Static Electricity

Static ElectricityElectrostatics

Static- not moving. Electric charges that can be collected an held in one placeExamples: sparks on carpet, balloon against hair, lightning, photocopierHistory: ancient Greeks made little sparks when rubbing amber with fur (Greek word for amber: elektron)

Electric charge, q, is measured in Coulombs, C. One Coulomb is charge is a dangerously high charge. An average lightning bolt has about 10 Coulombs of charge.

Atomic ViewProton: in nucleusPositive chargeq = + 1.6 x 10-19 CElectron: outside nucleusNegative chargeq = - 1.6 x 10-19 CProtons and Electrons have the same amount of charge but a proton has much more mass!Neutron: in nucleus, has no chargeMolecules2 or more atoms bonded togetherusually atoms and molecules are neutral, but if they have a net charge, they are called IONS

Behavior of chargesUnlike charges attractLike charges repelA neutral object will attract both positive and negative charges

Charles Coulomb, mid 1700s, studied and published papers about the electrostatic force between 2 charged objects.Ben Franklin was the first to use the terms positive and negative to describe electrical charge. Mid 1700s

Hmmm..+ + +- - -

Robert MillikanFirst determined the elementary charge- the charge on an electron or proton. (early 1900s)

MaterialsConductorsSubstances that have easily moveable electric chargesMost familiar conductors are metals that have free electronsPositive ions may also be mobileInsulatorsCharges cannot move easilyExamples: plastic, wood, glass

Semiconductor: used in computersConduction is an intermediate magnitude between a conductor and an insulator

Superconductor: NO resistance to the flow of electrons. So far, no material is a superconductor except at extremely low temperatures.

Water: insulator or conductor?PURE water does NOT conduct electricityImpurities or ions in water can allow conductionThe purer the water, the lower the conductivity(the conduction of electricity is called ELECTROLYTIC behavior- )Air: insulator or conductor?Usually an insulator, thankfullyWhen strong forces are present, electrons can be stripped from air molecules, creating ionsexample: lightning

LightningAn electrical discharge between the clouds and the ground or between two clouds.

As the electrons flow through the ionized air, they generate so much heat that a PLASMA is produced. We see that plasma and call it LIGHTNING!

The air around the lightning expands so rapidly from the heat that it creates a strong pressure wave of air molecules (thats sound!)We call that THUNDER!How much electrical charge is flowing through a lightning bolt?Typically around 10 Coulombs of charge.How many electrons, each with a negative charge of 1.6 x 10-19 C, does it take to have 10 C of charge?10 C / 1.6 x 10-19 C =6.25 x 1019 electrons !How many electrons are flowing in a 12 C lightning bolt?7.5 x 1019 electronsThe Earth is able to absorb much electrical charge.Touching a charged object to the Earth in order to discharge it is calledGROUNDING

Methods to electrically charge an objectConduction: Direct contact: will transfer electrons, such as touching your car door in the winterFriction: rubbing your feet against carpet, hair against a balloon

Induction: no direct contactStart with a neutral object. Then, bring an electrically charged object near, but not in contact with, a neutral object

The charges in the neutral object will be induced to separate to get closer or farther from the charged object.

If provided a pathway, the separated electrons will leave.

The object is now positively charged.

Static devicesElectroscope: the separation of metal leaves indicates the presence of static chargeVan de Graaff generator: charge is delivered by a rubber belt to a metal domeElectrophorus a device used to transfer electric charge

Coulombs LawCalculates the magnitude of the electric force between two charges Each charge experiences equal but opposite forces

where k is a constant, k = 9 x 109 Nm2/C2

Coulombs Law looks VERY similar to Newtons Universal Law of Gravitation

Similarities:Both act in a vacuumBoth are conservativeBoth are inverse square lawsBoth propagate with a finite speed, c, the speed of light

Differences:Electrostatic forces are stronger than gravitationalFE = FG 1036= age of the universe in seconds!!!!Gravity attracts on like charges, Electrostatic forces repel like charges and attract opposite chargesThere are NO negative gravitational chargesBoth laws are INVERSE SQUARE LAWSThe Force varies with the inverse of the distance squared.At twice the distance, 22 in denominator = the Force,At three times the distance, 32 in denominator,= 1/9 the ForceAt half the distance, (1/2)2 in denominator= 4 times the ForceNow if one CHARGE doubles. The Force doubles since they are directly related.Force is a VECTOR!Electric FieldA gravitational field surrounds all masses.

An electric field surrounds all charges.

The stronger the electric charge, the stronger the electric field surrounding it.

One way to measure the strength of a gravitational field is to release a mass in the field and measure how strength of the force exerted on it.One way to measure the strength of an electrical field is to release a charge in the field and measure the strength of the force exerted on it.So the strength of the electric field, E, is given by

Electric Field = Force charge

E = F qFor example:A 0.5 C charge experiences a force of 20 N when placed in an electric field.What is the strength of the electric field, E?E = F q =20 N 0.5 C =40 N/CThe electric field near a charged piece of plastic or styrofoam is around 1000 N/C.

