Transcript 13.1 Electric Charge and Force

Electrostatics
Electrical fields
Electrical energy
Electrical forces and Charge
Arise from the particles in atoms
Protons + Attraction and
Electrons – Repelling properties
Much stronger than gravitational force!
Over a billion times stronger!
Atoms usually have the same # of
electrons and protons so they are
electrically neutral
An atom that looses or gains an electron is
called ?
Ion
Charges: like charges repel; unlike charges
Charges attracting and repelling
Conservation of Charge
Electrons are neither created nor destroyed but are
simply transferred from one material to another.
Charge is conserved.
Examples:
Rubber rod is rubbed by a piece of fur, electrons transfer
from the fur to the rubber rod
Rub a glass or plastic rod with silk, you’ll find the rod
becomes positively charged (loses electrons)
Remember orbitals in chemistry? Which electrons
are transferred?
The charge of an object is always a whole number
multiple of the charge of an electron
Electrons cannot be divided into fractions of electrons
Coulomb’s Law
For charged particles or objects that are
small compared with the distance between
them, the force between the charges
varies directly as the product of the
charges and inversely as the square of the
distance between them.
F=k q1q2
d2
9 x 109
N·m2/C2
Quantity of
charge of
1st and 2nd
object
Distance
between them
Compare Gravity and Coulomb’s Law
Only attractive
G=small magnitude
Both are inversesquare laws
Attractive or
repulsive
K=large magnitude
Both are inversesquare laws
If a pair of charges of 1C each were 1 m apart, the force of
repulsion between the two charges would be 9 billion
Newtons (10 times the weight of a battleship!
Creating a charge:
By friction
Drag your shoes across a carpeted floor and
touch a doorknob
By touch
By induction
Using a ground; infinite
supply of electrons
Charge polarization
Creating a charge on an insulator
http://www.physicsclassroom.com/mmedia
Electric Field Lines
aka lines of force
Since electric fields have magnitude
and direction, it is a ___________
quantity.
Lines drawn to represent the electric
fields are always drawn pointing away
from the positive charge
Electrical Potential Energy
Recall gravitational potential energy
A charged object can have potential
energy by virtue of its location in an
electric field.
Electrical pe if a charged particle is
increased when work is done to push
it against the electric field of
something else that is charged.
Work done is equal to energy gained
by the charge.
Electric Potential
Explain why we refer to electrical
potential per charge
You may have multiple charges in one
location. Simply discussing the electrical
energy would be misleading without
talking about how many charges you had
Electrical potential energy in a
uniform electric field
ΔPE
A is moving to B in a
uniform electric field
The change in PE depends
upon the charge, the
strength of the electric
field and the
displacement
PEel
PEel
if –
if+
http://www.saburchill.com/physics/questions/quest02.html
problems
Why is the electrical potential energy not
changed if A moves perpendicular to the
uniform field lines?
That would be the same as
moving an object
horizontally and saying it
has the same gravitational
PE
Electrical potential energy with a
pair of charges
Key points in the formula:
Reference point for electrical potential
energy starts at infinity (because PE
goes to zero as the distance between
the charges goes to infinity)
PE is + when you have like charges
bec/ like charges repel, + work must be done
to bring them together
PE is – when you have unlike charges
Electricity in Circuits
Capacitor
Passive Electronic
device to store
energy in the
form of an
electrostatic field
Consists of 2
conducting plates
separated by a
dielectric
(insulator)
http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l
1-7
Voltage
Since electric potential is measured in
volts, it is commonly called
___________.
Electric potential = electrical potential
energy
charge
1 volt = joule
coulomb
Potential difference=Voltage
Cell
Covert chemical energy to electrical
energy
AA, AAA, D batteries have the same
voltage
+ end
Max Volts
-end
Zero Volts
A voltage sets charges in motion
“voltage supplies electrical pressure”
Moving electrons (negative to
positive)
Current
Rate the charges move through a
conductor
Battery = direct current
Unit = amp
In metals, moving electrons make up the
current
In gases or liquids, both + and – charges
are in motion WHY?
Current
The flow of electric charge
Ohm’s Law
Voltage - V
Volt
Battery
Potential difference
Current - I
Amps
Rate of flow
Resistance - R
Ohms
As resistance
, current
I=V/R
Resistance
The
resistance of
the filament
determines
the brightness
of the bulb
Moving electrons
Random movement if circuit is off
Marching analogy
Where do the electrons come from in a
circuit?
Free electrons in the wires
Why do the wires become hot when a
current is flowing through them?
Transfer of kinetic energy due to electron
speed
Electric circuit
An electrical device connected so
that it provides one or more complete
paths for the movement of charges
Batteries add the energy for the
electrons to move
Electrons move – to +
Conventional Current moves + to –
Explain an open and closed circuit.
Circuit Breakers and fuses
Safety device
Respond to an overloaded circuit by
opening the circuit
Acts like a switch
Reset
Fuses:
Must replace
Electric power
Current x voltage
Unit
Watts
Kilowatt-hour
Energy delivered in 1 hour at the rate of
1 kW
Series Circuit
One path
Resistors
share voltage
Current the
same
throughout
circuit
Parallel Circuit
More than one
path
Each path
receives maximum
voltage
Current could be
different in
different
branches
Kirchoff’s Current Law
All current flowing into a branch point
must flow out
Energy Transformations
Motor
Electrical Energy
Mechanical Energy
Generator
Mechanical Energy
Electrical Energy
Hydroelectric Power Plant
http://www.fwee.org/walktour/
Mechanical Energy (Water flowing and turning the
turbines)
Electrical Energy
How does electricity get to your house?
THE SPINNING SHAFT turns
magnets inside a stationary ring of
copper, moving electrons to
produce electricity
STEP-UP TRANSFORMERS
increase the voltage of electricity
produced by the generator.
TRANSMISSON LINES carry
electricity to substations in our
communities. The voltage is
decreased and the power is
distributed to homes and
businesses
How does electricity get to your house?
THE SPINNING SHAFT turns
magnets inside a stationary ring of
copper, moving electrons to
produce electricity
STEP-UP TRANSFORMERS
increase the voltage of electricity
produced by the generator.
TRANSMISSON LINES carry
electricity to substations in our
communities. The voltage is
decreased and the power is
distributed to homes and
businesses
Calculate resistance using the
graph.