STATIC ELECTRICITYWhat is electrostatics?

The study of electrical charges that can be collected and held.

Conservation of Charge

The total charge in the universe is constant. Individual charges cannot be created or

destroyed, just separated.

The Atom

• Positive charges remain fixed in the nucleus• Negative charges (electrons) are allowed to

move

What is an ion?

An ion is an atom that has a charge:

Positive ion- atom that has lost electron(s)

Negative ion- atom that has gained electron(s)

Static Electricity

The Electroscope

A device used to measure the type (and quantity) of charge.

Charging By Conduction:

• Charging a body by touching it with a charged rod

• A transfer of electrons takes place!

Static Electricity

Charging By Induction:

• Charging a neutral body by bringing another charged body near it.

• A separation of charge takes place!

• This charge is only temporary…when the charged body is removed, the charges within the neutral body will spread out again.

Separation of Charge

When a charged body is brought near a neutral body, the electrons move towards

the more positive end.

Attraction of unlike charges will cause a neutral body’s charges to separate when

brought near a charged body.

Because of this the neutral body will behave as a charged body.

Lightning

• Air is normally an insulator

• In the case where enough charge is built up between two bodies, charges may become discharged. (That is, negative will flow to positive)

• Electrons must be freed from the molecules in the air in order for a transfer to occur (Air must become a conductor)

Lightning Continued…

• With lightning and sparks, the charges between the bodies are great enough for this to occur. The air between the bodies forms a conductor called PLASMA where the electrons in the positively and negatively charged ions are free to move.

• The rapid expansion of the air due to the intense heat is greater than the speed of sound so you hear a loud “crack” of thunder.

• The light travels faster, so you see the lightning before you hear the thunder

Van de Graaff Generator

• Device that transfers large amounts of charge from one part to the top metal dome

• An electric motor does the work needed to increase the electric P.E.

• Charges are carried up via the belt

Van de Graaff Generator Cont.

• The metal needles at the bottom are positively charged.

• They attract electrons from the belt, leaving the belt positive.

• The conveyor belt brings the positive charges up to another set of needles which draws electrons from the dome to the belt

• The dome is then left positive

Van de Graaff Generator cont.

• Charge cannot build up on the dome indefinitely. Eventually, enough charge will build up to partially ionize the surrounding air and a spark (mini lightning) will result.

• 10,000 volts are necessary for a spark to occur through 1cm of air!

Electric ForcesHow do we describe and calculate the

forces of attraction or repulsion?

Coulomb’s Law • Describes the force b/t two charged bodies. The

magnitude of the force that a body with a charge q1 exerts on a second body with a charge q2, separated by a distance, d is:

F = k q1 q2

d2

Coulomb’s Law• k = 9.0 x 109 Nm2/C2 (Electrostatic Constant)

• q1 = charge of body one

• q2 = charge of body two

• d = distance b/t their centers of mass

Coulomb’s Law states:

1. Electric force varies inversely with the square of the distance b/t the two bodies

2. Electric force varies directly with the product of the two (or more) charges

The unit used for charge, q, is the Coulomb.

S.I. unit Symbol : C

1 C = the charge of 6.25 x 1018 e

The elementary charge, e, is that of one electron.

1 e = 1.6 x 10-19 C

To convert b/t C and e:

1 C = 1.6 x 10-19 C

6.25 x 1018 e 1 e

Electric Forces

When calculating electric force,

• If the two charges are the same, F = repulsive (+)

• If the two charges are different, F = attractive (-)

Note: Coulomb’s Law is an Inverse-Square Law…

Electric Fields

Michael Faraday (1791-1867) developed the concept of the electric field (a field is a region of space where a force is felt)

• A charge creates an electric field about it in all directions.

• If another charge is placed in the field, it will interact.

• In a field, interaction is local so interaction between particles at some distance is no longer required.

Electric Fields

The electric field – the vector sum of fields on individual charges.

• The ratio of the force exerted on a test charge to the charge:symbol: E E = F/qunits: N/CE = Electric FieldF = force on the chargeq = charge of the test charge

The Electric Field

The direction of the field is the direction of a force that would be exerted on a positive test charge.

The strength of the force depends on E and the size of the charge.

Rewrite the equation: F = E q

k q1 q2 = E q1

d2

Intensity of Electric Fields

Point Charge: the force follows an inverse-square law (Coulomb’s Law)

Rod: the force varies inversely with the distanceF

dParallel Plates: the force is uniform

F

d

Electric Field Lines

Electric Field Lines- lines that represent the direction and strength of the field.

• The direction of the field at any point is a tangent line drawn to the line at that point

• The strength is indicated by the spacing between the lines (close= strong)

• Positive field – lines point radially outward because of repulsive force

• Negative Field- lines point radially inward because of attractive force

Electric Field Lines

• Field lines never cross each other

• Electric fields are real, whereas electric field lines are imaginary

• You can feel the effects (and see them) of an electric field

• You cannot see the field lines

Electric Fields Near Conductors

• All of the charges move to the outside surface of a conductor

Hollow Sphere: -the electric field

outside of the conductor acts as though it were a point charge.