The electric field in a television picture tube is around 10,000 N/C.

The electric field at the location of the electron in a Hydrogen atom is 500,000,000,000 N/C!

The further you go from an electric charge, the weaker the field becomes.

The electric field around a charge can be represented byElectric field linesElectric fields exist, but electric field lines dont really exist but provide a model of the electric field.

Electric Field Lines

Electric field lines always point OUT of a positive charge and INTO a negative chargeTo indicate a stronger electric field, just draw MORE lines.The farther apart the lines, the weaker the field.Since the electric field, E, has both magnitude and direction, it is a vector.- 4q+2qThe electric field INSIDE a hollow conductor is ZERO even if there are charges on the OUTSIDE of the conductor!

Electric ShieldingThere is no way to shield from gravity, but there is a way to shield from an electric field.Surround yourself or whatever you wish to shield with a conductor (even if it is more like a cage that a solid surface)Thats why certain electric components are enclosed in metal boxes and even certain cables, like coaxial cables have a metal covering.The covering shields them from all outside electrical activity.

Faraday Cage

Are you safe from lightning inside your car?Why or why not?

Accelerating ChargesA charge placed in an electric field will experience an electric force, F = EqThis force will make the charge accelerate according to Newtons Second LawF = ma

What direction will a charge accelerate?

+-++++++++Positive charges will accelerate in the same direction as the electric field.Negative charges will accelerate in the opposite direction of the electric field.The Electric Field can also be determined by using Coulombs Law:

Electric Potential EnergyEnergy stored up between 2 charges separated by a distance d:

Unit: Joules

dChanging the Electric Potential EnergyIf you raise or lower a mass in a gravitational field, you change thegravitational potential energy, UG.

If you move a charge in an electric field, you change the electric potential energy, UE.Move a mass, mThrough a gravitational field, gA distance, hGravitational Potential Energy, mgh

Move a charge, qThrough an electrical field, EA distance, dElectrical Potential Energy, qEd

The work energy required to move a charge through an electric field is given by

W = qEd

+++++++++Two Ways to Find Electric Potential Energy ?

Are these the same thing???Which equation should be used??

Conversion of energyMoving a mass or moving a charge takeswork energythat is converted topotential energyWork = mghOr Work = qEd

If you release an object in a gravitational field,its gravitational potential energy is converted to kinetic energy.If you RELEASE a charge in an electrical field, its potential energy is converted to kinetic energy!UE = mv2E

-ExamplesWhat is the potential energy stored between 2 charges of 3 C and 4 C separated by 2 m?

5.4 x 1010 JIt takes 2.43 x 10-15 J of work to move an electron as distance of 2 m in an electric field. What is the strength of the field?

W = qEd

E = 7600 N/CThe electron is then released. What is the maximum velocity it will achieve?2.43 x 10-15 J = W = qEd = mv2

v = 7.3 x 107 m/sPre-AP ProblemsA 2 mC and a 3 mC are separated by 0.15 m. What is the potential energy? What is the Force they exert on each other?If you double the distance between 2 charges, what happens to the force they exert on each other? What happens to the potential energy?How much work is required to move a 4 mC charge of mass 0.02 kg 1.5 m in a 7500 N/C electric field? If the charge was then released from rest, what will be its maximum velocity?In a hydrogen atom, an electron orbits a proton at a distance of 0.53 x 10-10 m. What is the force between them? What is the potential energy?An electron reached a maximum velocity of 2.3 x 106 m/s moving through a distance of 1.4 m after being released from rest in an electric field. What is the strength of the field?It requires 6.7 x 10-12 J to move a proton through a 4000N/C field. How far was it moved? When released, what maximum velocity will it have?

If two charges are placed close to each other and held in place, there is an electric potential energy stored between them.++Two charges in an electric field at the same location will have twice as much electric potential energy as one charge;Five charges will have five time the potential energy, and so onIt is often convenient to consider the electric potential energy per charge.

+++++

The concept of the electric potential energy per charge has a special name-Electric PotentialUnit: Joule/coulomb.However, it gets its own unit called a volt.1 volt = 1 joule / coulombSince electric potential is measured in volts, it is commonly called Voltage.Electric Potential = Voltage

VoltageVoltage can be thought of as a kind of pressure- Electrical Pressure

Voltage is also called Electric Potential

Think of the water supply at your house- sometimes you have high water pressure-water flows quickly- and sometimes low water pressure- water flows slowly.With Higher Voltage (pressure), charges are able to flow more quickly

Rub a balloon on your hair and it becomes negatively charged, perhaps to several thousand volts. Does this mean that theres a lot of electrical energy?Well, the charge transferred to the balloon is typically less than a millionth of a Coulomb (Remember, one Coulomb is charge is a HUGE amount of charge)

Voltage = Energy / chargeEnergy = Voltage x chargeEnergy = 3000 V x 0.000001 CEnergy = 0.003 JThats not much energy!Theres a LOT of difference between Voltage and Energy!

