electrostatics for m.6
TRANSCRIPT
Electrostatics
Teacher : Piyanuch Plaon
Subject : Physics 4
Discovery of charge
Benjamin Franklin arbitrarily called the two kinds of charge positive and negative. In most cases, only the negative charge is mobile.
Properties of charge
Like charges repel, and unlike charges attract.
Charge is conserved, meaning it cannot be created or destroyed, only transferred from one location to another.
In all atoms, electrons have negative charge and protons have positive charge.
Insulators
In insulators, electrons are bound in “orbit” to the nucleus in each atom.When charge is placed on an insulator, it stays in one region and does not distribute.Wood, plastic, glass, air, and cloth are good insulators.
Conductors
In conductors electrons can move from atom to atom, thus electricity can “flow”. When charge is placed on a conductor, it redistributes to the outer surface.Metals (copper, gold, and aluminum) are good conductors.
Charging by Friction When insulators are
rubbed together, one gives up electrons and becomes positively charged, while the other gains electrons and becomes negatively charged.
Charging by Conduction When a charged conductor
makes contact with a neutral conductor there is a transfer of charge.
Electrons are transferred from the rod to the ball, leaving them both negatively charged.
Electrons are transferred from the ball to the rod, leaving them both positively charged.
CHARGING NEGATIVELY
CHARGING POSITIVELY
Step 1. A charged rod is brought near an isolated conductor. The influence of the charge object polarizes the conductor but does not yet charge it.
Step 2. The conductor is grounded to the Earth, allowing charge to flow out between it and the Earth.
Charging by Induction
Charging by Induction (cont.)
Step 3. The ground is removed while the charge rod is still nearby the conductor.
Step 4. The rod is removed and the conductor is now charge (opposite of rod).
Electric Forces and Electric Fields
CHARLES COULOMB(1736-1806)
MICHAEL FARADAY(1791-1867)
Electrostatic Charges
The charge of an electron (qe) is -1.6 x 10-19 CThe charge of an proton (qp) is 1.6 x 10-19 C
Electrostatic charge is a fundamental quantity like length, mass, and time.The symbol for charge is q. The SI unit for charge is called the coulomb (C).ATTRACTIO
N AND
REPULSION
The Electrostatic Force
The constant of proportionality, k, is equal to 9.0 x 109 Nm2/C2.
COULOMB’S LAW OF
ELECTROSTATIC FORCE
Fe kq1q2
r2
constant
distance
charges
electrostatic force
The electrostatic force depends directly on the magnitude of the charges. The force depends inversely on the square of distance between charges (another “inverse square law”)!
Electric Field Strength
g Fgm
E Feq0
DEFINITION OF
GRAVITATIONAL FIELD
DEFINITION OF
ELECTRIC FIELD
g field force
massE field
force
charge
SI unit of electric fieldnewton
coulomb
N
C
Electric field is a vector quantityE field points toward negative charges
E field points away from positive charges
q0 is a small, positive test
charge
Electric Field Lines
Density of field lines indicates electric field strengthDefinition of E Field for
single point charge
POSITIVE CHARGE
NEGATIVE CHARGE
E Feq0
kq0q / r
2
q0 E kq
r2
constant
distance
charge
electric
field
Single Point Charges
Electric Field LinesElectric fields for
multiple point charges
POSITIVE AND NEGATIVE POINT
CHARGES
TWO POSITIVE POINT
CHARGES
E kq
r2
EXAMPLE 1
EXAMPLE 2
E 9 109 Nm2 /C2 5 10 3 C
2 m 2
Electric Fields
Find the force on an proton placed 2 meters from the 5 millicoulomb charge in the problem above.E
Feq
Fe qE 1.6 10-19 C 1.13107 N/C 1.8110-12 N, to the right
Fe 9 109 Nm2 /C2 5 10 3 C 1.6 10-19 C
2 m 2 1.8 10-12 N, to the right
OR
Find the electric field strength at 2 meters from the 5 millicoulomb charge.