-the electric field inside is zero b/c all of the forces cancel

Example: Faraday’s Cage

Effect of the Shapes of Conductors

• Rounded – the field spreads out uniformly

• Pointed – the field concentrates at the point

• ALL of the charges stay to the outside surface – This explains why you are safe inside your car when the threat of electrical shock is near.

Application of Electric Fields

• When unlike charges attract, it requires WORK to separate them.

• The work done on the charge is stored in the charge as POTENTIAL ENERGY.

• WORK = POTENTIAL ENERGY

• The larger the charge, the larger the PE

• Conversely, it requires work to keep like charges together.

Application of Electric FieldsShapes of conductors continued…

• Electric charge will move to the outside of a conductor so that the like charges can be as far apart as possible.

• They want to LOWER THE ENERGY OF THE SYSTEM.

• When two objects that are charged, come into contact, charge will flow from one to another…HIGHER POTENTIAL TO LOWER POTENTIAL

Application of Electric Fields

• Charges want to move as far apart as possible to lower the energy of the system

• Charges will move to the lower potential

• Lower potential (V) – less charge per unit area

• Higher potential (V) – more charge per unit area

Electric Potential Difference

Electric potential difference – change in potential energy per unit charge

Work done in moving a test charge in an electric field divided by the magnitude of the test charge.

Symbol: V

Units: Volts or J/C

∆V = ∆PE/q or Work/q

Electric Potential DifferenceParallel Plates:

+ + + +high V low V

low V high V

_ _ _ _Electric Potential -potential energy of unit charge

We can describe the charges between the plates in terms of their PE (high or low V) referenced to the plates. Only changes in electric potential are meaningful.

Electric Potential Difference

Electric Potential Difference= Potential Difference = Voltage

V = Vb - Va

V = electric potential = PE/qV = electric potential difference

(potential difference or voltage = PE/q)

Equipotential

• When the electric potential difference between two or more positions is zero, those positions are said to be at equipotential.

• The positive charges at A and B are at the same potential b/c their distance from the positive plate is the same. The charge at C is different~it has a lesser potential.

Electric Potential In A Uniform Field

• Parallel Plates ~ for two parallel, oppositely charged plates, the field they create is UNIFORM, except near the edges.

• The potential difference (voltage) is then:V = E d E = electric field

d = distance b/t plates

V = pot. DifferenceUnits: Nm/C or J/C or V

V = E d = F d = Work = PE q q q

Application: Equipotential Lab

low V high V• How would an electron behave in the field?• It would “roll down hill”

high VSide view: low VSide view for parallel plates: -

+

Millikan’s Oil Drop Experiment

• An application of a uniform electric field b/t two parallel plates.

• Millikin used this to measure the charge of the electronElectricity , Electricity

Millikan’s Oil Drop Experiment

• Drop becomes charged because of friction through the atomizer.

• A single drop falls through the hole into an electric field.

• A negative drop will be attracted to the positive plate above.

• The electric field, E, is adjusted to suspend the drop in the field

Millikan’s Oil Drop Experiment

Fg = Fe

m g = E q

q = m g E

• The drop is too tiny to be measured by ordinary experiments

• The drop was first suspended, then the electric field was turned off and the rate of fall was measured

• q is always some multiple of 1.6 x 10-19 C

Storing Charges

• Capacitance – the ratio of charge stored to the electric potential difference

C = q

VCapacitor – a device designed to have a specific

capacitance Capacitor - Wikipedia, the free encyclopedia

These are useful in circuits because they store charge.

Storing Charges

• When you connect a capacitor in a circuit to a battery, the capacitor will charge until its potential difference equals that of the battery.

• Once you disconnect the battery, the capacitor will remain charged until it is attached to a conductive material.

• Usefulness: they supply(or discharge) much faster than a battery

• Ex…camera flashes HowStuffWorks "How Camera Flashes Work", computer buttons

Power conditioning

A 10,000 microfarad capacitor in a TRM-800 amplifier

Reservoir capacitors are used in power supplies where they smooth the output of a full or half wave rectifier. They can also be used in charge pump circuits as the energy storage element in the generation of higher voltages than the input voltage.

Capacitors are connected in parallel with the power circuits of most electronic devices and larger systems (such as factories) to shunt away and conceal current fluctuations from the primary power source to provide a "clean" power supply for signal or control circuits. Audio equipment, for example, uses several capacitors in this way, to shunt away power line hum before it gets into the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass AC currents from the power supply. This is used in car audio applications, when a stiffening capacitor compensates for the inductance and resistance of the leads to the lead-acid car battery.