
– Lose electrons- becomes more __________
– Gain electrons- becomes more __________
• Same charges _________ each other
• opposite charges _________ each other
E. Transfer of Charge
• When objects rub against each other electrons
are transferred from one object to the other
– Think about the balloon/sweater/wall example
from PHeT
• The flow of electrons goes from the
_________________________ object to
the more positive object
– This flow of electrons causes the overall
charges after the objects come into
contact to be ________________
Practice Question:
Oil droplets may gain electrical charges as they
are projected through a nozzle. Which quantity
of charge is not possible on an oil droplet?
1) 8.0 × 10-19 C
2) 4.8 × 10-19 C
3) 3.2 × 10-19 C
4) 2.6 × 10-19 C
Practice Question:
When a neutral metal sphere is charged by
contact with a positively charged glass rod, the
sphere
1. loses electrons
2. gains electrons
3. loses protons
4. gains protons
F. Law of Conservation of Charge
• Charge is transferred from the more negative
object to the more positive object until they
are both ________________________
+6 μC
+2 μC
G. Coulombs Law
• This law shows the relationship between
charges, the distances between them, and
the force they exert on each other
• F=
• Based on the equation for Coulombs Law:
– What is the relationship between force and charge?
– What is the relationship between force and
distance?
• In Coulombs law, the charges of the particles
determine if they will be attracted or repelled.
– Make sure you keep track of the charges:
• Protons are positive
• Electrons are negative
– If there are 2 like charges, there is a
_________________. The 2 particles are repelled
away from each other
– If there are 2 opposite charges there is an
_________________________. The 2 particles are
attracted towards each other
Example:
• A point charge A of +3x10-7 coulomb is placed
2.0x10-2 meter from a second point charge B of
+4x10-7 coulomb. Calculate the magnitude of the
electrostatic force that charge A exerts on charge
B.
Example:
• Calculate the electrostatic force that a small
sphere, A, with a net charge of +2microcoulombs
exerts on another small sphere, B, with a charge of
-3microcoulombs when the distance between
their centers is 10 meters.
H. Conduction vs. Induction
Conduction- direct contact between 2
objects with a transfer of charge
Charge on the rod will equal the new charge
of the object
Diagrams:
I. Induction
Induction- Transfer of charge without
direct contact
Objects need to be grounded
Diagrams:
Lightning
Induction
Electric Fields
A. Background
Electric fields- the regions around charged
particles where a force is exerted
Electric Field LinesImaginary lines along which a positive charge
would move
Starts at positive points and ends at negative
points
The closer to a point charge, the closer the force
1. Individual point
charges
2. Fields between a positive
and a negative charge
3. Fields between 2
negative charges
4. Fields between 2
positive charges
B. Parallel Plates
Field lines will always be parallel when
talking about parallel plates
If the plates are close together:
The electrostatic force, and the electrostatic field
strength will be the same at any point
A
B
A and B have the same force and field strength
C. Electric Field Strength
The strength of an electric field can be
determined using the formula:
E= Fe/Q measured in N/C
E= electric field strength
Fe= electrostatic force
Q= charge of particle moving in the field
Calculate the magnitude of the electric
field strength at a point in a field where an
electron experiences a force with a
magnitude of 1.0x10^-15 newtons
What is the magnitude of the electrostatic
force experienced by one elementary
charge at a point in an electric field where
the magnitude of the electric field
strength is 3.0x10^3 newtons/coulomb
D. Potential Differences
Potential difference (voltage)- the work
done to move a particle/charge
V=W/q
V= voltage (potential difference in volts,
J/C or eV/C)
W= work (in Joules, J)
Q= charge (in coulombs, C)
Voltage is created when an electron is
repelled through the electric field to
move away from the negative particle
Electronvolt (eV)- the amount of work,
or gain in potential energy
1eV = 1.6x10^-19J
Moving a charge of 3.2x10^-19C between
points A and B in an electric field requires
4.8x10^-18J of energy. Calculate the potential
difference between these points.
A helium ion with a charge of +2e is accelerated
by a potential difference of 5x10^3 volts. What
is the kinetic energy acquired by the ion?
Hint- PE and KE can be the same as work
E. Current
The amount of charge that flows through
a wire
∆𝒒
I=
𝒕
I= current (C/s or A)
Q= charge (C )
t= time (s)
Ammeter- measures current, and is
used to read amps
Symbol for an ammeter:
A
1. Conditions for an electric
current:
There must be a potential difference,
meaning a charge from either a cell or a
battery
Currents move from positive to negative
potentials
Symbols for potential differences:
Cell
Battery
Voltmeter
V
RB p124 #29, 30, 31
29. A total of 20 coulombs of charge pass a given
point in a conductor in 4 seconds. Calculate the
current in the conductor.
30. A wire carries a current of 2 amperes. How
many electrons pass a given point in this wire in 1
second?
31. Which condition must exit between 2 points in
a conductor in order to maintain a flow of charge?