E=1.13107 N/C, to the rightE
PE for Two Point Charges
PE kq1q2
r
Potential energy is zero at infinite distance
Potential energy is positive for like chargesPotential energy is negative for opposite charges
Potential Energy is force times distancePE Fed
kq1q2
r2r
charges
distance
electric
potentialenergy
constant
ExampleHow much electrostatic potential energy in a
hydrogen atom, which consists of one electron at a distance of 5.3 x 10-11 meters from the nucleus (proton).PE
kq1q2
r
(9 109 )(1.6 10 19 )(–1.6 10 19 )
5.310 11 4.35 10 18 J
Potential Difference (Voltage)
Potential Energy
Charge
V PEq
A volt (v) is the unit for voltage named in honor of Alessandro Volta, inventor of the first battery.1 volt
1 joule
1 coulomb
SI Units
source voltage (V)
common dry cell 1.5
car battery 12
household (US) 120
comb through hair 500
utility pole 4,400
transmission line 120,000
Van de Graaff 400,000
lightning 1,000,000,000
V J
C
A good analogy: potential is to temperature, as potential energy is to heat.
Electric potential is average energy per charge.
Potential difference is often called voltage.
Energy is a relative quantity (absolute energy doesn’t exist), so the change in electric potential, called potential difference, is meaningful.
Voltage is only dangerous when a lot of energy is transferred.Voltage, like energy, is a scalar.
Potential Difference for Constant Electric Field
V EdV PEq
qEd
q
voltage
E field
distance
Potential energy is often stored in a capacitor.
Most capacitors have constant electric fields.
Capacitors are made by putting an insulator in between two conductors.
Example
Calculate the magnitude of the electric field set up in a 2-millimeter wide capacitor connected to a 9-volt battery.V Ed 9 E(0.002) E 4500 N/C
Consider a test charge to measure potential
Potential Difference for Point Charge
V kq
r
charge
distance
potential
difference
constant
V PE
q0
kqq0 / r
q0
Example
V1 kq1
r
(9 109 )(6 10 9 )
0.3180 V
V2 kq2
r
(9 109 )( 4 10 9 )
0.4 90 V
V3 kq3
r
(9 109 )(10 10 9 )
0.5180 V
V V1 V2 V3 180 90 180 270 V
-4 nC
10 nC 6 nC
0.3 m0.4 m
find ∆V
here
CAPACITORS A basic capacitor has two parallel plates separated by an insulating material
A capacitor stores an electrical charge between the two plates
The unit of capacitance is Farads (F)
Capacitance values are normally smaller, such as µF, nF or pF
Basic capacitor construction
Dielectric material
Plate 1
Plate 2
The dielectric material is an insulator therefore no current flows through the capacitor
CAPACITORS
Storing a charge between the plates
Electrons on the left plate are attracted toward the positive terminal of the voltage source
This leaves an excess of positively charged holes
The electrons are pushed toward the right plate
Excess electrons leave a negative charge
+ -
+ _+ _
CAPACITORS
Types of capacitors The dielectric material determines the type of capacitor
Common types of capacitors are:MicaCeramicPlastic film
CAPACITORS
Variable capacitors are used in communication equipment, radios, televisions and VCRs
They can be adjusted by consumers by tuning controls
Trimmers are internal adjusted capacitors that a consumer cannot adjust
CAPACITORS
Fringing – At the edge of the capacitor plates the flux lines extend outside the common surface area of the plates.
CAPACITANCE
THE CURRENT : IC
Current ic associated with the capacitance C is related to the voltage across the capacitor by
Where dvc/dt is a measure of the change in vc in a vanishingly small period of time.
The function dvc/dt is called the derivative of the voltage vc with respect to time t.
CAPACITORS IN SERIES AND PARALLEL
- Capacitors, like resistors, can be placed in series and in parallel.- When placed in series, the charge is the same on each capacitor.
CAPACITORS IN SERIES AND PARALLEL
Placing capacitors in parallel the voltage across each capacitor is the same.
The total charge is the sum of that on each capacitor.