High Voltage does not necessarily mean that theres a lot of useful energy or that something is dangerous.High Voltage is not necessarily dangerous- a Van de Graaff generator can have more than 400,000 V, but theres not much charge that is transferred to you from the globe.

Low Voltage is not necessarily safe. Our houses are wired with 120V and you can be killed from that electricity.

Voltage (potential) is not the dangerous part of electricity. The dangerous part is how many charges are flowing- the current.

The Electric Potential (Voltage), V, changes as you move from one place to another in an electric fieldThe change in Potential (pressure), called the Potential Difference is given byDV = Ed

For example, the potential difference between two locations separated by 3 meters in a 4000 N/C electric field is given byDV = Ed = 4000 N/C x 3 m = 12,000 V

3 metersElectric FieldThe work energy required to move a charge, q, through an electric field, E, a distance d, is given by

W = qEd = qDV

Sometimes, a charge is said to be located at ground. The potential (voltage) at ground is zero. Vground = 0 Volts

+++++++++How much work is required to move a 3 C charge through an electric field of 2000 N/C a distance of 1.5 m?How much work is required to move 0.5 C of charge through a potential difference of 110 V?In a TV picture tube, an electron moves through a potential difference of 5000 V. How much work energy is required?It takes 2000 J of work to move a certain charge through a 400 N/C electric field a distance of 2 m. What is the charge?What is the potential difference between two points in a 3000 N/C electric field that are 0.3 m apart?What is the voltage between two points in a 4500 N/C electric field that are m apart? If the potential difference between two points in an electric field of 500 N/C is 220 V, how far apart are the two points?What is the strength of the electric field if there is a potential difference of 600 V at two locations that are 0.25 m apart?There is another unit for very tiny amounts of energy associated with atoms and sub-atomic particles. It is called an electron-Volt or eV.One electron-Volt is the amount of work energy required to move one electron through 1 Volt of potential difference.In other words, 1 eV = W = qDV = 1.6 x 10-19 C x 1V

So the conversion between eVs and Joules is1 eV = 1.6 x 10-19 J

The Electric Potential, V, due to a point charge, q, is given by

The potential will have the same sign as the charge- there can be a large positive and a large negative potential

qV = ??

At very great distances away from a charge d is very largeThe Potential, V, due to that charge is virtually ZERO.Potential due to more than one chargePotential is NOT a vector. (yea!!!).SoThe potential due to a group of point charges is given by

ExampleWhat is the potential halfway between 2 charges of 3mC and 4mC located 16 cm apart?787500V

What would be the potential if the 4mC charge were negative?-112500 V

The potential near a positive charge will be higher (its positive!) than the potential near a negative charge (its negative!).

Therefore a positive charge will accelerate from high to low VA negative charge will accelerate fromlow to high V+-Higher VLower V-Capacitors: Electric Energy StorageA device consisting of two conductors placed near, but not touching each other in which electric charge and energy can be stored.+-+Capacitors are Used incamera flashesdefibrillatorsComputers: tiny capacitors store the 1s and 0s for the binary codeMany keyboards have a capacitor beneath each key that records every key stroke.Virtually every electronic deviceLeyden Jar, the first capacitor

Dutch physicist Pieter van Musschenbroek of the University of Leyden

The capacitance measures how much charge can be separated in the capacitor per voltage and is measured in farads, F.

Capacitance, C =

-Parallel-Plate capacitorsthe capacitance is increased by placing an insulating material, a dielectric, between the two platesThe capacitance is given by

C =

A = area of plates eo = permittivity of free space = dielectric constant (air or vacuum, k = 1) d = distance between the plates

+-+Energy stored in a capacitorEnergy U = CV2

Substituting from C = q/V will yield other equations for U.-Parallel-Plate capacitors+-+The Electric Field inside a capacitor,

Move to a sturdy building or car. Do not take shelter in small sheds, under isolated trees, or in convertible automobiles. Get out of boats and away from water. Telephones lines and metal pipes can conduct electricity. Unplug appliances if possible and avoid using the telephone (unless it is an emergency) or any electrical appliances. Do not take a bath or shower. If you are caught outdoors and unable to find shelter: Find a low spot away from trees, fences and poles. If you are in the woods, take shelter under the shorter trees. If you feel your skin tingle or your hair stand on end, squat low to the ground on the balls of your feet. Place you hands on your knees, put your head down and try to make yourself the smallest target possible while minimizing your contact with the ground.